Vapor Compression Refrigeration

Vapor Compression Refrigeration:-

 

In a vapor compression cycle, the compressor raises the temperature of the refrigerant high enough that it can reject heat to the atmosphere, then the expansion valve reduces the temperature of the refrigerant low enough that it can absorb heat from the conditioned space. 

 

Compressors

Compressors are important since Win = Wcomp.

Compressing vapor requires more work than compressing liquid since:

dw = v dP  and vliquid << vvapor

Reciprocating Compressors

Small (1 to 50 tons)

Noisy

Turn on when compressing and completely off when idle

Often multiple reciprocating compressors staged on/off to meet loads.

Results in high part-load efficiency

Screw Compressors

Mid-sized (50-300 tons)

Quiet

Compress by squishing vapor between screws

At part-load, vapor flow reduced, but screw keeps rotating. Thus, poor part-load efficiency unless motor slowed with VSD

Centrifugal Compressors

Large (300 tons +)

Vapor compressed as spun to outside

Like screw compressors, don’t stop impellor at part-load. Thus poor part-load efficiency unless motor slowed by VSD.

Most centrifugal compressors are integrated into chiller with tube and shell heat exchangers for condenser and evaporator.

Evaporators

An evaporator is a heat exchanger that absorbs heat from the conditioned space.  While absorbing heat, the refrigerant “evaporates” from liquid to vapor.  An expansion value at exit of evaporator pushes vapor to super heat region prior to entrance into the compressor.  The two primary types of evaporators are:

           

Refrigerant–to–Air

Finned tube HX, Fins on air side since hair<<href 

Used in small to medium chillers

“Direct Expansion”, “DX” since refrigerant directly cools air as it expands

 

Refrigerant–to–Water

Tube-in-shell HX

Ref in shell, water in tubes

Used in large centrifugal chillers

“Chilled” water pumped from evap to air handling unit chilled water coils

 

Condensers

A condenser is a heat exchange that rejects heat from building and compressor to the environment.  While rejecting heat, the refrigerant “condenses” from vapor to liquid. The total heat rejected by the condenser is the sum of the heat from the building and the work added to the compressor.

Qcondenser = Qevaporator + Wcompressor

The two types of condensers are:

 

Refrigerant-to-Air

Finned tube HX, with fins on air side

Called “Air Cooled”

Tcondenser = Tair

Small to med chillers

 

Refrigerant-to-Water

Tube in shell with vapor on shell side

Condenser water cooled in cooling tower

Tcondenser water < Tair, due to evaporation

Colder condenser temperature increases efficiency

Increased efficiency is worth price of cooling towers in big  chillers