Where to sense pressure? Liquid or gas??

Ok I'm working on my heat pump project and it has come time to attach the Head pressure modulated water flow valve. The question where do I take my pressure measurement? I think liquid would be the best as it gives me a true differential, but gas gives a rapid response, but I have pressure surges and dips where with liquid I'd have a much smoother curve.

Same with the HP cut out, Liquid or gas.

I'm thinking Liquid for my self, but now will there be any issue with the diaphragm flooded? I don't think it would as hydraulic pressure transmits very good! So either way I get excellent pressure measurement.

What do you all think.

The discharge goes directly from the compressor to the HX via a very short rout so little is likely to plug or go wrong in it providing that as it is Liquid seems to be better for smooth curves in pressure and true liquid head.

We normally sense both HP switch and pressure control on the liquid line, to prevent compressor pulsations and have a HP reading slightly lower (would let me gain 0.5 barg on the HP switch).

Ok cool, I was most certainly thinking the liquid head would be the best point, but just wanted some re-assurances :)

Well , concerning HP protection, I should say, allways on the compressor.
When connected to the liquid, if you shut off the discharge valve or there becomes an abstruction in it or in the discharge line of the condensor, then the HP will not sense this. Same for LP connectio: on the compressor.
Seen it already several times that the sieve at the entrance of the LP service valve becomes clogged. If the LP is then connected on the 1/4 of teh service valve, then the LP cut-out doesn't see that the compressor is running in vacuum. If it's connected to the coimpressor itself, then the compressor is protected.

Regarding the pulsations of th HP switch: if you look inside the 1/4 connection of almost any HP switch, then you will see some sort of orifice or restriction in it. This is to flatten out the HP pulsations which will occur. So the constructor has foreseen this pulsations and has inserted this in-build safety for it.

This is a Hermetic rotary system, there is no access to the valves, & it directly connects to the HX.

I'm not sure if I'll be using a receiver or not, if I do I'll mount the HP cut out to the receiver and for the head modulated flow valve to the Liquid line.

My vote is the same as Peter. I always use the compressor fittings and dislike liquid connections.

Cap tube connections are notorious for leaks and if connected to a high pressure liquid line you have the chance to loose the refrigerant quickly.

On this system there are no compressor fittings!

As for the sensing ports I'll be using Ts and reducers & the final port will be brazed in with a length of factory installed 1/4 stub.

The I would suggest a location close to the compressor if the system is that small. Run some small tubing out from the suction and discharge line (for the Schrader valves) to a convenient location that is accessible.

Our mileage may vary, and of course I respect all of the possible choices. Still, I think that liquid lines are less prone to cause faults to switches and transducers, thanks to lower pulsations and hits, and more stable readings.

I think I'm set on the Liquid line for the head flow control valve, as this way I get true head pressure the TXV will see. I may choose to put the HP cut out at the discharge area, but assessing any of the possible risks due to the eXtreme shortness of the discharge to condenser is so short it makes more sense to place it along side the flow valves pick up.

For charging perposes I will be puting out 1/4 runners to the side of the case.

Quote:

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Originally Posted by The MG Pony

I think I'm set on the Liquid line for the head flow control valve, as this way I get true head pressure the TXV will see. I may choose to put the HP cut out at the discharge area, but assessing any of the possible risks due to the eXtreme shortness of the discharge to condenser is so short it makes more sense to place it along side the flow valves pick up.

For charging perposes I will be puting out 1/4 runners to the side of the case.

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Gentlemen, it doesn't matter whether the sensor for the pressure operated water valve is taken from a gas or liquid port. THERE ARE ONLY TWO PRESSURES IN A REFRIGERATION SYSTEM. Suction and discharge. The Liquid line is almost the same as the Discharge if pipes are sized reasonably well. The difference is in the enthalpy and temperature. there are some variations from pressure drops but these are not great. The important thing is to arrange so that they cannot be shut off easily.

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Originally Posted by fippo

G... is taken from a gas or liquid port. THERE ARE ONLY TWO PRESSURES IN A REFRIGERATION SYSTEM. Suction and discharge. The Liquid line is almost the same as the Discharge if pipes are sized reasonably well.

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But LP is not always the same as sump pressure and a pressure switch is there to protect the compressor, so on the compressor.
If you read back my answer, then you can see that there's a difference.
Why otherwise the manufacturer is making special connections on the compressor itself?

A pressure (but also a temperature) has to be measured at the place where it is needed, not where it suits one best (or is the easiest to mount).

A HP safety cut-out switch has to be connected on the HP side of the compressor. (because it's purpose is the cut out the compressor when the compressor produces a pressure higher then a preset value).

I agree that in normal operation conditions the discharge pressure will be at (about ) the same level as the (HP) liquid pressure. But a safety switch should cut out the compressor in 'abnormal' operating conditions ... someone forgetting to open the discharge valve of the compressor, ... a check valve not reacting quick enough because it is full of condensed liquid or oil ...

A HP switch (or condensor fan speed controller) for controling a minimum pressure towards the TXV should be connected on the liquid line ... for the same reason.

I posted on
http://www.refrigeration-engineer.co...194#post133194 a new thread about pressure testing

Quote:

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Originally Posted by fippo

THERE ARE ONLY TWO PRESSURES IN A REFRIGERATION SYSTEM. Suction and discharge.

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This is one of the wrongest assumptions I've ever heard. In fact, that's exactly the opposite. I.e., in a common discharge header, measuring a discharge pressure before check valve and service valve or after those, would considerably differ in pressure AND in the corresponding temperature.
Secondly, in any good refrigeration circuit calculation software, all dp's are accounted for and have to be specified for a correct calculation
Finally, if your assumption would by any mean be right, then why bother buying TXV with external equalisation?

i would be putting it on the high side of the system before any device, after the txv is no good as you could have a fault TXV running at high pressure and the system will not stop in time.
if your 4 way valve does not move all the way across, same thing you will get a build up of pressure that will not stop the compressor.
depending if your refrigerant is pre expanded, if this is the case when your EXV fails closed you will have a low pressure in your liquid line.
this will depend if you are expanding at your fan coil or at your condensing unit.
I would use the HP on the high side to ensure it works when it is supposed to.

Hi, you can get differential pressure volumes if you use refrigerant with temp glide( for example R407C). If you want measure, you should take data from liquid line

For subcooling measure you have take data from liquid line(for R407C)

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Originally Posted by Darmas

For subcooling measure you have take data from liquid line(for R407C)

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When you measuring subcooling you use saturated liquid temperature on P/T chart and when measuring superheat you use saturated gas temperature on P/T chart.
Pressures you take where you have access points. Closer to temperature measuring point is better or you must consider pipe (and other elements) pressure drop from measuring points..