CHILLERS AND THE REFRIGERATION CYCLE Flashcards
1.1
Describe the purpose of Chillers
Purpose
Chillers are heat transfer devices used to cool water in a Chilled Water system. Chilled Water systems provide cooling for the HVAC (Heating, Ventilation, and Air Conditioning) Air Cooling Units (ACUs) used to maintain spaces cool for either equipment or personnel concerns. Chillers absorb heat from the Chilled Water and reject the heat to a Cooling Water system.
1.2
Describe the refrigeration cycle of a Chiller.
Basic Chiller
1 → 2: The low pressure, high temperature, saturated Freon vapor from the Cooler is drawn into the suction of the Compressor and is compressed; increasing both the pressure and temperature of the gas.
2 → 3: The high pressure and temperature, super-heated Freon vapor is discharged from the Compressor and enters the top of the Condenser. It interacts with the upper Cooling Water tube bundles and starts to transfer sensible heat to the Condenser Cooling Water system until it becomes a high pressure and temperature, saturated vapor.
3 → 4: As the high pressure and temperature, saturated Freon vapor continues to interact with the Cooling Water system it will transfer latent heat to the Condenser Cooling Water and start condensing until it becomes a high pressure, low temperature, saturated liquid.
4 → 5: The high pressure saturated Freon liquid drains from the Condenser into the Expansion Valve, an enlarged volume tank, which causes the refrigerant’s pressure to drop until it becomes a liquid / vapor mixture.
5 → 1: The low pressure and temperature, liquid/vapor Freon mixture is driven out of the Expansion Valve into the Cooler where it interacts with the Chilled Water system. The Freon absorbs latent heat from the Chilled Water, causing it to flash into a low pressure high temperature saturated vapor, while the Chilled Water is cooled.
1.2 DRAWING BASIC
Describe the refrigeration cycle of a Chiller. (DRAWING BASIC)
Basic Chiller
1.3
Describe Compressor Surge.
Centrifugal compressors exhibit a “surging” characteristic if the differential pressure between the Cooler and Condenser (a.k.a. Lift) becomes greater than the capacity of the Compressor. Surging is a momentary gas flow reversal from the Condenser, through the Compressor, back to the Cooler. This reversal of flow reduces the differential pressure of the system below the capacity of the Compressor, allowing the system to operate normally again. However, the cause of the high D/P will repeatedly occur, causing the reversal of gas flow again. Continued surging may cause compressor bearing damage due to overheating.
Surging reverse loads the thrust bearings in the Compressor shaft and raises amperage of the motor. It will heat up the Compressor and, if continued for a period of time, may ultimately cause high bearing temperatures. The Compressor thrust assembly and bearings may also become damaged and cause further damage to the gearbox.
1.4
Describe Refrigerant Stacking.
“Stacking” is a condition that could occur with changes in Lift (difference in pressure between the Condenser and the Cooler) outside the normal parameters of the Chiller. Adequate differential pressure is required to drive the liquid refrigerant from the Condenser, through the Flash Economizer, back to the Cooler. Stacking is the abnormal accumulation of refrigerant in the Condenser caused by a decrease in the differential pressure of the system.
1.5
Describe the purpose and operation of a PVGS Chiller’s Inlet Guide Vanes.
Purpose
Inlet Guide Vanes control the volumetric flowrate of refrigerant vapor leaving the Cooler and entering the Compressor to control the cooling capacity of the Chiller.
Operation
The guide Vanes throttle in response to the temperature of the Chilled Water leaving the Cooler, being supplied to the HVAC heat loads. Lowering Chilled Water temperature causes the Inlet Guide Vanes to throttle closed. Since the Compressor suction lowers the pressure in the Cooler, aiding in flashing liquid refrigerant into a vapor, throttling the Guide Vanes closed will draw less vapor from the Cooler. This will raise the pressure in the Cooler, causing less liquid refrigerant to flash to a vapor. Since the phase change of refrigerant from liquid to vapor is the dominant heat draw from the Chilled Water, reducing this causes the refrigerant to absorb less heat from the Chilled Water. Therefore, the temperature of the Chilled Water leaving the Cooler will raise.
1.6
Describe the purpose and operation of a PVGS Chiller’s Compressor.
Purpose
Compression of the refrigerant vapor raises the temperature of the vapor above that of the Condenser’s Cooling Water.
Operation
The two-stage Compressor is driven by a 4.16 kV Motor through a reduction gear assembly. The first stage draws low pressure, high temperature refrigerant vapor from the Cooler and compresses it. Then it moves to the second stage inlet, where Flash Gas from the Thermal Economizer joins it. The second stage completes the compression of the vapor mixture to a high pressure and temperature, superheated vapor. Since the vapor from the Flash Economizer’s Flash Gas Chamber only has to pass through half the compression cycle to reach Condenser pressure, there is a savings in power and a gain in efficiency. This compression of the refrigerant vapor raises the temperature of the vapor above that of the Condenser’s Cooling Water.
1.7
Describe the purpose and operation of a PVGS Chiller’s Condenser.
Purpose
The Condenser rejects the heat picked up by the refrigerant in the Cooler from the Chilled Water to the Cooling Water system.
Operation
The Condenser is a two pass, shell and tube heat exchanger, with refrigerant on the shell side and Cooling Water on the tube side. Cooling Water passing through the tubes cools and condenses the superheated refrigerant vapor entering the Condenser.
Thermal Economizer
The bottom 12% of the Condenser is partitioned from the part above it, with openings at it ends. The coldest Cooling Water enters the Thermal Economizer to subcool the liquid refrigerant. This increases efficiency in the refrigeration cycle, reducing the power/ton requirement.
1.8
Describe the purpose and operation of a PVGS Chiller’s Flash Economizer.
Purpose
The Flash Economizer provides an intermediate pressure barrier between the Condenser and the Cooler, maintaining the pressure difference between the two. This regulates the refrigerant flow from the Condenser to the Cooler.
Operation
It consists of “High Side” and “Low Side” Float Valve Chambers, a Flash Gas Chamber, and the Main Chamber. Through the coordinated action of the two float valves, the Flash Economizer maintains an intermediate pressure and temperature, as compared to the Condenser and Cooler pressures.
There are two paths for liquid refrigerant from the Condenser to flow into the Flash Economizer, one into the Flash Gas Chamber and the other into the High Side Float Valve Chamber.
The High Side Float Valve Chamber serves as a level controller for liquid refrigerant in the Condenser, with the level of the entry tube in the Condenser placed high enough so that the Condenser’s Thermal Economizer is always filled with liquid refrigerant.
The high pressure, subcooled liquid refrigerant from the Condenser Thermal Economizer enters into the Flash Gas Chamber via the High Side Float Valve, as controlled by the liquid refrigerant level High Side Float Chamber. As the subcooled liquid refrigerant enters the larger volume chamber the pressure drops and some of it flashes to a vapor. The flashing process absorbs energy, “Latent Heat of Vaporization,” from the surrounding liquid refrigerant which further cools it. The flash gas is directed to the inlet of the Compressor 2nd stage to raise efficiency of the Chiller.
The Low Side Float Valve, controlled by liquid level in the Main Chamber, allows refrigerant to leave the Flash Economizer and enter the Cooler. It also maintains the pressure differential between the Main Chamber and the Cooler.
1.9
Describe the purpose and operation of a PVGS Chiller’s Cooler.
Purpose
The Cooler transfers heat from the Chilled Water used to cool the HVAC heat loads to the refrigerant.
Operation
The Cooler is a two-pass, shell and tube heat exchanger, with refrigerant on the shell side and Chilled Water on the tube side. Low pressure and temperature liquid refrigerant absorbs heat from the Chilled Water returning from its HVAC heat loads. The Chilled Water enters the Cooler at 60-64°F, depending on the Chilled Water system, and exits at ~44°F to be supplied back to the HVAC heat loads.
1.10
Describe the purpose and operation of a PVGS Chiller’s Pump Down Unit.
Purpose
Each Chiller package is provided with a Pump Down Unit which is used to transfer refrigerant to and from the refrigerant storage tanks, mounted below the Chiller, as necessary for maintenance.
Operation
The pump down unit consists of:
* Pump down compressor and motor
* Condenser
* Storage tanks (2)
* Piping and valves
Evolutions using the Pump Down Unit are controlled by HVAC personnel
1.2 DRAWING BASIC
Describe the refrigeration cycle of a Chiller. (DRAWING BASIC)
Basic Chiller
1.2 DRAWING
Describe the refrigeration cycle of a Chiller. (DRAWING)
Chiller
