View Full Version : 2 pipe and 3 pipe HG defrost.
08-18-2005, 12:46 AM
I was reading Icemeister's post about the doomed system. I don't have any experience with that kind of stuff, so I'll take his word for it that it is a mess.
My question is, what is 3 pipe and 2 pipe hot gas defrost? Never have dealt with defrost, so I don't know the difference, pros cons etc.
Just trying to learn more.
08-18-2005, 06:59 AM
I take a stab at it and hope Ice corrects me if I'm wrong. 3 pipe is like a Krame thermobank where there is a separate hot gas line to the evap. just past the txv before the distributer. 2 pipe is reverse cycle defrost like a heat pump.
[Edited by selfemployed on 08-18-2005 at 07:02 AM]
08-18-2005, 05:47 PM
There are two major categories of hot gas defrost refrigeration system. One is the three pipe system and the other is the two pipe system.
The three pipe system is most commonly used in walk-in freezers to defrost one or more evaporators (and the drain pans)
These systems have the typical liquid feed and suction return as one would have on any system. The third pipe is the HG line which is teed off after the compressor discharge line and runs to the evaporator, often running through a loop in the drain pan before entering the evaporator just after the TXV via a side inlet distributor.
The HG is controlled by a solenoid valve that opens during defrost and the liquid line solenoid closes. There's a lot more to it than that, but there's the basics.
The two-pipe system is most commonly used in supermarket systems with parallel rack system with many evaporators. Each evaporator circuit (which may in itself have multiple coils in parallel) has a suction and liquid line running back to a central header assembly fitted with liquid line, HG and suction stop solenoid valves to control the flow.
During normal refrigeration the LLS and suction valves are open. For defrost the suction valve closes and the HG valve opens dumping HG down the suction line to the evaporators in that circuit. At the coils, there's a bypass and check valve around the TXV so the condensing liquid can return back to the header via the liquid line. The liquid feed pressure is reduced (or the HG pressure is increased to allow for that liquid to return.
Variations of the Above
The Kramer Thermobank system is actually a unique hybrid of the three pipe system as it actually only uses two pipes, but acts on the three-pipe principle. It just requires a lot more control valves and stuff. Kramer utilizes the liquid line as the HG line as well.....thus eliminating the need for a third line.
Lastly, there is in fact a true heat pump style HG defrost system being manufactured by Russell Coil. Again, rather unique.
As I said, there is a lot more to all this, but that's covers the basics.
08-18-2005, 08:09 PM
Sorry for the misinformation on the kramer. Guess it's been to long since I worked on one :o
08-19-2005, 11:38 AM
Thanks for the info. Looks like I am ready to be a supermarket tech. Go buy me a bottle of R-134, maybe a little R-135 and I'm on my way.
08-19-2005, 05:57 PM
Say where you are :D Pretty stuff you build.
08-19-2005, 06:13 PM
If hot gas defrost systems with single compressors come alive again as a result of my posts, I'll call you to fab the systems for me. :D
You really do nice work.
08-20-2005, 12:37 AM
08-23-2005, 08:17 PM
Originally posted by selfemployed
Sorry for the misinformation on the kramer. Guess it's been to long since I worked on one :o The real old Thermobank systems did in fact use a Three pipe set up...............
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