Pump down and suction pressure switches: legacy?

[Gary]

Being a radical departure from standard design practices, I would expect that the HX with TXV sensing bulb at it's outlet would be the least understood part of my design, so perhaps I should elaborate:

During the run cycle, this maximizes subcooling/minimizes flashing thus increasing the efficiency of the coil.

Placing the TXV sensing bulb at the HX oulet ensures that the superheat reaching the compressor inlet is the same as if the HX were not there (while maximizing subcooling).

During the off cycle, The HX forms a U shape with the coil, trapping any liquid in the coil and HX.

During defrost the HX, being within the refrigerated space, becomes the coldest part of the system; the part to which the liquid must migrate.

[Gary]

The other non-standard part of this design is the delay of the LLS.

Just as charging vapor into a system from a container does not allow liquid transfer, delaying the opening of the LLS ensures that the liquid can be drained from the coil and HX only in vapor form.

[NoNickName]

Please feel free to rephrase my posts to reflect the politically correct and acceptable version of the technical discussion, as long as the main message is untouched.
I don't feel guilty for hurting the sensibility of the audience.

For Gary, although I appreciate your efforts in explaining how you would design a pd system, actual experience says the state-of-the-art is else way. It could be a new chapter for a troubleshooting handbook.

[Gary]

Please feel free to rephrase my posts to reflect the politically correct and acceptable version of the technical discussion, as long as the main message is untouched.
I don't feel guilty for hurting the sensibility of the audience.

For Gary, although I appreciate your efforts in explaining how you would design a pd system, actual experience says the state-of-the-art is else way. It could be a new chapter for a troubleshooting handbook.

I should add that if you don't agree with my radically new design then you must be stuck in the stone age.

[Blueboy]

May I suggest that properly designed compressors have run for many years trouble free and have pumped down hundreds and hundreds of times. It sounds to me like we are talking about a sub-standard product here. I have worked of systems big and small and have seen the damage that can occur when a solenoid valve fails and a compressor is smashed up by liquid. There are horses for courses and in many situations today a correctly set up pump down system is a simple and cost effective way of ensuring that the compressor is protected from liquid and high starting torque.

By a good quality product maintain it well it will last a long time.You get what you pay for

[Gary]

May I suggest that properly designed compressors have run for many years trouble free and have pumped down hundreds and hundreds of times.

My experience is the same. That's why I say that the very worst that can be said of pumpdown systems is that they may (or may not) be less than ideal, depending upon your level of faith in the manufacturers operating envelope.

They are certainly not a bad strategy, nor even a sub-standard strategy. But less than ideal? Maybe.

[NoNickName]

I should add that if you don't agree with my radically new design then you must be stuck in the stone age.

Oh, no, I support you.

Blueboy: we were talking about a concept, not about a product. It just started as a personal experience, but I wanted to enlarge it to meta-level.
There are engineers and sub-standard engineers, nonetheless what we are discussing on this forum is best-effort pratice.

[US Iceman]

If we accept that the envelope describes the ideal conditions, then we can say that operation outside of the envelope (however brief), while not necessarily "wrong" is "less than ideal" and should be minimized.

I would agree with that.


In fact, I have been guilty of operating compressors outside their envelopes at very low condensing pressures. Manufacturer did not recommend it, but it worked just fine.

The envelopes are the generally recognized allowable limits. For instance; Copeland does not catalog compressors for cascade systems, but they still sell them! The end result is though, someone is taking responsibility for it.

I did on those compressor above. It's important to understand what the limits are and try to stay within that envelope unless you are willing to buy new compressors!