Quote Originally Posted by Glenn Moore View Post
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...