Chillers and AC units with direct expansion (DX) of refrigerant are the two main type of equipment used for
air conditioning. Chillers provide cooling of the chilled water which is then used in the air cooling coils for
air conditioning, while DX units provide cooling by direct expansion of the refrigerant in the air cooling
coils. The air cooling coil in DX units and the chilled water heat exchanger in chillers are also called an
"Evaporator" since evaporations of the liquid refrigerant occurs in these components.

In general DX units are used for relatively small capacity systems and chillers are used for relatively large
capacity system. Typically a DX system is not used for systems larger than 100 ton capacity. Use the cooling
load software given on the "Hvac Software" link above to determine the required capacity. Further
variations of applications are as follows.

Chillers are available in the following major categories, based on the type of compressor and size of the
application. Typically chillers are package units on one skid, with the water piping system being external to
the unit. The package unit includes the compressor, condenser, evaporator and associated components.

  • Centrifugal compressor chillers (used for medium to large capacity applications)
  • Screw compressor chillers (used for medium capacity applications)
  • Reciprocating compressor chillers (used for small to medium capacity applications)
  • Scroll compressor chillers (used for small to medium capacity applications)

DX Units are available in following major categories, based on the type of the compressor and packaged or
split design:

  • Reciprocating or scroll compressor package units (condenser, compressor & evaporator are on the
    same skid).
  • Reciprocating or scroll compressor split systems (condenser, compressor, evaporator are located at
    separate locations - many systems have the condenser + compressor on the same skid as condensing
    unit, and the evaporator is located separately)

The condenser for both DX units and chillers can be air cooled or water cooled for relatively smaller sizes.
For the larger units the condenser is typically water cooled.

Typical Chiller & Direct Expansion AC Unit Diagrams (click on the links below)

Design, Operation and Maintenance Guidelines and Standards for Chillers and AC units -

Further guidelines for the Chillers and AC units design, operation and maintenance are provided for the
Members. These include
guidelines and standards for equipment selection & design, maintenance,
troubleshooting, operation, refrigerant analysis, etc.
Please click on the Members Resources button below
to access these additional guidelines.
In case you are not a member, please click on the Membership button
below to see how you can become a member, and a summary of all the various resources available to you
as a member.