View Full Version : Hot Gas Reheat

11-11-2014, 08:51 PM
Hey Guys,

I was wondering if I could get some advice as to the best methods of controlling and piping hot gas reheat coils. Here are a few questions/thoughts that I have.

I am tapping the discharge of my compressor and running to a stepper motor modulating valve. From there, the refrigerant cycles through my reheat coil and is dumped back into the discharge line through a check valve. Using this method, I have no real means to force X amount of refrigerant into the reheat coil even if the modulating valve is 100% open. Therefore, I thought about putting the modulating valve on the inlet of the condenser and a solenoid on the incoming side of the reheat coil. This way I can open the solenoid and throttle in as much refrigerant as required using the modulating valve. Do you guys see an easier/better way to do this? Do you guys have a good way of ensuring oil return back to the compressor and not getting stuck in the reheat coil while the circuit is shut down?

14-11-2014, 06:07 PM
bump it up

14-11-2014, 10:06 PM
Bump away Tanks ... I aint got a scooby doo what it is your doing

Explain some more and someone might be able to help

The Viking
15-11-2014, 01:09 PM
The "standard" would be to have 100% of the hot gas through the heat exchanger 100% of the time and then to modulate/control on the secondary side.


Glenn Moore
15-11-2014, 07:43 PM
There are several ways to control the system it depends on what you need to achieve.
If you are controlling the reheat coil temperature then you can fit your stepper valve on the inlet of the reheat coil and then fit a differential pressure regulator at the inlet to the main condenser. So when the stepper valve throttles to close on the reheat coil the differential valve on the condenser starts to open due to the differential pressure increasing between the discharge pressure and the liquid outlet pressure.
If you fit something like a Danfoss ICV 3 valve at the condenser inlet using a pilot differential CVPP valve and also a EVMNO solenoid pilot , the ICV valve could modulate on the differential pilot control , and when the RE heat is not required the solenoid can be used to fully open the ICV valve. You need to fit check valves at the coil outlets to prevent liquid back up under certain conditions.

17-11-2014, 06:29 PM
Thank you very much Glenn for your help

Glenn Moore
17-11-2014, 10:21 PM
Hi tanksboard
Your very welcome hope it helps Glenn

18-11-2014, 01:21 PM
Hi tanksboard
Your very welcome hope it helps Glenn

Hey Glenn,

I am having trouble finding any information on the differential pressure regulator. Do you have any documentation that explains how it operates? I've never worked with one and don't fully understand its operating principles. Thanks again for your help!

Glenn Moore
18-11-2014, 09:44 PM
Hi tanksboard
A differential pressure regulator does what it says ie it keeps a pressure difference between 2 points. In your case for instance the Stepper control valve is in the discharge line to the re heat coil , the diff regulator is in the main condenser discharge line.
When the stepper valve to the re heat coil is wide open (max reheat) the pressure drop across the re heat coil, and the main condenser coil is very small ie discharge to condenser outlet. So we set the diff pressure regulator to say 1 Bar Differential pressure. ( So the main condenser needs to see a 1 Bar differential pressure between its inlet pressurer and its outlet pressure.
So when the re heat is working fully open, there is little pressure difference between the inlet pressure and the outlet pressure of the re heat coil and the condenser coil. So the differential control valve on the condenser coil stays closed. ( ie no pressure differential)
As the re heat coil reaches its control point the stepper valve starts to throttle to a lower opening degree. This causes the discharge pressure to rise and the liquid outlet to fall, at this point the Differential Valve on the Main Condenser see the differential pressure increasingand as it gets to the 1 Bar setting the Diff Valve starts to open to keep the Differential pressureacross the main condenser at 1Bar . So the more the stepper valve closes off the re heat coil the more gas must be put through the main condenser coil.
As the stepper valve starts to re open to the reheat coil the differential pressure is reduced by the increase in the flow through the re heat coil, and the Diff Valve then starts to close as the diff pressure has gone below the 1 Bar set point.

If you go on the Danfoss web site and look for info on the ICV Valve ,the ICM + ICAD valve and the pilot valves CVPP(HP) you can download more info on these valves and their functions. If you need any more help let me know Kr Glenn