Heatcraft Refrigeration Products Condensing Units H-IM-CU User Manual

Liquid lines should be sized for a minimum pressure drop to prevent 

“flashing”. Flashing in the liquid lines would create additional pressure drop 

and poor expansion valve operation. If a system requires long liquid lines 

from the receiver to the evaporator or if the liquid has to rise vertically 

upward any distance, the losses should be calculated to determine whether 

or not a heat exchanger is required. The use of a suction to liquid heat 

exchanger may be used to subcool the liquid to prevent flashing. This 

method of subcooling will normally provide no more than 20

˚

F subcooling 

on high pressure systems. The amount of subcooling will depend on the 

design and size of the heat exchanger and on the operating suction and 

discharge pressures. An additional benefit from the use of the suction to 

liquid type heat exchanger is that it can help raise the superheat in the 

suction line to prevent liquid return to the compressor via the suction line. 

Generally, heat exchangers are not recommended on R-22 low temperature 

systems. However, they have proved necessary on short, well insulated 

suction line runs to provide superheat at the compressor.

Hot Gas Defrost systems can be described as reverse cycle, re-evap., or 

alternating evaporator.  Please see manual H-IM-HGD for Mohave™ systems.

Install all refrigerant components in accordance with applicable local and 

national codes and in accordance with good practice for proper system 

operation. The thermostatic expansion valve must be the externally 

equalized type. It can be mounted inside the unit end compartment. Mount 

the expansion valve bulb on a horizontal run of suction line as close as 

possible to the suction header. Use the clamps provided with the valve to 

fasten the bulb securely so there is a tight line-to-line contact between the 

bulb and the suction line. Suction and hot gas connections are made on the 

outside of the unit.
Suction lines should be sloped towards the compressor at the rate of one 

(1) inch per ten (10) feet for good oil return. Vertical risers of more than four 

(4) feet should be trapped at the bottom with a P-trap. If a P-trap is used, the 

expansion valve bulb should be installed between the unit and the trap.

The hot gas unit coolers can be used in reverse cycle hot gas defrost systems 

using multiple evaporators connected to one condensing unit. Generally, not 

more than one-third of the system defrosts at one time.  During the reverse 

cycle defrost, the reversing valve, located in the compressor discharge line, 

diverts hot gas through the suction line to the evaporator.
See the piping view in the Reverse Cycle Defrost Piping diagram. The suction 

line check valve directs the hot gas through the drain pan loop which 

prevents condensate in the pan from freezing. The hot gas exits the loop at 

the pan loop outlet header and enters the evaporator through the check 

valve assembly. As the hot gas defrosts the coil, heat is removed from the 

hot gas and eventually it condenses into a liquid and exits the coil at the 

distributor side port. The liquid then flows through the check valve of the 

thermostatic expansion valve bypass assembly, around the thermostatic 

expansion valve, and into the system liquid line. The liquid refrigerant then 

feeds other evaporators on the cooling cycle, evaporates, and returns to the 

compressor through their suction lines.

4,000

1/2

1/2

1/2

1/2

1/2

5,000

1/2

1/2

1/2

1/2

1/2

6,000

1/2

1/2

1/2

5/8

5/8

7,000

1/2

1/2

5/8

5/8

5/8

8,000

1/2

5/8

5/8

5/8

5/8

9,000

1/2

5/8

5/8

5/8

5/8

10,000

1/2

5/8

5/8

5/8

5/8

12,000

5/8

5/8

5/8

7/8

7/8

14,000

5/8

5/8

7/8

7/8

7/8

16,000

5/8

5/8

7/8

7/8

7/8

18,000

5/8

7/8

7/8

7/8

7/8

20,000

5/8

7/8

7/8

7/8

7/8

25,000

7/8

7/8

7/8

7/8

1-1/8

30,000

7/8

7/8

7/8

1-1/8

1-1/8

35,000

7/8

7/8

1-1/8

1-1/8

1-1/8

40,000

7/8

1-1/8

1-1/8

1-1/8

1-1/8

45,000

7/8

1-1/8

1-1/8

1-1/8

1-1/8

50,000

7/8

1-1/8

1-1/8

1-1/8

1-1/8

60,000

1-1/8

1-1/8

1-1/8

1-3/8

1-3/8

70,000

1-1/8

1-1/8

1-3/8

1-3/8

1-3/8

80,000

1-1/8

1-1/8

1-3/8

1-3/8

1-5/8

90,000

1-1/8

1-3/8

1-3/8

1-5/8

1-5/8

100,000

1-1/8

1-3/8

1-3/8

1-5/8

1-5/8

Note:  Use next larger hot gas line size for -20

0

F. and lower suction temperatures.

It is imperative that with the alternating evaporator hot 

gas defrost system, no more that 25% of the operating 

refrigeration load be in defrost at any time.

The three pipe system (sometimes called re-evap.) uses three pipes: one for 

liquid line, one for suction line, and one for hot gas line. In addition, a re-

evaporator accumulator is used at the suction outlet of the evaporator. The 

hot gas is taken from the discharge line between the compressor and the 

condenser, through a hot gas solenoid valve, then to the evaporator drain 

pan circuit, distributor tee, through the coil. See the Three-Pipe Defrost

Piping Diagram for typical piping at the evaporator coil.

In the alternating evaporator hot gas defrost system, a third line is taken 

off the compressor discharge line as the re-evap system. It is piped with 

solenoids at each evaporator, so that hot gas defrost is accomplished on one 

or more evaporators while the remaining evaporators continue to function 

in a normal manner. The liquid from defrosting evaporators is reintroduced 

to the main liquid line and it is necessary that 75% or greater capacity be 

retained in the normal refrigeration cycle to offset the capacity that is being 

removed by the units on the hot gas defrost.