Hot gas defrost

[GENERALCOOL]

Dear Friends,

Would you please read sentences at photo below inside of yellow rectangle and share your opinions with me ?

Usually on hot gas defrost, We transmit hot gas form discharge to inlet of evaporator and finally this gas goes to suction of compressor.
But I can not understand what EMERSON says. On photo you see discharge gas has been transmitted to output of evaporator and by solenoid valve (Suction stop solenoid valve) suction pipe has been isolated.
By this situation how defrost could happen ??

Thanks.
1543515436


http://www.generalcooltrade.com

[RANGER1]

GENERALCOOL,
I think document you posted is incorrect & to basic for correct description, as not showing all the control valves needed.
Below is a few documents which may help.
Would say all hot gas defrost systems would have to have a suction trap/slop pot to separate liquid in suction line before, during & after hot gas defrost.
Pressure in evaporator has to increase above 0 deg C to melt ice

http://www.colmaccoil.com/media/42058/optimizing-hot-gas-defrost.pdf

http://files.danfoss.com/technicalinfo/dila/01/DKRCI.PE.FT0.A1.02_ICFD_White%20Paper_Effective%20and%20cost-efficient%20hot%20gas%20defrost%20methods.pdf

[Brian_UK]

If you only defrost one evaporator at a time then that scheme will work, hot gas goes backwards through first evaporator and then mixes into liquid feed of second evap.

[RANGER1]

If you only defrost one evaporator at a time then that scheme will work, hot gas goes backwards through first evaporator and then mixes into liquid feed of second evap.


That would be on page 46 of this Danfoss book

https://assets.danfoss.com/documents/DOC137786416219/DOC137786416219.pdf

[passandscore]

Well, I think I just learned something. Thanks RANGER1

[RANGER1]

Well, I think I just learned something. Thanks RANGER1

Cannot say I have seen this method applied though!

[passandscore]

Me neither.

[Glenn Moore]

This method of defrost was often used in UK supermarket cabinets, and worked well. As the cabinets still refrigerating we’re having their evaporators superheat controlled by their Tevs so there is no need for suction accumulators or slop pots. In the condenser outlet (as in the Danfoss book page 46) a PM valve or nowadays an ICV Valve with a CVPP Differential pilot and an EVM solenoid pilot in parallel was used to create a Pressure difference between the liquid pressure and the hot gas pressure, so during refrigeration the EVM pilot is open and so is the ICV Valve allowing normal liquid flow, but during a cabinet defrosting, the EVM pilot would be forced closed, causing the CVPP pilot to control a differential pressure between the condenser outlet liquid pressure and the hot gas pressure. This pressure differential was required for the hot gas / condensate from the defrosting evaporator to enter the main liquid line and allow a hot gas flow and hence the evap to defrost.
One of the largest supermarkets, in place of the PM or ICV valve plus the pilots due to costs would use an NRVA non return valve with a much stronger spring fitted, so the plant always ran with a biased differential to the hot gas so that the defrost functioned correctly.
We had to stock several special heavy springs to suit NRVA 20, 25 and 32 to match the system plus at least 2 heavy spring versions to suit the plant requirements,from memory 0.8Bar and 1.4Bar .
A simple solenoid valve could be fitted in the suction line, but often a “special” valve called a PKVS valve was fitted , it had a solenoid function to force it closed during defrost and had a back pressure valve function whilst in refrig mode. Sporlan made one for both the States and Canada I think it was called a CDA .
These valves and the defrost method was the norm in most supermarkets and worked extremely efficiently
The Danfoss KVQ electronic back pressure valve could also be used in the suction as it would give very accurate temperature control with the benefit of a high stable humidity in the cabinet and could be forced closed for defrost with this method . The modern KVS stepper Valve can also be used for these combined functions .
Supermarket is probably the worse area of fridge to work in but the R&D that goes on in the industry is often generated from this area, as ideas are constantly changing

[Goober]

This method of defrost was often used in UK supermarket cabinets, and worked well. As the cabinets still refrigerating we’re having their evaporators superheat controlled by their Tevs so there is no need for suction accumulators or slop pots. In the condenser outlet (as in the Danfoss book page 46) a PM valve or nowadays an ICV Valve with a CVPP Differential pilot and an EVM solenoid pilot in parallel was used to create a Pressure difference between the liquid pressure and the hot gas pressure, so during refrigeration the EVM pilot is open and so is the ICV Valve allowing normal liquid flow, but during a cabinet defrosting, the EVM pilot would be forced closed, causing the CVPP pilot to control a differential pressure between the condenser outlet liquid pressure and the hot gas pressure. This pressure differential was required for the hot gas / condensate from the defrosting evaporator to enter the main liquid line and allow a hot gas flow and hence the evap to defrost.
One of the largest supermarkets, in place of the PM or ICV valve plus the pilots due to costs would use an NRVA non return valve with a much stronger spring fitted, so the plant always ran with a biased differential to the hot gas so that the defrost functioned correctly.
We had to stock several special heavy springs to suit NRVA 20, 25 and 32 to match the system plus at least 2 heavy spring versions to suit the plant requirements,from memory 0.8Bar and 1.4Bar .
A simple solenoid valve could be fitted in the suction line, but often a “special” valve called a PKVS valve was fitted , it had a solenoid function to force it closed during defrost and had a back pressure valve function whilst in refrig mode. Sporlan made one for both the States and Canada I think it was called a CDA .
These valves and the defrost method was the norm in most supermarkets and worked extremely efficiently
The Danfoss KVQ electronic back pressure valve could also be used in the suction as it would give very accurate temperature control with the benefit of a high stable humidity in the cabinet and could be forced closed for defrost with this method . The modern KVS stepper Valve can also be used for these combined functions .
Supermarket is probably the worse area of fridge to work in but the R&D that goes on in the industry is often generated from this area, as ideas are constantly changing

Yup, I remember working on these types of systems back in the early 90's. Like a lot of things, was the go to system back then. We bit complicated to work on, but once you got the hang of it, no issues.

Wasn't very good though, when evaps froze up on occasion, as could never get prolonged defrost heat for long enough. Nothing like hard wiring defrost heaters and isolating solenoid valve to aid defrosting that reach in freezer a block of ice...