View Full Version : Very low evaporator pressure

17-01-2006, 11:54 PM
I'm looking for information on the problems associated with running a suction pressure down to zero or even in a vacuum to achieve the desired temperature. Example; A small medium temp (35-41*f) self select type display case with 1/2hp 5800 btu 134A condensing unit. 1/4 ton TEV blower coil. My boss has continued to have me lower the controling "cut out" pressure to get the case to the acceptable temperature of 35*f, which sometimes was 0 psi gauge. I am sure this is not the answer to the problem and feel I need more facts to present my case for mods and a product I can feel confident in.

If you have already given me clues to this end in another thread,I appologize and thank you once again.


18-01-2006, 12:54 AM
35F is close to 0C, 0psi gauge is coresponding to -28C satuaration temp for r134a. i think the evap temp can be a little higher to keep the evap pressure above atmosphere. Marc's recommendation is practical or you can match a bigger evaporator if necesary.


US Iceman
18-01-2006, 01:27 AM
Hi Ken,

I have attached a pressure-temperature curve for you to use on this problem.

If you are trying to keep a 35F air temperature in the display case, the evaporating temperature should be about 20-25F, depending on how the evaporator was designed. At 20F, the suction pressure at the compressor should be about 18 psig. AT 25F, the suction pressure should be about 23 psig.


...1/2hp 5800 btu 134A condensing unit. 1/4 ton TEV...

If the compressor has a capacity of 5800 BTU/hour, the TXV should have a capacity roughly equal to this (based on the liquid feed pressure to the TXV and the design evaporating pressure). The evaporator should also have a capacity equal to 5800 BTU/hour.

I doubt the 1/4 Ton TXV has sufficient capacity to maintain the 5800 BTU/hour capacity of the compressor. IN this case, the suction pressure would have to be ran lower to produce 35F.

If you look on the chart 0 psig is equal to about -14F for the evaporating pressure. This is way too low. Going into a vacuum is even worse.

18-01-2006, 07:17 AM
Waw Ken, even help at night:cool:

Setting a LP safety switch below 0 psi isn't the cure for the problem.
If you set it that low, you could also remove it, why is it otherwise there?
It's installed there specially and solely preventing air and moisture entering the system.
Ammonia systems goes sometimes lower but those have special air purgers.

And look once what capacity you gain in that low working pressure area with R134a. Almost nothing.

This is like others say a wrong design. Too small evaporator and/or compressor if superheat is OK.

I fear both are too small or heat losses of the cabinet are too big for the installed unit.
Could be that the design of the cabinet is wrong.
Do you have the possibilty to look once with an infrared camera for heat losses?
We once did it to compare heat losses for a Koxka with a Linde.

Increasing fan speed helps almost nothing.
We once installed on a evaporator 4 pieces 6 blades, high twist of the blades instead of the originally 4 blades low twist (almost 70 % more air flow)
It increased the evaporating temperature with 1 to 2 K

Renato RR
18-01-2006, 08:46 AM
TEV is to smol (check the superheat).
1/4 ton = 12000/4 =3000BTU
Compressor 5800BTU.
Is it posible to lower condensing temp?


18-01-2006, 10:31 PM
You're right Marc.
What's the face velocity over the evaporator? Must be something around 2 m/s

19-01-2006, 10:11 PM
Thanks... Let's begin with Marc,

Maximise coil wetting by reducing the superheat without creating valve hunt.

Is "valve hunting" when I see the gauge needle rise and fall while watching to set the cut out presure? Am I understanding that a steady start to finish cycle ( a steadily falling gauge ) is indicative of an efficient and properly feeding valve?

Not sure how to do those Quotes either

20-01-2006, 09:01 AM
Is running a system in a vacuum really a problem?

US Iceman
20-01-2006, 08:07 PM

It is common to find low temperature ammonia systems operating in a vacuum. Almost any other system it is very uncommon.

The reason is the refrigerants will have a positive evaporating pressure when operating in low temperatures. Ammonia is one of those that operates in a vacuum at very low temps.

This is also one reason why cascade systems can reach extremely low temperatures with a positive suction pressure. The refrigerants used in cascade systems are selected to provide a positive evaporating pressure at the very low temperatures.

21-01-2006, 12:36 PM
Is running a system in a vacuum really a problem?
No, as long as you have no leaks.

21-01-2006, 01:24 PM
Can't purge valves be fitted to systems running negative pressures? And if so please elaborate.

21-01-2006, 02:13 PM
Hi Dogam...at home:p
How will you make the diffrence between air leaving the valve and refrigerant? You may not let escape refrigerant out of a system.
And when will you need to purge, especially if there's no leak in the system?

21-01-2006, 02:35 PM
i....don't know. I just over heard someone speaking about an air purge on negative pressure systems. Something about refrigerant being heavier than air and since a positive pressure was needed for the valve to open.............................?????:)

21-01-2006, 03:11 PM
If your system is below atmposheric pressure, then both refrigerant and air are below atmospheric pressure.

An air purger takes some mixture refr/air from a place where you can expect the most air in the system and re-condens it so that the non-condensibles remain in a storage cilinder with float (mechanically or electroncially) which are then purged to the air.
I only saw it on a NH3 systems.

22-01-2006, 01:38 AM
ahh true? Can this be applied to forane systems? Or is the threat of loosing small amts of refrigerant to atmos enough to have it regulated out by govs?

22-01-2006, 01:58 PM
Thanks for the Danfoss site lots of good stuff there. It would almost seem that by adjusting the valve to a non-hunting position the SH would be just about right.
Go Pittsburg, Ken

22-01-2006, 02:32 PM
Peter 1,

What's the face velocity over the evaporator? Must be something around 2 m/s
In laymans terms what is m/s. Is this a standard which can be applied to all or most situations. Go Pittsburg, Thanks , Ken

Johnny Rod
23-01-2006, 12:40 PM
The place for a non-condensible purger is on the high side (on or before the condensor) so pressures would be positive, regardless of evap temp/pressures surely? These are fitted to some fluorocarbon systems but in my limited experience they tend to be no longer functioning or have even been removed, couldn't give you a good reason why. These jobbies are refrigerated as Peter describes so that loss of refrigerant is minimal.