58 Refrigeration

Learning Objectives

  • Explain fundamentals of refrigeration
  • Describe the identification, classification, and physical properties of refrigerants
  • Describe the basic layout of compression refrigeration systems
  • Describe the construction and operation of refrigeration compressors
  • Describe the design of refrigeration evaporators and condensers
  • Condenser operation and maintenance
  • Troubleshoot cooling tower operation
  • List the functional components of air conditioning systems
  • Describe the operation of air conditioning systems

Essential Question

How is air cooled?

Introduction

From the simplest act of opening a window for fresh air to the intricate environmental controls of large establishments like hospitals and museums, the HVAC (heating, ventilation, and air conditioning) field is vital yet often underestimated in modern building structures. In this introductory text, we will explore the fundamental concepts of cooling, as well as the principle of refrigeration and its connection to air conditioning.

Refrigeration is a thermodynamic process that entails extracting heat from one location and transferring it to another. According to the laws of thermodynamics, heat naturally moves from areas of higher temperature to lower temperature. This can be achieved by using ice in a cooler or through mechanical means. The capacity of any refrigerating system is determined by its ability to transfer heat from the refrigerated medium.

Refrigeration Theory

Definition: “the thermodynamic process of lowering the temperature of a substance below the temperature of its surroundings and maintaining this substance at the lower temperature” (PanGlobal, 2017, 15-3)

This process is the process of transferring heat from the entity into an entity with a cooler temperature (this is called a refrigerant). When there is a loss of heat, it makes the temperature get lower and the entity to become cooler. Sensible heat is the heat that is lost from the first entity. This heat is absorbed by the refrigerant, BUT it does not increase the temperature. It will actually change the state of the refrigerant. This absorbed heat is called latent heat. 

Example

An example of this principle is an icebox:

Refrigerant In an icebox, the refrigerant is the ice. As the ice melts, it absorbs heat from the air and the items stored in the icebox, cooling them down.
Latent Heat When the ice absorbs heat and melts, it undergoes a phase change from solid to liquid, absorbing latent heat in the process. This latent heat absorption keeps the icebox interior cool.
Sensible Heat The air inside the icebox and the items stored in it transfer sensible heat to the ice as they come into contact with it. This sensible heat transfer lowers the temperature of the air and the items, contributing to the cooling effect.
Heat Transfer Medium The air inside the icebox acts as the heat transfer medium. It circulates around the items stored in the icebox, facilitating the exchange of heat with the ice. The air carries heat away from the items and transfers it to the ice, where it is absorbed.
Role of Air The air inside the icebox plays a crucial role in transferring heat from the items to the ice. As the air circulates, it continuously exchanges heat with the items and the ice, helping to maintain a cool temperature inside the icebox.
Storage Compartment The storage compartment of the icebox provides space to store items that need to be kept cold. The cooling effect generated by the melting ice ensures that the items remain chilled while stored in the compartment.

In summary, the icebox example illustrates how the ice acts as a refrigerant, absorbing latent heat to melt and provide cooling. Sensible heat transfer occurs between the items stored in the icebox and the ice, facilitated by the circulating air. The air serves as the heat transfer medium, facilitating the exchange of heat between the items and the ice. The storage compartment provides a space to store items while benefiting from the cooling effect generated by the ice.

Disadvantages of Ice as a Refrigerant
  • The lowest temperature obtainable is 0°C (32°F)
  • Regularly add ice
  • Need to discard the water
  • Hard to control the temperature
Example

Another example of this principle is a felt-covered canteen used in dry, hot regions to keep drinking water cool.

  • Refrigerant: In this example, the refrigerant is the water inside the canteen. As water evaporates from the surface of the felt, it absorbs heat from the surrounding environment, including the water inside the canteen. This absorption of heat causes the temperature of the water inside the canteen to decrease, providing the cooling effect.
  • Latent Heat: When water evaporates from the surface of the felt, it undergoes a phase change from liquid to vapor. This process requires energy in the form of heat, known as latent heat. The latent heat absorbed from the surrounding environment during evaporation is what cools the water inside the canteen.
  • Sensible Heat: As the water inside the canteen cools down, it transfers sensible heat to the felt and the air surrounding the canteen. This sensible heat transfer occurs as the water loses heat to its surroundings, further contributing to the cooling effect.
  • Heat Transfer Medium (Air): The air surrounding the felt-covered canteen acts as the heat transfer medium. As water evaporates from the surface of the felt, it releases heat into the surrounding air. The air then carries this heat away from the canteen, helping to maintain a cool temperature inside.

In summary, the felt-covered canteen example demonstrates how water acts as the refrigerant, evaporating from the surface of the felt and absorbing latent heat from the surroundings to provide cooling. Sensible heat is transferred from the water to the felt and the surrounding air, contributing to the overall cooling effect. The air serves as the heat transfer medium, carrying heat away from the canteen and helping to maintain a cool temperature inside.

Evaporation 

Refrigeration by evaporation can happen by using vaporizing liquids (refrigerants). They have a low boiling point at atmospheric pressure. The evaporator is the container in which the refrigerant is vaporized.

Example

An example of refrigeration by evaporation is the cooling of R-12 (dichlorodifluoromethane) which boils at -28.8°C (-22°F).

Refrigerant R-12, also known as dichlorodifluoromethane, serves as the refrigerant in this example. As a refrigerant, R-12 is circulated through a closed loop system within a refrigeration device, such as an air conditioner or refrigerator. It undergoes phase changes to absorb and release heat, providing cooling within the system.
Latent Heat R-12 undergoes a phase change from liquid to vapor within the refrigeration system. This phase change requires energy in the form of heat, known as latent heat. When R-12 evaporates, it absorbs heat from its surroundings, causing the temperature within the system to decrease. This absorption of heat provides the cooling effect required for refrigeration.
Sensible As R-12 vaporizes and absorbs heat, it cools the surrounding environment within the refrigeration system. The surrounding air or fluid transfers sensible heat to the R-12, causing its temperature to rise. This sensible heat transfer occurs as heat from the surroundings is transferred to the refrigerant, further facilitating the refrigeration process.
Heat Transfer Medium
  • In a refrigeration system, the heat transfer medium can vary depending on the specific application. It could be air, water, or another fluid circulating within the system. The heat transfer medium carries heat away from the refrigeration system, allowing the refrigerant to absorb heat and provide cooling. In the case of air conditioning systems, for example, air serves as the heat transfer medium as it flows over the evaporator coils, transferring heat to the refrigerant.

Types of Refrigerants

There are many types of refrigerants used in the industry. This section will discuss the identification, classification and properties of refrigerants.

The following tables in this section are taken from PanGlobal (2017) under the Fair Use Policy.

Common Refrigerants

Refrigerant Number Chemical Formula Chemical Name Trade Name
R-11 CCl 3 F Trichlorofluoromethane R-11
R-12 CCl 2 F 2 Dichlorodifluoromethane R-12
R-22 CHClF 2 Chlorodifluoromethane R-22
R-113 CCl 2 FCClF 2 Trichlorotrifluoroethane R-113
R-114 CClF 2 CClF 2 Dichlorotetrafluoroethane R-114
R-717 NH 3 Ammonia Ammonia
Classification

Classified in the Canadian Standards Association B52 Mechanical Refrigeration Code according to their toxic and flammable categorization.

Toxicity Flammability Examples
Group A1 Non-toxic Non-flammable R-11 Trichlorofluoromethane

R-22 Chlorodifluoromethane
Group A2 Low toxicity Flammable R-142b 1-chloro-1. 1-difluoroethane

R-152a 1,1-difluoroethane
Group A3 Low toxicity Highly flammable R-600 Butane

R-290 Propane
Group B1 Highly toxic Non-flammable R-764 Sulphur dioxide
Group B2 Highly toxic Flammable R-717 Ammonia
Group B3 Highly toxic Highly flammable Very rare, special application only
Characteristics

An ideal refrigerant should have the following characteristics:

  • low boiling point at atmospheric pressure
  • high latent heat capacity, requiring a large amount of heat to convert from a liquid to a gas after the boiling point
  • fairly low condensing pressure
  • neutral scent
  • non-corrosive
  • non flammable
  • non explosive
  • cost effective

An example of a characteristics’ comparison of three refrigerants.

R-12 R-22 Ammonia
Chemical symbol CCl 2 F 2 CHClF 2  NH 3
Refrigerant no. R-12 R-22 R-717
Boiling point at atmospheric pressure -28°C (-21.6°F) -40°C (-41.4°F) -33.3°C (-27.9°F)
Evaporating pressure at -15°C (5°F) 182.6 kPa (26.5 psi) abs 292.7 kPa (42.9 psi) abs 236.2 kPa (34.3 psi) abs
Evaporating pressure (absolute) at -15°C (5°F) 182.6 kPa (26.5 psi) 292.7 kPa (42.4 psi) 236.2 kPa (34.3 psi)
Condensing pressure at 30°C (86°F) (absolute) 745 kPa (108 psi) 1191.9 kPa (173 psi) 1165.8 kPa (169 psi)
Net refrigerant effect 116.4 kJ/kg 162.9 kJ/kg 1103.5 (kJ/kg)
Flammable No No Slightly
Explosive when mixed with air No No Yes
Toxic  No No Yes
Corrosive effect on metals when water is present None None Attacks copper and copper alloys
Effect of contact with foods None None Spoils taste, may cause toxicity
Effective on lubricating oil Mixes with mineral oil mixes with mineral oil to some extent Does not mix
Odor None None Strong and offensive
Applications Household & commercial refrigerating and air conditioning Commercial & industrial refrigeration and air conditioning Industrial & commercial refrigeration
Physical Properties
Miscibility Definition: the ability to dissolve into oil and vice versa

In refrigeration, miscibility refers to the ability of refrigerants and oils to mix together uniformly when they come into contact. This property is crucial because refrigeration systems often rely on the circulation of refrigerant-oil mixtures to lubricate moving parts, enhance heat transfer, and maintain system efficiency. It means the ability of a refrigerant to dissolve or blend evenly with the lubricating oil used in the refrigeration system. Ideally, refrigerants should be miscible with oil to form a stable solution that can effectively lubricate compressor components and facilitate heat transfer. A refrigerant that is miscible with oil will ensure proper lubrication and minimize wear and tear on system components.

Conversely, miscibility in terms of oil dissolving in refrigerant refers to the ability of oil to dissolve or disperse within the refrigerant. While refrigerants are primarily gases or liquids, they can still absorb small amounts of oil, especially during compressor operation. However, excessive oil entrainment in the refrigerant can lead to poor heat transfer, reduced system efficiency, and potential damage to downstream components such as expansion valves and heat exchangers. Therefore, it is important to select refrigerants and oils that exhibit good miscibility to ensure optimal performance and longevity of refrigeration systems.
Leakage Tendency Definition: leakage tendency refers to how likely a substance is to escape from a sealed system or container over time.

It indicates how prone a substance is to migrate through barriers or openings, leading to unintended loss or release into the surrounding environment. In refrigeration systems, for example, refrigerant leakage can occur due to factors such as operating pressure, viscosity, and density. The leakage tendency of a substance can be related to its molecular mass. Generally, substances with lower molecular masses may have higher leakage tendencies because their smaller molecules can more easily pass through barriers or openings in a sealed system. Conversely, substances with higher molecular masses may have lower leakage tendencies because their larger molecules are less likely to migrate through such barriers. However, other factors such as pressure, temperature, and chemical properties also play significant roles in determining the leakage tendency of a substance. Therefore, understanding the leakage tendency of substances is crucial for proper system maintenance, leak detection, and environmental management.
Odor Refrigerants can have various odors, depending on their chemical composition. Some refrigerants have no odor, while others may have a characteristic smell that can range from faint to strong. However, it’s essential to note that many refrigerants are odorless or have odors that are not easily detectable at low concentrations. Odors can also be masked by additives or contaminants present in the refrigerant. In some cases, refrigerant leaks may produce noticeable odors, which can help in detecting leaks and addressing potential safety concerns. A substance with a really strong odor is not that advantageous because this can cause a public incident or commotion. Overall, the odor of refrigerants can vary widely, and it’s essential to follow proper safety protocols when handling them.
Toxicity Refrigerants are categorized into groups according to their toxicity. Generally, they are not very toxic in low concentrations. However, when they come in contact with a flame or become very toxic. A halide torch can be used as a tool to detect leaks and repairs can be made using soldering or brazing, however ventilation must be available.
Flammability/Explosiveness Among the refrigerants studied, ammonia is the only one that is combustible, and that too in concentrations between 16%-25% by volume in air.
Moise Reaction Different refrigerants accumulate different amounts of moisture. When a refrigerant is at a high temperature, it can absorb more moisture, compared to when it is at a lower temperature. So, when a warm refrigerated saturated with moisture cools down, it will produce free water.

Moisture accumulation should be avoided:

  • It can cause ice to form between the valve and valve seat of the regulation or expansion valve when the liquid pressure and temperature is reduced when it goes to the evaporator side of the valve. This will create improper operation pf the valve.
  • It can cause acid to form which will result in corrosion, sludge in the compressor crank case and/or deterioration of the motor insulation in semi-hermetic or hermetic compressors.

Refrigeration Systems

Definition: A refrigeration system is a mechanical setup that transfers heat from one place to another, creating a cooling effect. It typically involves components like compressors, condensers, evaporators, and refrigerant lines, working together to achieve cooling.

Compression Refrigeration Systems

A compression refrigeration system is a type of refrigeration system that operates by compressing a refrigerant gas to increase its pressure and temperature. The compressed refrigerant is then condensed to a liquid state, releasing heat in the process. The high-pressure liquid refrigerant passes through an expansion valve, where it undergoes a rapid expansion, leading to a decrease in pressure and temperature. This low-pressure, low-temperature refrigerant then absorbs heat from the surroundings, cooling the desired area or substance. Finally, the refrigerant returns to the compressor to restart the cycle. Compression refrigeration systems are widely used in various applications, including air conditioning, refrigeration, and industrial processes.

There are five principal parts in a simple closed-cycle compression refrigeration system:

  1. evaporator
  2. compressor
  3. condenser
  4. liquid refrigerant control or regulating valve
  5. liquid receiver

Some other components can also be found such as drier, sight glass, etc.

Construction

Here’s an overview of the construction of such a system:

Compressor The system begins with the compressor, which is usually located outside the refrigerated space. The compressor is responsible for compressing the low-pressure refrigerant vapor into a high-pressure gas. It may be powered by electricity or other sources of mechanical energy.
Condenser After compression, the high-pressure, high-temperature refrigerant gas flows into the condenser coil, which is often located outside the refrigerated space or in a separate unit. In the condenser, the refrigerant releases heat to the surrounding environment and condenses into a high-pressure liquid.
Expansion Valve The high-pressure liquid refrigerant then passes through an expansion valve or capillary tube, where its pressure is rapidly reduced. This sudden drop in pressure causes the refrigerant to expand and partially vaporize, leading to a decrease in temperature.
Evaporator The low-pressure, low-temperature refrigerant liquid-vapor mixture then enters the evaporator coil, which is typically located inside the refrigerated space. Here, the refrigerant absorbs heat from the surrounding environment, causing it to evaporate fully into a low-pressure vapor. This absorption of heat cools the refrigerated space.
Refrigerant Lines The various components of the refrigeration system are connected by refrigerant lines, which transport the refrigerant between the compressor, condenser, expansion valve, and evaporator.
Control Systems The refrigeration system may also include control systems, such as thermostats and pressure sensors, to regulate the operation of the compressor and other components based on temperature and pressure conditions.

Overall, the construction of a compression refrigeration system involves assembling these components in a specific configuration to create a closed-loop cycle that efficiently removes heat from the refrigerated space, providing cooling as desired.

Operation

A compression refrigeration system is a type of refrigeration system that operates based on the compression and expansion of a refrigerant. Here’s a simplified overview of how it works:

Compression The process begins with the compressor, which compresses the low-pressure vaporized refrigerant into a high-pressure, high-temperature gas. This compression increases the energy and temperature of the refrigerant.
Condensation The hot, high-pressure refrigerant gas then flows into the condenser coil, where it releases heat to the surrounding environment and condenses into a high-pressure liquid. This phase change from gas to liquid releases heat energy, causing the refrigerant to cool down.
Expansion The high-pressure liquid refrigerant then passes through an expansion valve or capillary tube, where its pressure is rapidly reduced. This sudden drop in pressure causes the refrigerant to expand and partially vaporize, leading to a decrease in temperature.
Evaporation The cold, low-pressure refrigerant liquid-vapor mixture then enters the evaporator coil, located inside the refrigerated space. Here, the refrigerant absorbs heat from the surrounding environment, causing it to evaporate fully into a low-pressure vapor. This absorption of heat cools the refrigerated space.
Return to the Compressor The low-pressure vaporized refrigerant then returns to the compressor to restart the cycle.

This continuous cycle of compression, condensation, expansion, and evaporation allows the compression refrigeration system to remove heat from a refrigerated space and maintain low temperatures. Compression refrigeration systems are widely used in various applications, including air conditioning, refrigeration, and industrial processes, due to their efficiency and effectiveness in cooling.

Refrigeration Compressors

Refrigeration compressors play a crucial role in compression refrigeration systems and are essential for various applications. The compressor’s role in a refrigeration system involves drawing in the vapor refrigerant from the evaporator, which is low in temperature and pressure, and subsequently increasing both its pressure and temperature. This can have the following results:

  • Maintain low pressure in the evaporator so that the liquid refrigerant can boil off or evaporate at a boiling temperature that is below the temperature of the medium. So, the refrigerant can absorb the heat from the medium, which requires cooling.
  • Maintaining high pressure of the refrigerant vapor entering the condenser so that the new saturation temperature is higher than the cooling medium, which results in the heat being rejected into the cooling medium.

There are three main classifications:

  1. Reciprocating Compressors: These compressors use a piston-cylinder arrangement to compress the refrigerant vapor. They operate with a reciprocating motion, where the piston moves back and forth within the cylinder to compress the refrigerant.
  2. Rotary Compressors: Rotary compressors utilize rotating mechanisms, such as vanes, scrolls, or screws, to compress the refrigerant vapor. They achieve compression through the rotational motion of these components, which traps and compresses the refrigerant as it moves through the compressor.
  3. Centrifugal Compressors: Centrifugal compressors employ centrifugal force to compress the refrigerant vapor. They utilize a rotating impeller to accelerate the refrigerant to high speeds, generating a centrifugal force that compresses the vapor against the compressor casing.
Construction

Overall, compressors in refrigeration systems are constructed with precision-engineered components that work together to compress refrigerant vapor and facilitate the refrigeration process. The specific design and construction vary depending on the type of compressor and its intended application.

Reciprocating Compressors Rotary Compressors Centrifugal Compressors
Cylinder: Reciprocating compressors feature one or more cylinders where the compression process takes place. These cylinders contain a piston that moves up and down to compress the refrigerant vapor. Compressor Body:

  • The compressor body, also known as the housing or casing, provides structural support and houses the internal components of the compressor.
  • It is typically made of sturdy materials such as cast iron or aluminum to withstand the mechanical stresses and pressures generated during operation.
 Impeller: Centrifugal compressors utilize a rotating impeller to accelerate the refrigerant vapor to high speeds. The impeller consists of blades or vanes that impart kinetic energy to the refrigerant.
Piston: The piston is a movable component within the cylinder that creates compression by reciprocating motion. It seals the refrigerant vapor within the cylinder during compression strokes. Crankshaft:

  • The crankshaft is a central component of the rotary compressor that converts the rotary motion of the motor into reciprocating motion.
  • It is typically made of hardened steel and features precision-machined bearing surfaces to ensure smooth rotation and minimize friction.
  • The crankshaft is connected to the motor shaft and transfers motion to the pistons or rotors within the compressor.
 Diffuser: After leaving the impeller, the high-speed refrigerant vapor enters a diffuser, where its velocity is converted into pressure. The diffuser slows down the refrigerant and redirects it towards the outlet.
Crankshaft: The crankshaft converts the rotary motion of the motor into reciprocating motion for the piston. It connects to the piston through a connecting rod. Pistons and Connecting Rods:

  • In some types of rotary compressors, such as reciprocating compressors, pistons and connecting rods are used to compress the refrigerant vapor.
  • Pistons are cylindrical components that move up and down within the cylinder bore, creating compression by reciprocating motion.
  • Connecting rods connect the pistons to the crankshaft and transmit motion from the crankshaft to the pistons, converting rotary motion into linear motion.
 Casing: The compressor casing surrounds the impeller and diffuser, providing structural support and maintaining proper alignment of components.
Valves: Reciprocating compressors have inlet and discharge valves that control the flow of refrigerant vapor into and out of the cylinder during compression cycles. Suction and Discharge Valves:

  • Suction and discharge valves control the flow of refrigerant vapor into and out of the compressor during the compression cycle.
  • Suction valves allow refrigerant vapor to enter the compressor cylinder during the suction stroke, while discharge valves permit the compressed vapor to exit the cylinder during the discharge stroke.
  • These valves are typically made of durable materials such as stainless steel or hardened steel and are designed to provide reliable sealing and efficient flow control.
 Bearings: Bearings support the rotating shaft of the impeller and ensure smooth operation of the compressor. They reduce friction and allow the shaft to rotate freely.
Three types: open, hermetic (airtight), and serviceable or semi-hermetic Safety Head:

  • The safety head, also known as the pressure relief valve, is a safety device designed to prevent overpressure in the compressor.
  • It automatically opens when the pressure inside the compressor exceeds a predetermined threshold, allowing excess pressure to be safely released.
  • The safety head helps protect the compressor from damage and ensures safe operation under varying operating conditions.
Compressor Bearings:

  • Compressor bearings support the rotating components of the compressor, such as the crankshaft or rotors, and allow them to rotate smoothly.
  • They are typically made of materials such as bronze, steel, or ceramic and are designed to withstand high loads and speeds.
  • Bearings are lubricated to reduce friction and wear, ensuring long-term reliability and efficiency of the compressor.
Operation
Reciprocating Compressors Rotary Compressors Centrifugal Compressors
Suction Stroke: The piston moves downward inside the cylinder, creating a vacuum that draws gas or air into the cylinder through an intake valve. Intake Phase: Gas or air enters the compressor through an inlet port while the rotary mechanism is in motion. Inlet Phase: Gas or air enters the compressor through an inlet port and flows into the center of a rotating impeller.
Discharge Stroke: The piston moves downward again, pushing any remaining gas or air out of the cylinder through the outlet valve. Compression Phase: Inside the compressor, the gas or air gets trapped between the rotating element (such as screws, vanes, or lobes) and the compressor housing. As the rotating element spins, it reduces the volume of the gas or air, thus increasing its pressure. Rotation: The impeller spins rapidly, imparting kinetic energy to the gas or air particles. As the gas or air moves radially outward from the center of the impeller, it gains speed and kinetic energy.
This cycle repeats continuously to maintain the desired pressure level. Reciprocating compressors are commonly used in various applications, including refrigeration, air conditioning, gas pipelines, and industrial processes, due to their ability to provide high pressure and relatively low flow rates. Discharge Phase: Once the gas or air is compressed to the desired pressure, it is forced out of the compressor through a discharge port. Compression: As the gas or air moves outward, it encounters a diffuser or volute casing surrounding the impeller. This casing converts the kinetic energy of the gas or air into pressure energy, causing the gas or air to slow down and increase in pressure.
Repeat: The process continues in a continuous cycle, with gas or air continuously entering the compressor, getting compressed, and then exiting at a higher pressure. Discharge: The high-pressure gas or air exits the compressor through a discharge port located at the outer edge of the volute casing.
Repeat: The process repeats continuously, with gas or air continuously entering the compressor, being accelerated and compressed by the impeller, and then exiting at a higher pressure.

Evaporators

Definition: the part of the refrigeration system where the liquid refrigerant is vaporized by the absorption of heat from the medium to be cooled. There are two types of evaporators: dry/direct expansion evaporator and flooded evaporator.

Dry/Direct Expansion Evaporator Flooded Evaporator
  • A dry/direct expansion evaporator in a refrigeration system cools by absorbing heat from a substance or space.
  • It receives high-pressure refrigerant from the compressor, reduces its pressure via an expansion valve, and circulates it through coils in direct contact with the substance.
  • The refrigerant evaporates, cooling the substance, before returning to the compressor to repeat the cycle.
  • The dry/direct expansion evaporator is called “dry” because the refrigerant evaporates directly in the evaporator coils without the need for liquid refrigerant to be present.
  • A flooded evaporator in a refrigeration system is a type of evaporator where the refrigerant completely fills the evaporator coils or heat exchanger.
  • In this design, the refrigerant level is maintained above the heat transfer surface, ensuring that the coils are fully submerged.
  • As the refrigerant absorbs heat from the substance or space to be cooled, it evaporates, turning into a vapor.
  • The vaporized refrigerant is then drawn into the compressor to be compressed and circulated back into the system.
  • Flooded evaporators are commonly used in industrial refrigeration systems due to their efficiency and ability to handle high heat loads.
Construction Classification
  • Bare Tube Evaporator:
    • Consists of bare metal tubes arranged in a coil or bundle configuration.
    • Tubes are typically made of materials like copper, aluminum, or stainless steel.
    • Heat transfer occurs directly between the substance to be cooled and the outer surface of the tubes.
  • Plate Surface Evaporator:
    • Comprises a series of flat plates with fluid passages between them.
    • Plates are usually made of materials like stainless steel or aluminum.
    • Heat transfer occurs across the surface area of the plates as the refrigerant flows through the passages.
  • Finned Tube Evaporator:
    • Features metal tubes with extended surfaces, known as fins, attached along their length.
    • Tubes are typically made of materials like copper or aluminum, while fins are often made of aluminum.
    • Fins increase the surface area available for heat transfer, enhancing the evaporator’s efficiency.
  • Shell and Tube Evaporator:
    • Consists of a cylindrical shell containing multiple tubes arranged parallel to each other.
    • Tubes are usually made of materials like copper, steel, or titanium, while the shell is typically made of steel.
    • Refrigerant flows through the tubes, while the substance to be cooled circulates around the outside of the tubes.
    • Heat transfer occurs through the tube walls, facilitating efficient cooling of the substance.

Condensers

Definition: the part of the refrigeration system where the superheated refrigerant vapor is cooled until it is condensed to a liquid and is subcooled. This process occurs after the refrigerant has absorbed heat from the substance or space being cooled in the evaporator and then compressed by the compressor.

There are generally three types of condensers: air-cooled, water-cooled, and evaporative.

Air-Cooled Condenser Water-Cooled Condenser Evaporative Condenser
  • It transfers heat from the high-pressure vapor refrigerant to the surrounding air for condensation.
  • It is commonly used in refrigeration and air conditioning systems where water availability or cost is a concern.
  • Finned Tubes: These are tubes with extra surfaces (fins) attached. They increase the area for heat transfer, allowing the refrigerant to release heat into the air and turn into a liquid.
  • Fans/Blowers: Mounted on the condenser unit, they blow air over the finned tubes, helping to cool them down. This airflow carries away the heat from the refrigerant, aiding in the condensation process.
  • Circulation Systems: These ensure that air flows efficiently over the condenser coil. They use ducts or louvers to direct airflow and ensure even heat exchange across the coil.
  • Ducts: Channels that guide airflow to and from the condenser unit. They help control the airflow and make sure the heated air is expelled away from the unit.
  • Three basic designs: double tube, shell and coil, and shell and tube.
    • Double Tube: Two tubes, one inside the other. Refrigerant flows through the inner tube, while cooling water flows around the outer tube.
    • Shell and Coil: Refrigerant flows through a coil inside a shell, with cooling water circulating around the coil to cool the refrigerant.
    • Shell and Tube: Bundles of tubes are housed within a shell. Refrigerant flows through the tubes while cooling water surrounds them in the shell.
  • The function of a water-cooled condenser in a refrigeration system is to transfer heat from hot refrigerant vapor to cooling water, causing the refrigerant to condense into a liquid.
  • This maintains the refrigeration cycle by removing heat energy from the refrigerant, allowing it to circulate back to the evaporator as a cool liquid to absorb more heat.
  • The cooling water absorbs the heat from the refrigerant and carries it away, usually dissipating it through a cooling tower.
  • Condensing Coils: These coils are designed to facilitate the condensation of the hot refrigerant vapor. They provide a large surface area for heat transfer between the refrigerant vapor and the surrounding air or water.
  • Hot Refrigerant Vapor: The refrigerant vapor enters the condensing coils after being compressed by the compressor. It carries heat energy absorbed from the evaporator coils in the refrigerated space.
  • Compressor: The compressor pressurizes the refrigerant vapor, raising its temperature and pressure. This hot, high-pressure vapor then enters the condensing coils for condensation.
  • Receiver: The receiver is a storage vessel located after the condenser coils. It collects and stores the condensed liquid refrigerant before it is sent to the expansion device and evaporator coils.
  • Cooling Coils: In an evaporative condenser, cooling coils are used to lower the temperature of the condensing coils. Water is sprayed or circulated over these coils, causing evaporation. As the water evaporates, it absorbs heat from the condensing coils, helping to condense the refrigerant vapor.
Operation and Maintenance

A condenser may malfunction if the cooling water on the waterside surface is dirty or if there is an oil coating on the refrigerant side surface of the coils. It is important to properly use compressor lubricating oil and maintain proper refrigerant velocities to keep everything clean.

Non-condensable gasses

  • Example: air
  • This reduces its capacity which causes high compressor discharge pressures and temperatures.
  • The refrigerant needs to be removed and be replace with new refrigerant.

Scale Formation

  • If there are scale forming chemicals in the cooling water, it can be deposited on the waterside surfaces of the condenser.
  • Scaling reduces the rate of heat transfer through the coil walls or tubes.
  • This reduces the efficiency of the condenser and the refrigeration system in general.
  • Tubes should be inspected regularly from the inside and outside so they can be cleaned immediately.

Oil Deposits

  • The lubricating oils that are applied sometimes accumulate and create deposits on the refrigerant side of the coils and tubes.
  • Oil will not breakdown if the system is being maintained and the temperature is not too high.
  • Proper operation will ensure that it lasts a longer time.

Cleaning

  • Take the unit out of service.
  • Circulate an acid or chemical solution through the unit.
  • Consult a qualified chemist or the manufacturer to ensure proper cleaning steps are being taken when working with chemicals.
  • Shell and tube condensers that have straight tubes can be cleaned mechanically (or mechanical cleaners or brushed through the tubes).

Other Considerations

  • Installing a water softening system (economical)
  • Implement a regular maintenance schedule for condenser cleaning, inspection, and servicing. Follow manufacturer recommendations and industry best practices for maintenance tasks.
  • Ensure proper airflow across the condenser coils in air-cooled condensers. Remove any obstructions or debris that may impede airflow and maintain adequate ventilation around the condenser unit.

Refrigeration Accessories

There are certain accessories that are used to aid or inspect the equipment. Each serves an important purpose. This section will cover some of the most common accessories that are used with refrigeration systems.

Pressire Gauges

 They show the pressure inside an evaporator and condenser, as well as the saturation temperatures. There are at least two pressure gauges in the refrigeration system. The first is connected to the high-pressure side and the second is connected to the low-temperature side. The gauges are also connected to the suction and discharge valves of the compressor. The pressure gauges should have pulsation dampeners, so the pointers don’t flicker due to the pulsations in pressure. Pressure gauges are not installed in low-capacity systems, such as residential appliances or window air conditioners or even in low-capacity commercial systems.

“The CSA Standard B52-09, Mechanical Refrigeration Code, Clause 5.8.3 that requires when a pressure gauge is permanently installed on the high side of a system, the gauge must be constructed for not less than 1.2 times the design pressure” (PanGlobal, 207, 20-3)
Oil Separators The oil level in a compressor must be above its minimum level because there is always some oil that leaves the compressor with the high-pressure vapor, so there is a change too much oil leaves the compressor. So, it is important to separate the oil from the vapor and have it return to the compressor crankcase. IT is also important to avoid an accumulation of oil in the condenser because the oil coated on the tube surfaces will prevent proper heat transfer, which leads to high compressor discharge pressure and increased power consumption.

As the refrigerant vapor enters the separator casing, it undergoes a spinning motion. This spinning action, driven by centrifugal force, causes the oil droplets suspended in the vapor to accumulate along the walls of the vessel. Eventually, the collected oil drains downward to the bottom of the separator.
Suction Strainers

 Sometimes little bits of foreign matter can be brought into the compressor with the refrigerant vapor. This can cause damage to the compressor parts. A suction strainer is used in the suction line of the compressor so these particles can be removed from the vapor before they enter the compressor. There is a fine-mesh screen basket that can also be removed for cleaning.
Sight Glass and Moisture Indicators

 A sight glass is installed in the liquid refrigerant to observe the liquid flow. It usually has one or two lenses. When the refrigerant level drops, the sight glass may show bubbles or foam instead of a clear, continuous flow of refrigerant. This indicates that there is insufficient refrigerant in the system, potentially due to a leak or a change in pressure.

The chemical dot under the sight glass, often referred to as a sight glass moisture indicator, functions similarly to a moisture indicator elsewhere in the system. It contains a desiccant that changes color upon exposure to moisture. When the refrigerant passes through the sight glass, any moisture present in the system is absorbed by the desiccant, causing the dot to change color. This color change serves as a visual indication to maintenance personnel that moisture may be present in the refrigerant, alerting them to potential issues with the system.
Heat Exchanger

 A heat exchanger is used to transfer heat from the warm liquid flowing to the evaporator to the hot vapor that is taken from the evaporator. There are two advantages of using a heat exchanger:

  1. Lowering the refrigerant’s liquid temperature prevents flashing due to pressure drops and reduces vapor formation during expansion, boosting system efficiency by around 2-3%.
  2. When the workload changes quickly, some liquid from the evaporator might enter the suction line. The heat exchanger raises the vapor temperature, causing any liquid in the suction line to evaporate before it reaches the compressor. This prevents damage to the compressor.

Cooling Towers

Definition: cooling towers are used to lower the temperature of the cooling water and equipment by using heat transfer.

Heat transfer occurs through two main processes: sensible heat transfer, which involves heating the air passing through the tower, and latent heat removal due to the evaporation of approximately 5% of the water. The evaporation process extracts additional heat from the remaining water.

Water requiring cooling is dispersed within the cooling tower using spray nozzles and splash bars. This method maximizes evaporation by increasing the water’s surface area exposed to the air. The following methods can be used for air circulation:

  • fans
  • convection currents
  • induction effects from water sprays

Alternatives:

  • a once-through system with water from a lake or a stream.
  • a natural water body or a cooling pond to cool the heated system.

Cooling towers are frequently used as heat exchangers in situations where water is limited or costly, and when a temperature close to the ambient wet bulb temperature, typically within 4-6°C, is needed.

The cooling tower operates by pumping water to its top, where it’s sprayed down in a fine mist. As this mist descends through the tower, air passing through causes some of the water to evaporate, carrying away heat in the process. This evaporation reduces the temperature of the remaining water. Meanwhile, some of the heat is also absorbed by the airflow passing through the tower’s basin before being circulated back into the system.

Factors that contribute to the heat transfer rate:

  • Velocity of air and water during contact
  • Water surface area that comes in contact with the air
  • Contact time between the air and water
  • Relative humidity of the air (inlet water temperature vs. inlet air wet bulb temperature)
Components
  • Inlet water distributing box or sprays
  • Packing or fill
  • Air moving equipment
  • Inlet air louvres
  • Drift eliminators
  • Cooled water basin

Inlet Air Louvres:

Types
Natural Draft Cooling Tower a natural draft cooling tower helps remove heat from the condenser by utilizing convective air movement. Warm water from the condenser is distributed over baffles at the top of the tower and cascades downward, exposed to ambient air flowing upward. As the water evaporates, it absorbs heat from the remaining water, cooling it before returning to the condenser. This process facilitates efficient heat transfer, allowing the refrigeration system to operate effectively.
Mechanical Draft: Forced Draft The forced draft tower houses the fan, basin, and piping internally. The tower’s framework is covered by aluminum, galvanized steel, or seamless plastic panels. A forced draft cooling tower works by using a fan at its base to draw air upward through the tower, where it passes over condenser coils containing hot refrigerant vapor. Heat is transferred from the vapor to the air, causing the refrigerant to condense. The cooled refrigerant exits the tower, while the air carries away the absorbed heat.
Mechanical Draft: Induced Draft An induced draft cooling tower works by using a fan at the top of the tower to pull air through the tower. As the air passes over condenser coils containing hot refrigerant vapor, heat is transferred from the vapor to the air, causing the refrigerant to condense. The cooled refrigerant exits the tower, while the air carries away the absorbed heat.
Maintenance and Operational Problems
Type Maintenance Potential Operational Problems
Natural Draft
  • Clean all parts to ensure they do not have scale, algae or dirt.
  • Clean the water basin, check for leaks.
  • Clean the suction screen.
  • Check that the float control valve is working.
  • High water discharge temperature from the tower (elevated wet bulb air temperature, high water solids concentration, airflow restrictions, and worn or dirty nozzles)
  • Reduction in water flow (algae, scale, dirt)
  • Tower basin or sump overflowing (clogged sump screen/pipes or leaking float control valve)
Mechanical Draft: Forced Draft and Induced Draft
  • Lubricate the fan motor every 3 months, or as instructed by the manufacturer.
  • Lubricate the V-belt driven fan shaft monthly and check the tension on the belt.
  • Check tower bolts monthly.
  • Check the float valve monthly.
  • Clean and repaint any external corroding metal surfaces every year. Inspect the interior of the tower.
  • Clean and paint the fan blades annually and inspect for cracks.
  • Clean fans and balance if/when high vibration levels are detected.
  • Consult a water treatment specialist if there is scale or algae forming.
  • Corrosion of structural components or heat exchange surfaces due to exposure to water and air can weaken the tower structure and reduce its lifespan.
  • Accumulation of scale on the heat exchange surfaces can reduce heat transfer efficiency and increase energy consumption.
  • Inadequate airflow or water circulation can cause the tower to overheat, leading to reduced cooling capacity and potential equipment damage.
  • Cracks in the cooling tower should be promptly identified and repaired to prevent further damage and ensure the safe and efficient operation of the system.
Troubleshooting a Cooling Tower

The following troubleshooting guide is taken from PanGlobal under the Fair Use Policy.

Trouble Cause  Remedy
Excessive water drift
  1. Faulty drift elimination
  2. Overpumping
  • Check to be sure all louvres, eliminator sections, or slats and splash retainers are in place.
  • Top deck assemblies (on early models) and wood fill splashbars must be intact and level. There must be no tendency to channel water. Fill packs must be intact and in place with no sagging or gaps between packs.
  • Metering orifices of hot water basins must be in place and clean of debris.
  • There must be no leaks under hot water basin sides.

2. Reduce water flow to tower to design conditions or use larger metering orifices.
Motor will not start
  1. Power not available at motor terminals
  2. Wrong connections
  3. Low voltage
  4. Open circuit in motor winding
  5. Motor or fan drive stuck
  6. Rotor defective
  • Check power at starter. Correct any bad connections between the control apparatus and motor.
  • Check starter contacts and control circuit. Reset overloads, close contacts, reset tripped switches or replace failed control switches. check vibration switch.
  • If possible, not on all leads at starter, make sure overload and short circuit devices are in proper condition.

2. Check motor and control connections against wiring diagrams.

3. Check nameplate voltage against power supply. Check voltage at motor terminals.

4. Check stator windings for open circuits.

5. Disconnect motor from load and check motor and gear reducer for cause of problems.

6. Look for broken bards and rings.
Unusual motor noise
  1. Motor running single-phase
  2. Motor leads connected incorrectly
  3. Ball bearings
  4. Electric unbalance
  5. Air gap not uniform
  6. Rotor unbalance
  7. Air gap not uniform
  8. Rotor unbalanced
  9. Cooling fan hitting guard
  1. Stop motor and attempt to start it. Motor will not start if single phased. Check writing, controls, and motors.
  2. Check motor connections against wiring diagram on motor.
  3. Check lubrication. Replace bad bearings.
  4. Check voltage and currents of all three lines. Correct if required.
  5. Check and correct bracket fits.
  6. Rebalance.
  7. Reinstall or replace fan.
Motor runs hot
  1. Wrong voltage or unbalanced voltage
  2. Overload
  3. Wrong motor r/min
  4. Bearings over greased
  5. Rotor rubs stator bore
  6. Wrong lubricant in bearings
  7. One phase open
  8. Poor ventilation
  9. Winding fault
  10. Bent motor shaft
  11. Insufficient grease
  12. Deterioration or foreign material in grease
  13. Bearings damaged
  14. Incorrect fan blade pitch
  1. Check voltage and current of all three lines against nameplate values.
  2. Check fan blade pitch. See fan service manual. Check for drag in fan drive train as from damaged bearings.
  3. Check nameplate against poser supply. Check r/min of motor and gear ratio.
  4. Remove grease reliefs. Run motor up to speed to purge excessive grease.
  5. If not poor machining, replace worn bearing.
  6. Change to proper lubricant. See motor manufacturer’s instructions and electric motor manual.
  7. Stop motor and attempt to start it. Motor will not start if single-phased, check wiring controls and motor.
  8. Clean motor and check ventilation around motor.
  9. Check with ohmmeter.
  10. Straighten or replace shaft.
  11. Remove plugs and re-grease bearings.
  12. Flush bearings and re-lubricate.
  13. Replace bearings.
  14. See fan service manual for blade pitching instructions.
Motor does not come up to speed
  1. Voltage too low at meter terminals because of line drop
  2. Broken motor bars
  1. Check transformer and setting of taps. Use higher voltage on transformer terminals or reduce loads. Increase wire size or reduce inertia.
  2. Look for cracks near rings. A new rotor may be required. Have motor service company check motor.
Wrong rotation (motor)
  1. Wrong sequence of phases
  1. Change any two of the three motor leads
Gear reducer noise
  1. Gear reducer bearings
  2. Gears
  1. If new, see if noise disappears after one week of operation. Drain, flush, and refill gear reducer. See gear reducer service manual. If still noisy, replace.
  2. Correct tooth engagement. Replace badly worn gears. Replace gears with imperfect tooth spacing or form.
Unusual fan drive vibration
  1. Loose bolts and cap screws
  2. Unbalanced drive shaft or worn couplings
  3. Fan
  4. Worn gear reducer bearings
  5. Motor
  6. Bent gear reducer
  1. Tighten all bolts and cap screws on all mechanical equipment and supports.
  2. Make sure motor and gear reducer shafts are in proper alignment and “math marks” properly matched. repair or replace worn couplings. Rebalance drive shaft by adding or removing weights from balancing cap screws. See drive shaft service manual.
  3. Be sure blades are properly positioned in correct sockets. Check match numbers. Make certain all blades are as far from center of fas as safety devices permit. All blades must be pinched the same. See fan service manual. Clean off deposit build-up on blades.
  4. Check fan and pinion shaft endplay. Replace bearings as necessary.
  5. Disconnect load and operate motor. If motor still vibrates, check for bent shaft or unbalanced rotor. Replace shaft or rebalance rotor.
  6. Check fan and pinion shafts with dial indicator. Replace if necessary.
Fan Noise
  1. Loose fan hub cover
  2. Blade rubbing inside of fan cylinder
  3. Loose bolts in blade clamps
  1. Tighten hub cover fasteners.
  2. Adjust cylinder to provide blade tip clearance.
  3. Check and tighten if necessary.
Scale or foreign substance in water
  1. Lack of or insufficient blow-down
  2. Poor water treatment
  1. Consult a water treatment specialist.
  2. Consult competent water treatment specialist.
Wood deterioration
  1. Improper water treatment or microbiological attack
  1. See tower manufacturer sales representative of water treatment specialist.

Air Conditioning Systems

Air conditioning systems play a crucial role in maintaining good indoor air quality (IAQ) by ensuring a continuous supply of clean air that is free from contaminants.

Purpose: Air conditioning systems are engineered to uphold indoor conditions that ensure the comfort of occupants and support the intended purpose of the building.

Functional Components 
Heating
  • Offset the heat loss to the atmosphere surrounding the building through the exterior walls and roof and through floor slabs on grade and basement walls.
  • Heat should be distributed to all areas evenly to ensure that comfortable conditions are maintained.
Cooling
  • Offset the heat gain through the exterior shell of the building from solar radiation, internal sources of heat, and the atmosphere surrounding the building.
  • Cooling should be distributed to all areas evenly to ensure that comfortable conditions are maintained.
Ventilation
  • Outside air is necessary to replenish the oxygen supply indoors.
  • Outside air is necessary to mask any odors.
  • Outside air is necessary to provide makeup air for the exhaust systems in the bathrooms. This maintains acceptable air balance.
Humidification
  • Low humidification requires adding moisture to the building in order to maintain acceptable conditions for human comfort.
  • The moisture will minimize the effects of static electricity, which will also contribute to the correct operation of electronic equipment.
Dehumidification
  • This is the process of removing excess moisture to maintain a humidity level that is acceptable for the occupants and building.
  • In humid areas such as swimming pools to prevent structural damage in the wintertime (frost forming in exterior walls).
Air Circulation
  • Movement is required for dissipation of latent heat from occupants.
  • Movement is required to prevent a buildup of odors to develop in stagnant areas.
Filtration
  • Removal of contaminants (dust, dirt, smoke, pollen, lint) from the recirculated air and from the outside air.
  • Factors that play into this:
    • geographical location, weather, occupancy type, building layout, construction.
  • Smaller residence: only heating and ventilation is required, so electric baseboard heaters are installed and satisfy the requirements.
  • Larger residence: more complex system is required with many components (listed in this table).
Operation

Air conditioning systems operate by removing heat and humidity from indoor air to create a comfortable and controlled environment. Here’s a simplified description of how they work:

  1. Air Intake: Warm air from the indoors is drawn into the air conditioning system through return air ducts.
  2. Filtration: Before reaching the main components, the air passes through filters to remove dust, pollen, and other particles, improving indoor air quality.
  3. Cooling: The air passes over a set of cold coils containing a refrigerant. The refrigerant absorbs heat from the air, causing it to cool down. This cooled air is then circulated back into the indoor space.
  4. Dehumidification: As the air cools, moisture in the air condenses on the cold coils. This removes humidity from the air, making it feel more comfortable.
  5. Exhaust: The heat absorbed by the refrigerant is expelled outside the building through a condenser coil, and the refrigerant is circulated back to the evaporator coil to repeat the cooling process.
  6. Thermostat Control: The temperature and humidity levels in the indoor space are monitored by a thermostat. Once the desired temperature is reached, the system cycles off until the temperature rises again.

Overall, air conditioning systems provide a controlled indoor environment by cooling and dehumidifying the air, maintaining comfortable temperature and humidity levels for occupants.

Copyright © 2017 PanGlobal. All rights reserved. Under Fair Use Policy.

PanGlobal. (2017). In Refrigeration. Low Pressure Boiler Components & Operation (2nd ed.). PanGlobal Training Systems Ltd.

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SIGA Maintenance Worker Prep Copyright © by hrycaym. All Rights Reserved.

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