Heatcraft Refrigeration Products H-IM-CU Liquid Lines, Hot Gas Defrost Systems, Three Pipe System

Liquid Lines

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

Hot Gas Defrost systems can be described as reverse cycle, re-evap., or alternating evaporator. Please see manual H-IM-HGDfor Mohave™ systems.

Refrigerant Piping

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.

Reverse Cycle System

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.

Three Pipe System

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.

Alternating Evaporator System

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.

IMPORTANT:

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.

Hot gas line sizes for R-22, R404A and R507

System Capacity		Equivalent Discharge Length (Ft.)
BTU/Hr	25	50		75	100	150
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 -200F. and lower suction temperatures.

REVERSE CYCLE DEFROST PIPING THREE-PIPE DEFROST PIPING

TXV

EVAP. COIL

CHECK

VALVE

CHECK VALVE

PAN LOOP

LIQUID

LINE

HEAT – X

CHECK VALVE	SUCTION
	LINE

TXV CHECK

VALVE

PAN LOOP

HEAT – X

EVAP. COIL

HOT GAS LINE

LIQUID LINE SUCTION LINE