What happens when a refrigerant vapor is compressed?
Vapor-compression uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere.
How does cooling occur in a direct expansion vapor compression refrigeration system?
The compressor takes in a low temperature, low pressure refrigerant vapor, and compresses it into a high temperature, high pressure vapor. This lowers the temperature of the refrigerant stream. The refrigerant at the exit of the expansion valve is a low temperature two-phase fluid.
What is the difference between a direct expansion and flooded evaporator?
What is the difference between a direct expansion and flooded evaporator? In a flooded evaporator, the refrigerant is not fully evaporated and heated, while in a direct expansion evaporator, the refrigerant is fully evaporated and heated at a high temperature.
What is the function of a direct expansion evaporator?
An evaporator or direct expansion (DX) coil works on the “refrigeration effect”. Cooling occurs when a fluid under pressure and at a temperature above its normal boiling point, has the pressure reduced. This makes the fluid want to boil or “flash” to a gas.
Which of the following is considered an advantage of a flooded evaporator?
Characteristics of flooded systems. An advantage of flooded evaporators is that the potential problem of poor refrigerant distribution in the evaporator is reduced. The refrigerant is 100% liquid, and a liquid stream is much better distributed between the channels compared with the two-phase mixture of DX systems.
What is the most common type of system used in traditional supermarkets?
The most commonly used refrigeration system for supermarkets today is the multiplex direct expansion (DX) system. All display cases and cold store rooms use direct expansion air- refrigerant coils that are connected to the system compressors in a remote machine room located in the back or on the roof of the store.
What is the advantage of a cased coil secondary loop system?
86-8) What is the advantage of a case coil secondary loop system? The primary refrigerant is used for cooling the glycol in the chiller and is not delivered to the individual cases.
What is the disadvantage of a parallel heat reclaim system that has the reclaim coil located in the air handler?
A disadvantage to this location is the heat rejected from the reclaim coil needs to satisfy both the compressor head pressure requirements and the air handler demands.
How does air from air conditioning vents affect an open display case?
How does air from air conditioning vents affect open display cases? It disrupts the case’s air curtain, causing them to rise in temperature. Incorrect product placement can disturb the case’s air curtain.
What should a technician do to isolate the root cause of a malfunction?
What should you do to isolate the root cause of the malfunction? Perform tests to isolate the cause. recommend corrective action. determine if the components are receiving voltage.
What types of prime movers can be used for centrifugal chillers?
What types of prime movers can be used for centrifugal chillers? DC motors, AC motors, AC motors with variable frequency drives, diesel engines, gas turbines, and steam turbines.
What is the advantage of using a pressure relief valve to protect a receiver instead of a fusible plug?
It will not allow the evaporator pressure to drop below its setpoint, even if the suction pressure is lower. What is the advantage of using a pressure relief valve to protect a receiver instead of a fusible plug? The pressure relief valve resets itself.
What happens when a fusible plug in a cylinder melts?
The fusible plug is made of a metal alloy designed to melt. Once the plug melts away, the gaseous chlorine will spew out of the small hole. As the temperature of the released chlorine near the hole drops and chlorine icing slows the leak, the pressure inside the cylinder is also decreasing.
Which device metal melts when water goes down by permissible limit?
A fusible plug is a threaded metal cylinder usually of bronze, brass or gunmetal, with a tapered hole drilled completely through its length. This hole is sealed with a metal of low melting point that flows away if a pre-determined, high temperature is reached.
What is the state of refrigerant leaving the receiver?
Normally, the refrigerant leaving the receiver is picked up below the interface toward the bottom of the receiver, allowing the refrigerant to leave in a subcooled state. Want more HVAC industry news and information?
What is the difference between a liquid receiver and a suction accumulator?
The difference between a liquid receiver and a suction accumulator is that: Receivers only allow liquid to leave while accumulators only allow vapor to leave. Suction line. The refrigerant is a high-pressure, warm, subcooled liquid at the inlet of the metering device.
What is the purpose of the receiver soft plug?
The receiver dryer’s function is to store refrigerant for the system during periods of low cooling demand and filter out moisture and particles that can be harmful to the system.
Where should you remove the refrigerant in a system that has a condenser below the receiver?
Refrigerant should be removed from the condenser outlet if the condenser is below the receiver. In a building that has an air-cooled condenser on the roof and an evaporator on the first floor, recovery should begin from the liquid line entering the evaporator.
Where must you connect a hose in order to recover liquid refrigerant?
TYPE II
| Question | Answer |
|---|---|
| 6. When recovery is first started, at what phase is the refrigerant removed to save recovery time, | C. Liquid phase |
| 7. Where must you connect the hose in order to recover liquid refrigerant, | C. Liquid line |
How does an evacuated tank remove liquid from a system?
How does an evacuated tank remove liquid from a system ? The high- pressure liquid in the receiver is sucked into the evacuated cylinder. The high – pressure liquid in the receiver is sucked into the evacuated cylinder.
What happens to a system that leaks while in a vacuum?
If the system has a leak the vacuum gauge will continue to rise until atmospheric pressure has been reached. However if the system is vacuum tight but still contains moisture the rise will level off when the vapor pressure equalizes in the system typically between 20,000 and 25,000 microns between 72º and 80º F.
What happens if you have too much oil in AC compressor?
The compressor will be robbed of some of its crankcase oil and will run with a lower-than-normal oil level. This may score or ruin mechanical parts in the compressor.
What happens if too much oil in AC system?
If you add too much oil to the system, it may pool in various places of the system and can coat the inner walls of the evaporator and condenser coils. This will diminish their ability to absorb or dissipate heat, thereby bringing down the overall cooling performance.
How long should AC vacuum hold?
5-45 minutes