Understanding the basic concepts of refrigeration is important. Even though air conditioning and refrigeration systems may look different or perform different functions, they all have the same operating principles.
Heat is present everywhere in nature. If we do not have heat, we have cold. Cold, in simple terms, is just a lack of heat.
Refrigeration is a process that transfers heat from a place or object where it is not wanted to a place where it is not objectionable. A refrigerant is the fluid that circulates through a refrigeration system and carries the heat.
The ability of heat to do work depends on temperature and the quantity of heat, which is known as heat content.
Temperature is the measure of the hotness or coldness of an object or substance and is measured with a thermometer.
Most common thermometers measure on the Fahrenheit or Celsius scale. On the Fahrenheit scale, water boils at 212° and freezes at 32°. On the Celsius scale, water boils at 100° and freezes at 0°.
Heat content is the amount of heat energy contained in a substance and is measured by the British thermal unit (Btu). One Btu is the amount of heat needed to raise the temperature of one pound of water one degree Fahrenheit.
A substance can exist as a solid, liquid, or gas, depending on its heat content. For example, water can exist as ice, liquid water, or steam.
As water is heated, its temperature rises. This rise in temperature that can be measured with a thermometer is called sensible heat.
As water begins to boil and turn to steam, additional heat is added but the measured temperature stays constant. This added heat without an accompanying temperature change is called latent heat.
The combined sensible heat and latent heat added to an object or substance is known as total heat.
Once the boiling water changes completely to steam, a thermometer will again register sensible heat as the steam temperature rises. This rise in temperature of a vapor above its boiling point is called superheat.
It takes much more heat to cause a change of state. For example, it takes 970 Btus to change a pound of water to a pound of steam.
The heat gained or lost when changing from ice to water or water to ice is called the latent heat of fusion. The heat gained when changing from water to steam is called the latent heat of vaporization. The heat given up when steam condenses back to water is called the latent heat of condensation.
Another term used in refrigeration is subcooling. It is the temperature of a liquid when it is cooled below its condensing temperature.
Pressure in substances can be exerted in different directions. Gases such as refrigerant tend to exert pressure equally in all directions.
The air in our atmosphere exerts a downward pressure of 14.7 PSI. This is known as atmospheric pressure. This same pressure can support a column of mercury nearly 30high. These values are used as standards in HVAC work.
Gauge pressure is normally used in HVAC work. On a gauge pressure scale, the atmospheric pressure of 14.7 PSI is used as the zero pressure point. Pressures below zero are expressed in inches of mercury vacuum.
Absolute pressure is gauge pressure plus atmospheric pressure. To find absolute pressure, add 14.7 to the gauge pressure.
Pressure and temperature in HVAC are directly related. The temperature at which a liquid or gas changes state depends on the pressure.
As pressure decreases, the temperature at which a liquid such as water boils also decreases. If pressure increases, boiling will occur at a higher temperature.
Pressure also affects fluids by causing them to flow from a high-pressure area to a lower pressure area.
Many instruments are available to measure temperature and pressure in HVAC systems. Different types of thermometers are available to measure temperature.
Thermometers are available for different applications. Electronic thermometers use a thermocouple or thermistor type of probe and are used when extreme accuracy is required.
Some newer electronic thermometers use a non-contact infrared probe to measure temperature.
The gauge manifold set is the most frequently used item of HVAC equipment. It contains pressure gauges, hoses, and valves to control refrigerant flow.
Digital versions of the gauge manifold are available that provide more precise pressure readings.
Manometers are used to precisely measure lower pressures. They are commonly used to measure duct pressure and gas furnace manifold pressure.
All mechanical refrigeration systems contain the same basic components. An evaporator absorbs heat into the refrigerant, a compressor creates a pressure differential for refrigerant flow, the condenser rejects the heat being carried in the refrigerant to the outdoors, and an expansion device provides a pressure drop that allows the refrigerant to boil.
Refrigerant piping called lines, connect the system components. Included are a hot gas line, liquid line and suction line.
The refrigeration cycle is based on the following principles. When liquid changes to a gas it absorbs heat. The boiling point of a liquid can be changed by the pressure exerted on the liquid.
In the evaporator, warm air passes over tubes containing low-temperature, low-pressure liquid refrigerant. As refrigerant passes through the tubes, heat is absorbed by the refrigerant, causing it to boil and change to a vapor.
From the evaporator, the refrigerant vapor moves to the compressor where it is compressed to a high-temperature, high-pressure vapor. From there, it moves through the hot gas line to the condenser.
In the condenser, the hot gas loses heat and condenses into a liquid. Further cooling below the condensing temperature (subcooling) takes place. The subcooled liquid is then fed to the expansion device.
The expansion device lowers the pressure and temperature of the liquid refrigerant and feeds it to the evaporator where the cycle repeats.
The refrigeration cycle in an air conditioning system containing R-22 refrigerant operates with certain temperature and pressure characteristics.
Refrigerants are used in HVAC systems to move heat within the system. The most common refrigerants are ammonia and fluorocarbon refrigerants.
As a result of recent laws and environmental concerns, the old “R” designation for refrigerants has been dropped in favor of one that more accurately reflects the chemical composition of the refrigerant. R-22, for example, is now called HCFC-22.
Ammonia has been used as a refrigerant for many years in ice making and food processing. Because of its hazardous nature, the use of ammonia is tightly regulated.
Refrigerants used in HVAC are classified as fluorocarbons. They contain organic compounds called hydrocarbons and chemicals called halogens.
Fluorocarbon refrigerants are regulated by law due to their potential to damage the environment. Refrigerants that only contain the halogen fluorine (F), and no chlorine (C), do not damage the environment.
CFC and HCFC refrigerants are being phased out in the United States. CFC-12 was the first to be phased out in 1996.
Refrigerants come in disposable, returnable and refillable metal containers.
Disposable containers are for one-time use and must not be refilled. When empty, they should be recycled as scrap metal.
Returnable cylinders go back to the manufacturer for refilling. They are not to be refilled in the field or used as a refrigerant recovery cylinder.
Refillable cylinders are usually used with refrigerant recovery or recycling units. Recovery cylinders are painted gray with a yellow top.
Refrigerant cylinders are color- coded based on the type of refrigerant they contain.
Color Codes Used for Some Common Refrigerant Containers
CFC-11 Orange CFC-12 White
CFC-13 Dark purple CFC-500 Yellow
CFC-502 Purple HCFC-22 Green
HCFC-123 Gray HCFC-124 Dark green
HFC-410A Pink HFC-134A Light blue
Refrigerant recovery cylinders—Gray with yellow top
Safety glasses and gloves must be worn when handling refrigerant because liquid refrigerant can freeze or burn the skin. Work in well ventilated areas because concentrations of refrigerant can cause suffocation.
The compressor is the keystone of the refrigeration system. It creates the pressure differential that causes refrigerant to move through the system by taking low temperature and low-pressure gas and converting it into a high-temperature and high-pressure gas.
Compressors can be open-drive, welded hermetic or semi-hermetic. Open-drive compressors have the drive motor separate from the compressor.
Semi-hermetic and hermetic compressors have the drive motor enclosed within the compressor shell.
A reciprocating compressor has a piston or pistons moving back and forth in a cylinder to compress the refrigerant. It is the most common type of compressor.
Reciprocating compressors are often housed in a welded steel shell and are called welded hermetic compressors.
Rotary compressors are often used in room air conditioners and other small systems. A shaft with an off-center rotor turns or rolls around the cylinder, compressing the refrigerant gas.
Scroll compressors operate in a unique way. An orbiting scroll inside of a fixed scroll squeezes the refrigerant into an ever-smaller pocket.
The scroll compressor does not have suction or discharge valves but does have a check valve on the discharge side to prevent reverse rotation.
Scroll compressors produce unique and unusual sounds when starting up and shutting down. These sounds are normal and not an indication of a problem.
Screw compressors are used in larger industrial and commercial applications. They compress refrigerant by squeezing it between a pair of spinning rotors.
As the rotors turn, the gas is squeezed into progressively smaller compression stages before exiting the compressor.
Centrifugal compressors are used in commercial and industrial settings such as high-rise office buildings, where large capacity is needed.
Centrifugal compressors use an impeller spinning at very high speed to compress the refrigerant. Often, impellers are put in series to create a greater pressure difference and to pump more vapor.
Condensers are used to remove heat from refrigeration systems. They take high-pressure, high-temperature refrigerant gas from the compressor and change it into a high-temperature, high-pressure liquid.
The condensing medium, either air or water, must be lower in temperature than the refrigerant for condensing of refrigerant to occur. Condensers are grouped according to the medium used to carry heat away. Condensers can be air-cooled, water-cooled or evaporative.
Air-cooled condensers use a fan to push or pull air through the coil. They are widely used in residential and light commercial systems up to 50 tons.
Fin-and-tube condensers have tubes passing through rows of metal fins. The tubes transfer heat from the refrigerant to the fins. Air flowing over the fins removes the heat.
Plate condensers function the same as fin-and-tube condensers except the tubes pass through plates that surround and make contact with them, providing a large surface area for heat transfer.
Water-cooled condensers are more complicated and expensive than air-cooled condensers. They are also more difficult to service and maintain but offer the advantage of operating at lower and more stable condensing temperatures. They are available in several configurations.
Tube-in tube condensers are made so that refrigerant passes through a tube in the opposite direction of the water that is flowing through an outer and surrounding tube.
Shell-in-tube condensers have refrigerant tubes encased in a water-filled metal shell. The condenser can be horizontally or vertically mounted.
Shell-and-coil condensers are similar to shell-and-tube condensers except the refrigerant tubing is coiled within the surrounding metal shell.
Plate-and-frame or plate heat exchangers consist of metal plates on a metal frame with refrigerant and cooling fluid flowing through in separate circuits.
Cooling towers are used to cool the water that is circulated through a water-cooled condenser. The natural-draft cooling tower depends on natural air currents to help cool the water and does not require a fan.
Another cooling tower design, called a forced-draft tower, employs a fan to help cool the water.
Evaporative condensers first transfer heat to water and then from the water to the outside air. It is essentially a water-cooled condenser and cooling tower in one package.
Evaporators absorb heat from the conditioned space by transferring the heat from air or water to the refrigerant. Direct-expansion and flooded are the two main types of evaporators.
Direct-expansion evaporators are the most commonly used. While air passes over the coil, refrigerant is metered into the coil. It boils to a vapor as it passes through.
In a flooded evaporator, refrigerant can be circulated through the evaporator more than once. A surge chamber at the exit of the coil collects any liquid refrigerant, which is then circulated back through the evaporator.
Evaporators are classified by the way they are constructed. Bare-tube evaporators, as the name implies, rely on the surface of the tubing and nothing else to transfer heat.
Finned-tube evaporators have tubes passing through metal sheets to increase the surface area for greater heat transfer.
Plate evaporators are like plate condensers. Tubes are attached to a plate to increase the surface area for greater heat transfer.
In some large systems, chilled water or brine is produced in a centrally located chiller and pumped to individual cooling coils. The water is used to absorb heat instead of refrigerant. The water is then pumped back to the chiller, heat is removed and the chilled water is circulated again.
The metering device is located at the evaporator inlet. It allows refrigerant to flow into the evaporator at a controlled rate and it drops the refrigerant pressure to lower its boiling point.
There are two main types of metering devices, fixed and adjustable.
Fixed metering devices have a fixed restriction or orifice to control refrigerant flow and are usually found in room air conditioners and residential systems. Due to their fixed size, higher pressure experienced at higher temperature will force more refrigerant through.
An adjustable metering device can adjust the flow of refrigerant to match the cooling load. Several types of adjustable metering devices are available.
A thermostatic expansion valve or TXV is an adjustable metering device that controls refrigerant flow by sensing the level of superheat in the suction line at the evaporator outlet.
A remote sensing bulb clamped to the suction line contains a vapor that expands and contracts with temperature. This vapor exerts pressure on a diaphragm that controls the size of the metering orifice.
A refrigeration system contains many other components that increase efficiency, safety, reliability and serviceability.
A filter-drier protects the system from foreign debris and moisture. It typically is installed in the liquid line ahead of the metering device.
Filter-driers are also available for the suction line and are designed for use in cleaning up a system after a compressor motor electrical burn out.
A sight glass is a tiny window into the refrigeration system. It is often installed in the liquid line to allow a technician to see the state of the refrigerant. Sometimes a moisture indicator that changes color in the presence of water is built into the sight class.
A suction line accumulator is a storage tank in the suction line near the compressor. It stores liquid refrigerant to prevent it from entering and damaging the compressor.
A crankcase heater is fastened to or inserted into the bottom of a compressor. It prevents liquid refrigerant from migrating to the compressor crankcase and causing damage.
Oil is used to lubricate the compressor but it can travel throughout the system and build up in places where it does not belong. An oil separator removes oil before it gets into the system and stores it for return to the compressor.
A heat exchanger can be used to help subcool liquid refrigerant before it enters the metering device or it can add heat to the suction gas to drive out any damaging liquid refrigerant. A heat exchanger in the compressor discharge line can also provide domestic hot water.
Receivers store excess liquid refrigerant created by varying cooling loads. A receiver is normally installed in the liquid line ahead of the metering device.
Service valves allow a service technician to measure system pressures, evacuate the system, add or remove charge and isolate various components from the refrigeration system.
Service valves that use Schrader valves require a special tool to remove the valve cores.
Mufflers are normally installed in the discharge line to dampen pulsations that could be carried through the refrigerant lines and cause noise complaints.
Controls are used to start, stop, regulate, and protect the components of a refrigeration system. Primary controls directly or indirectly start or stop the system. Secondary controls regulate and protect the system.
Primary controls include the thermostat, pressurestat, humidistat, and time clock.
The simplest thermostat use a bimetal element that expands when heated, causing a set of switch contacts to close. The switch contact may be open, or sealed in a mercury bulb.
Other thermostats may be activated by pressure applied to a bellows attached to a sensing bulb filled with refrigerant gas. The gas expands or contracts with temperature changes, putting pressure on the bellows and activating a switch.
Electronic thermostats use a thermocouple or thermistor to sense temperature changes. Microprocessor-controlled electronic circuits respond to these changes by providing an open or closed circuit to the HVAC equipment being controlled.
Pressurestats control the system using variations in pressure and can work on the high and low sides of the compressor. Pressure is applied to a bellows or Bourdon tube that activates switch contacts to turn equipment on or off.
Humidistats turn equipment on and off based on the humidity level. They contain special materials that expand and contract in response to moisture. The expansion and contraction is used to control switch contacts that in turn control the humidifier.
Time clocks can activate HVAC equipment. Thermostats often contain clocks or electronic timers to activate equipment or functions based on time. An example is automatically changing the temperature setpoint during times when an area is unoccupied.
Secondary controls regulate or protect the refrigeration system and are often referred to as operating controls. They include electrical devices and mechanical valves. Safety controls include overload protectors and pressure switches.
Condenser water valves regulate water flow in a water-cooled condenser to maintain head pressure.
Evaporator pressure regulator (EPR) valves maintain a constant pressure to maintain a constant temperature in the evaporator.
Check valves allow refrigerant to flow in one direction only. Check valves are common in heat pumps where refrigerant flow changes direction as a part of normal operation.
Pressure relief devices can take the form of a fusible plug or rupture disc. They allow pressure to escape before it builds up to a dangerous or explosive level.
An oil safety switch protects the compressor by shutting it off if oil pressure drops below an acceptable level.
A flow switch will shut the system down if air or water flow to a component is inadequate.
A good HVAC piping layout should provide refrigerant paths, avoid excessive pressure drops, return oil to the compressor, and protect the compressor.
Refrigerant lines should be kept simple and direct, with horizontal lines pitched in the direction of flow. Dips or low spots should be avoided because they trap oil.
It is important that the suction line be laid out and constructed to avoid excessive pressure drop.
A suction riser and trap should be used to help oil return to the compressor and promote free drainage of liquid refrigerant. The trap creates turbulence that helps the oil mix with the refrigerant.
If system capacity varies, the suction riser should be sized smaller to ensure that oil returns to the compressor.
A double suction riser should be used if a single riser is insufficient to ensure oil return. It will also prevent excessive pressure drop in the suction line.
To prevent liquid refrigerant from draining from the evaporator during the compressor off cycle, an inverted loop at least as high as the top of the evaporator should be installed.
The hot gas line should be laid out and constructed similar to the suction line to minimize pressure drop and ensure oil return to the compressor.
Oil return is not a critical issue in liquid line layout and construction. The liquid line size and liquid line accessories should be chosen to keep pressure drop in the liquid line below 4 PSI.
Insulation is normally applied to the suction line only to prevent excessive heat gain in the line and to prevent condensation.
The three major considerations in refrigerant piping layout are compressor protection, oil return, and pressure drop.