Discharge Desuperheating Good or Bad?

[Calvin Becker]

Real example: R22 100Tons/cir

Actual discharge surface temperature 5 cm upstream of injection inlet 78`C
Reduction discharge surface temperature 5 cm downstream of injection inlet 23`C

Shell & tube condenser, water cooled, 25% capacity, condensor TD 3K, constant flow, low head.

Would expect 1.5K diff!

Chiller available to view.

Any comments????

[Dan]

Hi, Calvin. I don't see it as either good or bad. Just overcoming some sort of inefficiency that can be better addressed in the design or repair of the equipment. What am I missing?

[Calvin Becker]

Hello Dan,
Having read many hundreds of posts made by yourself and Marc over the past few years... and saying that, I can thank you both for a few things that contributed towards my knowledge in fridge.

Posting the above was just a quest for further thought and insight into the subject. I dont think you are missing anything.
Just wondering if LI desuperheating into discharge vapour is effective or not.

[Dan]

Haha! Please don't stir me and Marc up again! Okay Marc, rain your mathematics on me! Professor, I expect you to back me up at least once after all these years.

Everybody else, for this discussion, Calvin is proposing that there could be an advantage to squirting liquid into the discharge line before it goes into the condenser.

I think it is crap. But if you have a failing condenser, it might lower discharge pressure. If you have something wrong, in other words, it might utilize the compressor to correct the discharge pressure problem with a capacity penalty. Beyond that, it doesn't make any sense to me.

If there is poor design, or age-and-condition issues, this will alleviate the immediate problem.

You can also squirt water on the condenser and improve COP. Or if you are already squirting water on the condenser you can squirt colder water on it and improve COP.

Why not just deliver all the liquid into the condenser and be done with it?

[Prof Sporlan]

Thermodynamically, one is trading Btus by taking some liquid downstream of the condenser and injecting it into the discharge line, albeit requiring some pump energy to accomplish.

The only plausible benefit the Prof sees in liquid injection is it may well increase the heat exchange effectiveness of the condenser, as a greater percentage of its surface will be seeing two phase flow. If this is the case, and if the condenser is able to reject more Btus per degree TD, then we could have greater subcooling and/or lower head pressures result.

At this point, the economics of this arrangement could be surmised. Say our normal condenser TD is 15°F, and we were able to lower it by 5°F, what would be our energy savings? To be fair, we need to subtract out the energy represented by the refrigerant flow being injected into the discharge line, and the energy used by the pump. And then any energy savings applied to an ROI calculation.

One would think a system with adequate condenser would not benefit much from this arrangement. A system lacking in condenser would be fair game for speculation...

[Gary]

It seems to me that de-superheating the discharge gas with liquid would be no different than adding a compressor head fan, or a small condenser in series with the main condenser, or just going to a larger condenser. If the condenser is ample, it accomplishes nothing.

[Calvin Becker]

Hello Gary,

I would agree. Adding surface are may reduce condenser TD. Your thoughts?

[Gary]

Exactly, Calvin. And that's how high efficiency condensing units are made, i.e. increased surface area.

[Gary]

Everyone has an energy saving scheme. Here's my crackpot idea:

Take a small humidifier. Place a few coils of copper tube in the bottom, and pipe this in series between the condenser and the liquid line. Blow outdoor air over the humidifier plates, drum, whatever. This should drop the SCT closer to wetbulb temperature.

[Dan]

Take a small humidifier. Place a few coils of copper tube in the bottom, and pipe this in series between the condenser and the liquid line. Blow outdoor air over the humidifier plates, drum, whatever. This should drop the SCT closer to wetbulb temperature.

Gary, can you expand on this idea? I don't understand. I recall air conditioners.. window units with belts that flung the condensate onto the condenser, and many other schemes that were not practical. Practicality is your specialty, so I will listen.

[Gary]

You might think of it as a small evaporative condenser, if that helps. It would need details worked out.

Would it be practical? As is, probably not. But with a lot of experimenting and modifications, perhaps it could evolve into something practical.