Unit 81

What is the function of a purge recovery unit?

A purge recovery unit will purge the air from the top of the condenser and recover the refrigerant on a continuous basis while the chiller is running.

What is the difference between a single stage and two stage centrifugal compressor?

A single stage centrifugal compressor only has one impeller while a two stage unit would have two impellers. Multiple stage units develop higher pressures as compared to single stage units.

How should refrigerant be transferred from a chiller?

Always begin the refrigerant transfer from a chiller unit using liquid recovery methods.

Why are low pressure refrigerants well suited for centrifugal compressor chillers?

Centrifugal compressors are able to pump large volumes of refrigerant as compared to reciprocating compressors. However they do not develop the high side discharge pressures of a reciprocating compressor. Since low pressure refrigerants do not require such high discharge pressures, they are ideally suited for centrifugal chiller applications.

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.

Explain the operation of lead and lag compressors.

During peak periods, both compressors would be running fully loaded. In the early evening as the load begins to decrease the chill water temperature will begin to drop. This will signal the lag compressor to begin unloading cylinders sequentially and run at a reduced capacity. At some point the load will decrease to the point that the lag compressor stops. If the load continues to decrease, then the lead compressor will begin unloading cylinders. Eventually when the lead compressor is fully unloaded the hot gas bypass will open.

What are some advantages and disadvantages to flooded coolers?

Flooded coolers have greater cooling capacities than dry type coolers however they require more refrigerant and have a greater potential for freeze-up as compared to direct expansion coolers.

Describe the difference between a flooded and a dry (direct expansion) type cooler.

In a flooded cooler the refrigerant surrounds the outside of the tubes and the chilled water is circulated through the inside of the tubes. A "dry" direct expansion type cooler will circulate the refrigerant through the inside of the tubes and the chilled water will surround the outside of the tubes.

What precaution should be taken whenever leak testing a low pressure chiller?

Never allow a low pressure chiller to reach or exceed the rupture disk setting when testing for leaks.

What can be considered an advantage of a screw type chiller as compared to a centrifugal chiller?

Screw compressors are used because of their high capacity for a small unit, and their continuously variable (stepless) capacity control, typically modulating from 100% to 10% of full capacity. Being positive displacement, screw compressors have piping flexibility to accommodate remotely located condensers and can be used with high pressure refrigerants.

What happens to the flash gas formed in a chiller economizer?

Since the economizer is at a higher pressure than the suction pressure the vapor can enter the compressor at an intermediate point or in the case of a staged compressor, in a later stage.

What precaution should be taken for coolers that are located outside?

The cooler shell (evaporator) must be protected from freezing. Electric heating elements are wrapped around the shell and then covered with a thick layer of insulation.

What is the most efficient method of capacity control for a centrifugal compressor?

The most efficient form of capacity control for a centrifugal compressor is to regulate the speed.

What is the purpose of an oil return system for a chiller?

The oil in many large commercial centrifugal and screw type chillers travels through the system and collects in the bottom of the cooler. Without some type of oil return system, the oil will not be able to be reintroduced into the lubrication circuit.

What is the purpose of a chiller economizer?

The purpose is to subcool the liquid refrigerant before it enters the cooler.

What could happen if you charge liquid refrigerant into a chiller that is under a deep vacuum?

The refrigerant will immediately flash and the chiller will freeze up and the tubes will crack to cause extensive damage to the chiller.

How can the centrifugal compressor speed be higher than the motor speed?

This is accomplished by using speed increasing gears.

What is the advantage of a screw compressor that utilizes three screws?

This shortens the overall length and increases the compression efficiency.

Why is the cooler shell and suction line insulated on a unit that is located indoors?

To prevent sweating.

What type of capacity control is used for a centrifugal compressor that is driven by a constant speed motor?

Variable inlet guide vanes.