Minimum discharge pressure for ammonia systems

[US Iceman]

Oversupply will lead to significant parasitic load on compressors, because only certain portion of the gas will condense and the rest will go through the coil, BPR to the suction line. This supply depends of coil size, refrigerated space temperature, amount of frost on the coil and etc.

NH3LVR, What Sergei is describing is the samething as the coil being in defrost (with the hot gas still on) after the coil has been defrosted. Once the frost is melted off, additional hot gas continues to heat the coil resulting in steaming and a lot of gas flowing out of the coil.

The gas flowing out of the defrost regulator adds to the compressor load (booster if the hot gas returns to the booster suction, or to the high stage if the condensate line is piped back to the intercooler).

Since the defrost pressure is around 75 to 90 psig ( 5.2 to 6.2 bar g), it would make sense to have the condensate piped back to either, a separate vessel with a separate recip. compressor, or a controlled pressure receiver. Whatever is close enough to work.

Pumping out ammonia evaporators is a pain for the reason stated. It's the same problem with trying to get a flooded out screw started. A little bit of ammonia takes a lot of heat.

[NH3LVR]

it would make sense to have the condensate piped back to either, a separate vessel with a separate recip. compressor, or a controlled pressure receiver. Whatever is close enough to work. Pumping out ammonia evaporators is a pain for the reason stated. It's the same problem with trying to get a flooded out screw started. A little bit of ammonia takes a lot of heat.

US Iceman;
I had never thought about the concept of using a separate Compressor. Has this been done before, could you elaborate?
On the subject of Flooded Screws, one of our Customers managed to fill up a Screw Booster with NH3 the other day. It took 3 days to get it going again. They now believe in the safety Shutdown on the LPR we wanted to install.

[US Iceman]

HI NH3LVR,

What I'm thinking about is to use a separate vessel (accumulator if you will) to collect the gas and liquid from the defrost regulators. If the suction pressure on the compressor attached to this vessel is running higher than any other house suction pressure, it just means the penalties for hot gas defrost would be much less.

In other words, we are taking the condensate up to as high as pressure as we possibily can. From that point, a separate compressor could run the suction pressure with no problem (especially a recip).

I've seen controlled pressure receivers run this way, so it should not be any big jump in logic to say try this for defrost.

The trick would be to size the compressor to handle the defrost loads accurately. This is just me thinking out loud. I have not done this for defrost, but I can't see why it wouldn't work.

On the subject of Flooded Screws, one of our Customers managed to fill up a Screw Booster with NH3 the other day. It took 3 days to get it going again. They now believe in the safety Shutdown on the LPR we wanted to install.

Ohhh, those are always fun jobs to get.

[Sergei]

Frequency of defrosting is another important factor of refrigeration plant optimization. Right now, here in Ontario, one cold storage has defrosting once per week, but another one has defrosting twice per day. I found that overdefrosting is common problem for many refrigeration plants. It is difficult to justify additional compressor installation, if you have defrosting once per week.

Sergei

[US Iceman]

I found that overdefrosting is common problem for many refrigeration plants. It is difficult to justify additional compressor installation, if you have defrosting once per week.

I would agree. Each facility has it's own personality in how it operates and what it needs. This also changes with the seasons (summer, winter, etc).

The concept I posted earlier is just one way of dealing with an issue. Just like in troubleshooting, there is always a cause and effect we need to account for in any instance.

[Bradford]

Frequency of defrosting is another important factor of refrigeration plant optimization. Right now, here in Ontario, one cold storage has defrosting once per week, but another one has defrosting twice per day. I found that overdefrosting is common problem for many refrigeration plants. It is difficult to justify additional compressor installation, if you have defrosting once per week.

I’ve been reading this post with great interest. What started out as a discussion on lowering head pressure and it’s implications has become a discussion on total system optimization! And it seems to be a topic that a lot of people are interested in. With the ever rising costs of energy and the push for a greener planet, I wonder if this would make a good topic for a forum or sub-forum.

What Sergei mentions about defrosts going from twice per day to once per week has become quite typical in plants that incorporate smarter controls and/or keener operators. Evaporators that require 2 defrosts per day during that one hot week in august can often get away with one or two defrosts during the winter months. This gets back to the problem of fixed setpoints often being the “worst case” setpoint. If two or three defrosts are needed in the summer, that’s where the pins are left in the time clock. If -25 suction kept the rooms cold in August, that’s where the suction is left all year. If one evaporator requires 145# for defrost, that’s where the pressure switches are set. It’s obvious that a lot of systems have a lot of room for energy usage improvements

Another great up-side to long durations between defrosts is you can float the head pressure lower for a longer period of time thus enhancing energy savings.

Brad

[US Iceman]

a discussion on total system optimization! ... I wonder if this would make a good topic for a forum or sub-forum.

That's good idea. Let me run it past the boss of RE for his comments.