Effect of high-side pressure drop on system

[desA]

A lot has been mentioned, on various threads, about the effects of low-side pressure drops.

For example:
1. TXV orifice size;
2. Distributor & capillary tubes;
3. Evaporator;
4. Suction line & fittings.

What effect does the high-side pressure drop (compressor discharge to TXV inlet) have on overall RHVAC system performance?

What are acceptable high-side pressure drops?

[chemi-cool]

Good point Des,

I allow 50 psi drop or I add high pressure control on the first condenser fan.

Other wise capacity drop, suction temp drop and frost start to appear on the fins.

[Magoo]

Hi DesA.
It comes down to degrees,[as in loses in discharge to TXV cycle ] and flow rates feeding the TX vav. TX vavs are designed on potential PD across vav, not rocket science. With out that constant PD the vav under feeds and compounds evap superheat control. Reason for loses is head separation loses+ pipe sizing loses. Varying operational conditions.
All comes back to my pet subject of TXV super heat and control of. TX vav selection and general system design balance for maximum efficient application. I seriously endorse MOP TX valves for system efficiencies.

magoo

[mad fridgie]

This brings us back to one of my earlier threads, I believe in many cases that we do indeed have a significant pressure drop (pd) on the high side. The pd can give the apperance that you have lots of liquid sub-cooling (if pressure measurements are taken a comp discharge, like most do) when if fact you have very little if the pressure is measured at the liquid line. Without know this fact you will tend to undersize your TXV, or you have to keep your head pressure somewhat higher than could be desired

[desA]

Thanks very much for the comments : chemi-cool, magoo, MF.

Let's expand this a little further.

When we talk about high-side pressure drop, we have basic design rules for pipe sizing that allow enough velocity to carry oil, but, are large-enough to keep line pressure loss within reasonable limits (ASHRAE & others).

These pressure drop limits are often given in temperature equivalents eg. 1-2K, for a particular line. This will mean different actual pressure drops for different refrigerants.

The condenser is often specified with a certain maximum design pressure drop.

The problem then comes when the system is connected together. Piping design eg. elbows, fittings, valves, filter-driers, sight-glasses, equipment connections, then all add in a pressure drop. Very often this pressure drop is way in excess of what was expected at design stage.

A few more questions:
1. What is the effect of uncondensed vapour carry-over from the condenser?
2. Does condenser sub-cooling guarantee a vapour-free liquid line?
3. Does liquid-line sub-cooling guarantee a vapour-free liquid line?
4. Can vapour carry-over exist yet the TXV still operate stably?
5. What is the effect on vapour-carryover with rising high-side pressure?
6. What is the effect on high-side pressure-drop, with rising high-side pressure?

[desA]

Thanks very much chemi-cool.


I allow 50 psi drop or I add high pressure control on the first condenser fan.

Can I ask where the 50 psi (3.45 bar(a) = 0.345 MPa(a) ) is measured? From compressor discharge, to TXV inlet?



Other wise capacity drop, suction temp drop and frost start to appear on the fins.

Ok. So you're guarding against system performance drop-off, as seen in refrigerant mass-flow reduction. Very good observations, thank you for this.

[desA]

I allow 50 psi drop or I add high pressure control on the first condenser fan.


Do you see the condenser pressure drop as being the main factor here?

[desA]

Hi DesA.
It comes down to degrees,[as in loses in discharge to TXV cycle ] and flow rates feeding the TX vav. TX vavs are designed on potential PD across vav, not rocket science. With out that constant PD the vav under feeds and compounds evap superheat control. Reason for loses is head separation loses+ pipe sizing loses. Varying operational conditions.
All comes back to my pet subject of TXV super heat and control of. TX vav selection and general system design balance for maximum efficient application. I seriously endorse MOP TX valves for system efficiencies.

magoo

Now, to take this point a few steps further.

For a well-balanced system, what would be reasonable % of total (1-7) for the following sections of a standard rhvac cycle, based on 'degrees' temp drop, kpa (psi):
1. Discharge line;
2. Condenser;
3. Liquid line (to TXV,inlet);
4. Across TXV (Txv,in-Txv,out);
5. 2-phase line (TXV,out, across distributor);
6. Evaporator core;
7. Suction line.

[desA]

This brings us back to one of my earlier threads, I believe in many cases that we do indeed have a significant pressure drop (pd) on the high side. The pd can give the apperance that you have lots of liquid sub-cooling (if pressure measurements are taken a comp discharge, like most do) when if fact you have very little if the pressure is measured at the liquid line. Without know this fact you will tend to undersize your TXV, or you have to keep your head pressure somewhat higher than could be desired

This is a very valid point, that is not well published at all, in the literature.

I do remember your thread some months back. Good point.

[NoNickName]

There is a white paper research published by a korean PhD (can't remember her name) who demonstrated that 46% of the pd in the high side is caused by the shut off valve.
We have compressors which can only fit in the ECA scheme if supplied without the high side shut off valve.

At the same time, a common mistake of installers is to regulate the fan speed by a pressure plug on the liquid line. The high pressure shall be read on the same high side shut off valve, to avoid HP trips at low fan revs.


EDIT: additionally, when comparing compressor performance and COP, reading the high and low pressure on the common headers instead than on the compressor, give sensibly different results, and affects COP evaluation.

[desA]

Many thanks NNN


There is a white paper research published by a korean PhD (can't remember her name) who demonstrated that 46% of the pd in the high side is caused by the shut off valve.
We have compressors which can only fit in the ECA scheme if supplied without the high side shut off valve.

This, I can quite believe.


At the same time, a common mistake of installers is to regulate the fan speed by a pressure plug on the liquid line. The high pressure shall be read on the same high side shut off valve, to avoid HP trips at low fan revs.

Very interesting point.



additionally, when comparing compressor performance and COP, reading the high and low pressure on the common headers instead than on the compressor, give sensibly different results, and affects COP evaluation.

This is very, very true. Good point.

[chemi-cool]

Its getting interesting,

I am not a design engineer and my best thermostat is my bare hands. I balance and tune most systems without any instruments, experience and patient I guess.

High pressure reading is always at the shot off valve.

Copper tubes behave different if there are gas bubbles in the liquid so its quite easy to feel it just before the TEV.

COP is changing all the time, every °C ambient or around the evaporator, dirt in the coils, changing fans to fans with different wing angle or speed will afect it so I leave it to the people using pencils.;)

[desA]

Copper tubes behave different if there are gas bubbles in the liquid so its quite easy to feel it just before the TEV.


I'm intrigued.

What do you look for in this situation?

[chemi-cool]

When the tube is full of flowing liquid, it feels stable and hardly have any vibrations.

When there are vapour bubbles in the tube, they change their size all the time. This is caused from liquid flowing, there is always some sort of turbulence inside the tube, [I have seen it once in a glass tube]

This is making tiny vibrations in the tube and when you know what it feels like, you can easily detect it.

Some times, when liquid line is long and you are charging a system, it actually visible.

[desA]

^ Thanks c-c.

This makes perfect sense & I understand what you are looking for. I've seen a similar effect by watching the pressure gauge hose off the liquid line, when there are vapour bubbles present in the liquid line. The hose can be seen to move at times, & can be felt per hand.

Have you noticed a different response from different types of condensers e.g. PHE, tube-coil?