Cascade compressor size question

[sdumper]

In general I try and use between a 1hp and a 1 1/4hp compressor on stage 1 and either a 1 or 3/4 hp on the second.

I am repairing a cascade tuned to hold 225 watts at -78c using r23 in second and using 1/2hp compressors on both the first and second. The customer needs the unit to hold 350 watts possibly even 400 watts. Im going to need to upgrade both those compressors right?

Sorry if this is a silly question but I have never used small compressors in a 2 stage before.

[Ozzfest05]

In general I try and use between a 1hp and a 1 1/4hp compressor on stage 1 and either a 1 or 3/4 hp on the second.

I am repairing a cascade tuned to hold 225 watts at -78c using r23 in second and using 1/2hp compressors on both the first and second. The customer needs the unit to hold 350 watts possibly even 400 watts. Im going to need to upgrade both those compressors right?

Sorry if this is a silly question but I have never used small compressors in a 2 stage before.

The first stage compressor will definetely need more power.
Second stage at 1/2 hp would be fine for tuning a standard 250-300 watt load.
A 400 watt load is hefty.

It sounds like a lot of work, you will also need to get a huge condenser on the first stage.
As the current will not handle the 1-1/2hp compressor.

Will you be changing second stage gas to ethelyne?

Good luck bud and please post pics of the unit when you receive it.

[n00b 0f l337]

It depends on the temperature you want. The dual 1/2's are probably fine if you can't get a proper holding on the first stage at a temp that will condense the second.

[sdumper]

Adam: -80c at 350 watts...

Do you think a 1/2 can handle that? I know you have experimented a lot with smaller compressors with good results. Im just worried about the load.


Ozzfest:
It has a good sized condenser but doesn't have a desuper at all. It also only has a single line of cap feeding the HX from the first. I dont use cap on first stages usually but when I have its been 2 lines.

[Ozzfest05]

I think it all boils down to time. Personally i dont think its possible with that first stage compressor (due to the high load).
Is it .031 cap? It must be bigger.
Time comes into play because the stars need to align for it to hold that load, tinkering with cap tube length etc.
Take into account warmer weather and effieciency loss over time I dont think you want it back in a few months for tuning.

Of course cost comes into play.
I would swap the first stage compressor and add an expansion valve .
May aswell add a dsuper coil to the mix.

[n00b 0f l337]

You will really want a TXV on that first to push the second. You can most likely manage -60~C @ 350-400W with r23 in there.

[sdumper]

Thanks guys all sound advice

[Kevin Hotton]

I believe there is a general lack of knowledge about refrigeration compressors. That being that they are positive displacement devices. To generate a given cooling duty will depend on the mass flow that the compressor can circulate (based in its displacement and volumetric efficiency). Using R23 at -80C I find the saturation pressure to be 16.5-psia. With an evaporator inlet quality estimate of 0.25 along with a volumetric compressor efficiency estimate of 0.75 (compressor rotational speed 3500rpm) and compressor inlet temperature of -40C, I calculate a second-stage compressor displacement of 11 cc/rev. I don't know what the displacement of a 1/2-hp rotary compressor is, but I suspect it is less than 11. Also, with the low temperature used in CPU coolers, the compressor power needed is low relative to what the compressor was designed for (standard AC cycle with much higher suction pressure).

[n00b 0f l337]

Hey Kevin, 11cc is about right for 1/2hp, closer to 10, but its about right. Especially if the goal is closer to -60C.

[sdumper]

Its actually not being used for cooling CPUs. Its being used to test tolerances on other chips but the concept is the same.
I ordered a 3/4 for the first and will swap the first with the burned out second. I will also use r170 for refrigerant.