6 - Basic Refrigeration Systems Flashcards
What is refrigerant?
Refrigerants are fluids that absorb heat inside the refrigerated space and release the heat outside.
What is the low side of a refrigeration system?
What are some alternate names for low side pressure?
The low side of the system is where heat is absorbed and removed from the refrigerated space. It is under low pressure.
Low side pressure is also known as suction pressure, or evaporator pressure.
What is the high side of a refrigeration system?
What are some alternate names for high side pressure?
The high side of the system is where heat is rejected out of the refrigeration system.
It is also known as discharge pressure, condenser pressure, and head pressure.
What are the two roles of the compressor?
First, it must create suction to draw heat-filled refrigerant vapor from the evaporator into the compressor. By constantly drawing vapor out of the evaporator, it creates low pressure there.
Second, the compressor compresses each quantity of refrigerant drawn in during suction, which increases both the pressure and temperature of the refrigerant.
What are the four basic components of a refrigeration circuit in order, starting from the heart of the system?
The Compressor, the condenser, the metering device, and the evaporator
How does the condenser reject heat?
By transferring heat the surrounding metal and air.
When does operational balance occur?
When the number of vapor molecules that condense into liquid equals the number of vapor molecules that the compressor pumps into the condenser.
What is a compressor?
A compressor is a device that removes heat-laden, low-pressure vapor refrigerant from the evaporator. It compresses vapor into a small volume at a high temperature and high pressure. It then discharges superheated vapor into the discharge line.
What is oil used for in the compressor?
Compressors are lubricated by oil. This oil is placed inside the compressor crankcase or housing. It is circulated to various compressor parts. In a hermetic system, this oil also lubricates the motor bearings.
What is an oil separator?
When the compressor operates, small amounts of oil are pumped out with the hot, compressed vapor. Small amounts of oil do no harm, but too much oil entering the condenser, metering device, evaporator, and filters interferes with their operation. Oil can be separated from refrigerant by placing an oil separator between the compressor exhaust and the condenser.
An oil separator is a tank or cylinder that contains a series of baffles or screens that collect oil. The oil, separated from the hot, compressed vapors, drops to the bottom of the oil separator. A float controls the needle valve to an oil return line connected to the compressor crankcase. When the oil level is high enough, the float rises and opens the needle valve. The pressure in the oil separator is considerably higher than the pressure in the compressor crankcase. The oil serves as a liquid seal to prevent refrigerant from entering the return line. The float closes the needle valve when the oil level in the separator drops.
What is the condenser?
A condenser is the part of a compression refrigeration system that releases heat from the vapor refrigerant and allows the vapor to condense back into liquid. Only liquid refrigerant should leave the condenser. A condenser removes the latent heat from the refrigerant vapor, by releasing the latent heat, the vapor condenses back to a liquid.
Condensers frequently transfer heat into one of two substances: air or water.
- Air cooled condensers are cooled by the flow of air. As hot, high-pressure refrigerant vapor flows through the condenser tubes, air around the condenser accepts the heat given up by the condensing refrigerant. Often, air-cooled condensers have fans that blow air over tubes to remove heat more quickly. These are called forced-air condensers.
- Water-cooled condensers are mainly manufactured in three different designs: shell-and-tube (cooling water flows through long, straight copper pipes that run along the inside of a long cylinder filled with hot refrigerant), shell-and-coil (water flows through a coil of copper tubing that winds around the walls of a shell filled with refrigerant), and tube-within-a-tube (This consists of two tubes. One tube is located inside the other. Water flows through the inner tube in one direction. In the other tube, refrigerant flows in the opposite direction of the water flow).
What is a pump down?
During system servicing, refrigerant is often pumped out of various system parts and into the liquid receiver where the refrigerant can be sealed off from the rest of the system. This is called a pump down.
What is the liquid receiver?
The liquid receiver is a storage tank for liquid refrigerant located on the high side between the condenser and the liquid line. Occasionally a liquid receiver is built into the bottom of a condenser. Having a liquid receiver makes the quantity of refrigerant in a system less critical. When a smaller amount of refrigerant is needed, the remaining refrigerant collects in the liquid receiver.
Most liquid receivers have service valves. A fine copper mesh in the liquid receiver outlet prevents dirt from entering the liquid line. Liquid receivers are usually found on larger HVACR systems that have a significant refrigerant charge. These systems use either low-side float or expansion valve metering devices. Systems with a capillary tube metering device do not use liquid receivers.
What is the liquid line?
A liquid line is tubing that carries liquid refrigerant from the condenser or liquid receiver to the metering device.
What is flash gas?
Flash gas is the instantaneous evaporation of some of the liquid refrigerant. Some flash gas is acceptable in the evaporator because it cools the remaining liquid refrigerant to the desired evaporation temperature. Excessive flash gas reduces the efficiency of the evaporator. When a system has an expansion valve as its metering device, flash gas in the liquid line can cause damage to the expansion valve.
What is a liquid line filter-drier?
A filter-drier collects moisture, dirt, metal, and other debris to prevent any from entering the metering device. The drying element in the filter removes moisture. This moisture might otherwise freeze in the metering device. Moisture is also harmful when mixed with oil in a system because it forms sludge and acid. Moisture is especially harmful to hermetic units.
What is a metering device?
A metering device controls the flow of refrigerant into the evaporator. A metering device is located between the liquid line and the evaporator. Its function is to lower the pressure of the refrigerant by restricting the passageway into the evaporator.
A metering device’s restriction allows only a small quantity of liquid refrigerant to pass. Since this small quantity of liquid refrigerant does not fill all the available space in the evaporator, pressure is reduced. The space that the liquid refrigerant does not fill is filled with vapor refrigerant that flash boils because of the reduced pressure. The metering device maintains this lower pressure in the evaporator.
What are some different kinds of metering devices?
Fixed orifice
Capillary tube
Thermostatic expansion valve (TXV)
Automatic expansion valve (AXV)
Electronic expansion valve (EEV)
Low-side float (LSF)
High-side float (HSF)
What is an evaporator?
An evaporator is a heat exchanging device that absorbs heat into its refrigerant.
What is an accumulator?
An accumulator is a tank in the suction line that prevents liquid refrigerant from flowing through the suction line and into the compressor. A typical accumulator has its inlet and outlet at the top. Any liquid refrigerant that flows into an accumulator falls to the bottom and must evaporate to enter the suction line.
What is the suction line?
A suction line is tubing that carries refrigerant vapor from evaporator to compressor. It must slope downwards to the compressor, otherwise pockets of oil could collect. It should be insulated in order to prevent moisture from the air from condensing on the cold tubing.
What is superheat?
The vapor refrigerant moving through the suction line is superheated. This means that the temperature of the refrigerant is above its condensation (saturation) point. This additional heat content can be measured in degrees because it is sensible heat.
What is a hot gas defrost valve?
What is a bypass valve?
