HVAC PRIME SOURCE, LLC

CHILLERS & DIRECT EXPANSION (DX) UNITS.

General
Chillers and DX units 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.

Chiller & Dx Unit Sizing Rules –

  • Dx Unit Type – Reciprocating or Scroll
    Capacity range for package units – 0.5 to 150 Tons
    Capacity range for split units – 1 to 70 Tons
    Power – 0.9 to 1.3 KW / Ton
    Min Capacity Turndown capability – depends on number of cylinders

  • Chiller Type – Reciprocating
    Capacity range – 15 to 100 Tons
    Power – 0.9 to 1.3 KW / Ton
    Min Capacity Turndown capability – depends on number of cylinders
    Chiller Type – Scroll
    Capacity range – 10 to 150 Tons
    Power – 0.9 to 1.3 KW / Ton
    Min Capacity Turndown capability – depends on number of cylinders

  • Chiller Type – Rotary Screw
    Capacity range – 70 to 500 Ton
    Power – 1 to 1.5 KW / Ton
    Min Capacity Turndown capability – 25%

  • Chiller Type – Centrifugal
    Capacity range – 200 Ton to 2000 Ton
    Power – 0.5 to 0.85 KW / Ton
    Min Capacity Turndown capability – 10%

  • Chiller Type – Absorption
    Capacity range – 100 Ton to 2000 Ton
    Heat – 12000 Btuh/Ton (gas/oil), 12000 – 18000 Btuh/Ton (steam/hot water).
    Min Capacity Turndown capability – 10%