question on hot gas and cool gas defrosts

[beachfronttech]

Some racks use hot gas defrost, with the refrigerant returning to the condenser by means of a pressure difference provided by a valve located in the discharge line. Some other racks use cool gas defrost (aka receiver gas), with the refrigerant returning to the liquid manifold by means of a pressure difference provided by a valve placed in the liquid line out of the receiver. These racks also use another valve (OPR) located between the discharge line and receiver, to maintain a constant pressure in the vessel
But there is this rack, with cool gas defrost, which has the three valves installed: discharge, liquid line and OPR. In such a case, why do we need a pressure differential between discharge and condenser during defrosts? Any comments appreciated.

[Josip]

Hi, beachfronttech :)


Some racks use hot gas defrost, with the refrigerant returning to the condenser by means of a pressure difference provided by a valve located in the discharge line. Some other racks use cool gas defrost (aka receiver gas), with the refrigerant returning to the liquid manifold by means of a pressure difference provided by a valve placed in the liquid line out of the receiver. These racks also use another valve (OPR) located between the discharge line and receiver, to maintain a constant pressure in the vessel
But there is this rack, with cool gas defrost, which has the three valves installed: discharge, liquid line and OPR. In such a case, why do we need a pressure differential between discharge and condenser during defrosts? Any comments appreciated.

See this.....http://www.refrigeration-engineer.com/forums/showpost.php?p=36744&postcount=2

follow the link.....maybe is not bad idea to search forums, as I remember there were a similar question with some schemes and explanation from Sporlan....sorry I'm not familiar with racks at all...;)

Best regards, Josip :)

[Gary]

In a hot or cool gas defrost system, the refrigerant flow is reversed through the evaporator. The hot/cool gas flows into the evaporator through the suction line, condenses in the evaporator and then the resulting liquid is pushed back through the liquid line to the liquid manifold.

In order to accomplish this reverse flow, the hot/cool gas must be at a higher pressure than the liquid manifold.

In a hot gas defrost system, the discharge gas pressure can be reduced downstream of the hot gas takeoff. This causes the hot gas to be at a higher pressure than the rest of the high side, including the liquid manifold... OR... the liquid pressure can be reduced upstream of the liquid manifold. Either strategy can be used for hot gas.

In a cool gas system, there is nothing to be gained by reducing the discharge line pressure, as this would also reduce the cool gas pressure. A cool gas system can only use the liquid reduction strategy.

My guess would be that the system has been converted from hot gas defrost (with discharge reduction) to cool gas defrost (with liquid reduction). This being the case, the discharge reducing valve should be wide open if not removed.

[Gary]

Hi, beachfronttech :)



See this.....http://www.refrigeration-engineer.com/forums/showpost.php?p=36744&postcount=2

follow the link.....maybe is not bad idea to search forums, as I remember there were a similar question with some schemes and explanation from Sporlan....sorry I'm not familiar with racks at all...;)

Best regards, Josip :)

That Sporlan diagram is for hot gas bypass, not hot gas defrost as commonly used in rack systems.

[Philip]

Hi,I am New to Rack system,and my Client asking me about the advantages and dis advantages of using rack system to multiple cold rooms with the same temperature requirements.

[Mark]

Some racks use hot gas defrost, with the refrigerant returning to the condenser by means of a pressure difference provided by a valve located in the discharge line. Some other racks use cool gas defrost (aka receiver gas), with the refrigerant returning to the liquid manifold by means of a pressure difference provided by a valve placed in the liquid line out of the receiver. These racks also use another valve (OPR) located between the discharge line and receiver, to maintain a constant pressure in the vessel
But there is this rack, with cool gas defrost, which has the three valves installed: discharge, liquid line and OPR. In such a case, why do we need a pressure differential between discharge and condenser during defrosts? Any comments appreciated.


Is there Heat Reclaim used?

[Josip]

Hi, Gary :)


That Sporlan diagram is for hot gas bypass, not hot gas defrost as commonly used in rack systems.

Thanks for answer......BTW can you post some link or maybe some scheme with example you are speaking about in your above post...it is never too late to learn something..;)....I know there are many executions in ***** systems which we (ammonia guys) never use:D

Best regards, Josip :)

[Gary]

The following shows both strategies for a hot gas defrost system:

http://www.sporlan.com/90-51.pdf

The cool gas system is essentially the same as the hot gas system using liquid reduction, except the defrost gas comes from the top of the receiver instead of downstream from the oil separator.

The advantage of cool gas over hot gas is less thermal shock to the evaporator.

[beachfronttech]

Thank you Gary for your precise explanation. I also thought that in this COOL GAS defrost system, the differential pressure valve located in the discharge line wasn´t really necessary, since we already have one in the liquid line. Regarding Mark´s question, there is no heat recovery in this system. Josip, there is plenty of information about hot gas defrost theory in the websites of Heatcraft and Tyler refrigeration. And Philip, I guess the main advantage of using parallel rack systems for large walk-in cooler and freezers vs individual compressors and condensing units lies in the potential energy savings. Under low load conditions, you might have ONE single compressor running, keeping proper temperatures in several walk-ins.

[Josip]

Hi, :)

Gary thanks for link...it was useful;)


.......Josip, there is plenty of information about hot gas defrost theory in the websites of Heatcraft and Tyler refrigeration........

Thanks, beachfronttech no need "theory" too much;) something I already learn and mostly I have to solve practical problems....I will stay with my friend ammonia:D it is more simple. In another hand having all of you here, it is easier to ask you:D

Best regards, Josip :)