Low ambient conditions for RACCU

[lippy]

Hello my fellow engineers,
Here is my problem. I have a small Copeland RACCU, R-134, 3HP. It is used to cool a dry room with a Munters dryer.
We use hot gas bypass into the dx coil in the room for modulation and heat because the room is to run at 77F. Well, I can't get enough hot gas to heat up the room. It seems I probably overestimated the moisture load as well as the heat load which the process equipment in the room would give off. The RACCU runs 24/7 with a single speed fan and the condenser has an Alco "head master" which is suppose to bypass hot gas from the compressor around the condensor to the receiver during low ambient situations. I'm not getting much "hot gas". I was wondering if a quick fix would be to put some cardboard over the fan intake. The problem has just started since the outdoor temps have dropped here in September into the 50F-60F.

 

[ClydeMule]

What's your head pressure ? From what I remember the Alco is set for 185 psi, so if it working right, the head pressure should not drop below that. Actually I think there is a 15% tolerance or so, so if it goes to 180 you are probably OK.

I want to make sure when to talk about the hot gas bypass for capacity control, you are not confusing it will the condenser flooding valve (headmaster) for head pressure control.

In essence, they both bypass hot gas, but for different functions. This confuses a lot of people.

Is this the first time this has happened? Is this a new machine?

I would recommend going to

http://www.sporlan.com,

and looking up the literature on TEV's and head pressure control valves.

The valves most similar to the Headmaster are the LAC, and the OROA. THey have another system which uses two separate valves, the ORI and ORD.

But to answer your question, the cardboard method may solve your problem, if low head pressure is your problem.

Good Luck,

Clyde

 

[lippy]

Thanks for responding.
I have both an Alco HP8 with 110 psi condensing pressure set point and a seperate modulating hot gas valve on a hot gas line from the compressor outlet to the distributor side port on the dx coil. Is it possible that for 134-a I should have higher setpoint specified. Thanks, Joe.

 

[imok2]

HeadMaster HP Operation
The HP control is a three-way modulating valve controlled by the discharge pressure. The charged dome exerts a constant pressure on top of the diaphragm. At high ambient air temperature, bypass gas entering Port B (by-pass connection) is allowed under the diaphragm where it counteracts the pressure of the dome charge. This upward push on the diaphragm allows the seat disc to seal against the top seat, preventing flow from Port B (discharge gas) while flow from Port C (condenser connection) is unrestricted As ambient air temperature falls, an uncontrolled air cooled condenser will exhibit a corresponding decrease in head pressure. As the discharge (bypass) pressure falls, it no longer counteracts the dome charge pressure and the diaphragm moves downward, moving the pushrod and seat disc towards the bottom seat. NOTE: This allows discharge (bypass) gas to be metered into the receiver, creating a higher pressure at the condenser outlet. The higher pressure at the condenser outlet
reduces the flow from Port C( the condenser and causes
the level of condensed liquid to rise in the condenser. The question is: is this working correctly or is the condenser connection “C” bypassing refrigerant to the receiver

. IMPORTANT: The condenser must be filled with liquid to point where a minimum head pressure is created for cold weather (low ambient temperature) conditions.
NOTE: Should the outdoor temperature fall below design conditions, additional refrigerant will be required.
. During summer operation the receiver must be sized to safely hold the total system charge. Good refrigeration practice states that the total system charge should not exceed 75% of the receiver capacity. Also you may need to go to HP14

 

[ClydeMule]

Modulating as in a motorized valve? What parameter controls your hot gas valve?

Yes 110 psi is right, the 185 psi is for R-22. Wasn't paying attention.

I don't really mess with hot gas much to be honest, but have used a ton of condenser flooding valves. I have never done hot gas with DX.

However, what bugs me is that you say you can't "heat up the room." The hot gas is not designed to heat the room, but to add load on the low side to keep the suction pressure from dropping below the setpoint of the HGBP valve. By bypassing it to the inlet it allows the TEV to desuperheat the gas before returning it to the compressor. On systems that bypass hot gas directly to the suction side of the compressor you have to use a desuperheating TEV to keep from burning up your compressor.

I guess you should restate your problem. Is the room to cold? RH too high?

If I was to take a stab at it from the info given I would say the suction pressure setpoint (assuming the valve is set up that way) is too low.

Hope this helps.

Clyde

 

[imok2]

Anoter solution is if adjusting the HG bypass results in deminished refrigerant flow then maybe installing a duct heater in he supply air may be needed

 

[lippy]

Our refrig systems are probably considered a little non-traditional. I am well aware of how the Alco HP valve works. The modulating hot-gas valve, not the Alco HP, is a motorized (Siemens SDR-3) and controlled from a little PLC (0-10vdc) with a room temp set point. We use a lot of hot-gas modulation because these are usually rooms with temps from 40-70F +/- 1F control throught the room. There is also alot of part-loading situations.

Yes the room is too cold at 68F. The set point is 77F. I believe we are constantly hot gassing but not enough to warm up the room on these cold ambient outdoor temp days.
I would like to do it without adding electric heat. Thanks for hangin with me, Joe.