Superheat?

[cretan]

I have no problem checking superheat on a WI cooler but,I am having trouble understanding how to check superheat on a low temp unit.Can someone tell me how to do this properly?

[nike123]

What is WI cooler?

[cretan]

It is short for walk-in cooler.tried to save a couple key strokes:)

[nike123]

When you say low temperature unit do you think on walk in coldroom with temperature below -18.
Good piping practice require fitting a schrader valve near the TXV bulb at evaporator connections to measure saturation pressure. That is place where you need to take pressure and pipe temperature measurements for calculating superheat.
Could you explain what is your problem with measuring?

[cretan]

Yes,I meant a walk-in freezer unit.With a walk-in cooler I take the suction pressure and temp and am able to figure out superheat.With a freezer the gauge readings are in the negative and that is where I get mixed up.Can't figure how to use the P/T chart with a freezer.Can you explain to me the ways of properly checking superheat?

[nike123]

Yes,I meant a walk-in freezer unit.With a walk-in cooler I take the suction pressure and temp and am able to figure out superheat.With a freezer the gauge readings are in the negative and that is where I get mixed up.Can't figure how to use the P/T chart with a freezer.Can you explain to me the ways of properly checking superheat?

Depending on P/T chart, you could have pressures in, say, Bars gauge or Bars absolute (or PSi gauge or PSI absolute). If you have absolute pressure/temperature chart, you need to convert reading of your gauge to absolute pressure.
For example: Your gauge reads -0,3 Barg (-4,35 PSIg). Notice that gauge pressure should have suffix g at end of unit. Sometimes, for absolute pressures, suffix a is used at the end of unit.
That converted to absolute pressure is 0,7 Bar (10,15 PSI), and if your P/T chart is for absolute pressure, that is your pressure for finding corresponded saturation temperature.
If your P/T chart doesn't have negative pressures then it is absolute pressure in that chart.
In fact, negative pressure doesn't exist. It is convention that pressure below atmospherics is negative.
So, if you are using P/T chart with gauge pressures, you dont need to convert anything, just find on chart corresponding temperature to your gauge reading (no matter negative or pozitive, all are there), or if you are using P/T chart with absolute pressures, then you need to convert your gauge pressures to absolute, regardless of their negative or positive pressures at gauge.
Example:
-1 Barg = 0 Bar
7 Barg = 8 Bar
-5 PSIg = 9,5 PSI
0 PSIg = 14,5 PSI
232 PSIg = 246,5 PSI

[cretan]

Probably a stupid question but.......At 32 psi I my gauges show 0 on the 502 range.If psi on the gauge is below 32,is the reading on the 502 range in the negative?

[wambat]

32 is the temperature and "0" is the guage pressure of 502 :rolleyes:

[SteinarN]

Probably a stupid question but.......At 32 psi I my gauges show 0 on the 502 range.If psi on the gauge is below 32,is the reading on the 502 range in the negative?

Yes. At 32 psi you have a saturation temperature of 0*F. At lower pressure you have a "negative" temperature, at 10psi you have a temperature of -29*F.

[cretan]

I know I sound like a novice but superheat on a freezer is the only thing that I can't figure out.I am ok with a walkin cooler since the sat temps are positive.How would I convert temp and pressures to find the super heat on a freezer?Is it best done by the compressor or the evaporator?

[US Iceman]

How would I convert temp and pressures to find the super heat on a freezer?


The same way you do on anything else. If the negative temperatures are throwing you for a loop... don't let them.

When you subtract a negative number it is the same as adding.

Example:
0 minus -10 = 10 degrees

Since zero degrees is warmer than -10, the temperature difference is 10 degrees.

You want to measure for the superheat at the evaporator. That is where it is being controlled.

The temperature conversions are the same above zero as you would do below zero.

[cretan]

So if I have suction line temp at compressor of 23f and suction pressure of 21psi,which converts to -14f my superheat is 37.Is this correct?

[SteinarN]

So if I have suction line temp at compressor of 23f and suction pressure of 21psi,which converts to -14f my superheat is 37.Is this correct?

Yep, that's correct. But that is your compressor superheat. To get your evaporator superheat you must take the temperature at evaporator outlet.

[nike123]

Yep, that's correct. But that is your compressor superheat. To get your evaporator superheat you must take the temperature at evaporator outlet.

And pressure too, if lines are long enough to make pressure drop of significance to measurement! ;)

Or, if lines length affecting pressure measurement , calculate pressure drop in lines, measure temperature at evaporator outlet, add line pressure drop to pressure measured at compressor and convert that to saturation temperature. That temperature subtracted from measured temperature at evaporator outlet is evaporator (useful) superheat.:cool:

[cretan]

So using the suction pressure at the compressor,take the pipe temperature at the evaporator outlet and convert the same way for evaporator superheat?What is the correct range for evaporator superheat?

[SteinarN]

5 to 8*C is normal evaporator superheat. Compressor superheat will nomally be slightly higher due to heat flow into the suction line between compressor and evaporator. If you have a suction line heat exchanger, then the compressor superheat will be much higher, maybe 30 to 40*C on a freezer.

Be aware of the temperature measuring error when measuring evaporator outlet temperature. It can be hard to get a correct reading dependent on your equipment. You still want to make an overall asesment of the operation of the system, like evaporation temperature compared to freezer air temperature, no sign of liquid flodback to the compressor and so on.

[scruffy]

Cretan,

I think your getting stuck on the minus minus bit of the equation. Hopefully seeing the equation will help you.

Superheat equals the difference between the TX valve bulb temperature and the saturated suction pressure (at the evap) converted to a temperature.
Superheat = Bulb temp - SST.

Eg1. Bulb Temp = 1oc
SST = -5oc
Superheat = 6 oc.
Therefore SH= 1oc --5oc = 6oc.

Mathematically minus minus equals a plus.(- - = +)

Eg2. Bulb Temp = -24oc
SST = -30oc
Superheat = 6oc
Therefore SH = -24oc --30oc = 6oc.

Remember a minus minus equals a plus!

[nike123]

Therefore SH = -24oc --30oc = 6oc.

Remember a minus minus equals a plus!

Scruffy, litlle help for ° symbol.
Pres and hold Alt Gr key and in same time hit 5, then you simply type C, or hit space bar if you want only symbol.
It is litle confusing doing mathematics with those o after numbers. ;)

[US Iceman]

And you also want to make sure the evaporator coils are clean; no ice, dirt, the fans are running, etc. Let the system run for a while to stabilize, then check the temperatures and pressures.

If you cannot measure the pressure at the evaporator outlet, then you can do as nike123 suggests or set the superheat by using the compressor suction superheat. 37° is a lot of superheat, but probably in the range of what I might suspect for a supermarket (again long lines and higher pressure drops).

If you can adjust the valve to reach a target of 15° (F scale) superheat at the compressor the superheat should be fairly close to what is needed at the evaporator.

[Susanthaj]

Hi.
I have an urgent question and I do not know how to post it as a question.

I would like to have a chart for LRA to RLA.

[nike123]

Hi.
I have an urgent question and I do not know how to post it as a question.

I would like to have a chart for LRA to RLA.


For what?:confused::confused:

Go there http://www.refrigeration-engineer.com/forums/newthread.php?do=newthread&f=5
and ask question with lot more details then this two short acronyms.

[cretan]

I finally understand superheat for a freezer thanks to you guy's.It was one thing that i was struggling with.So as long as superheat is 15-20 at compressor no need to worry about floodback?If not how do I check for floodback?

[SteinarN]

I finally understand superheat for a freezer thanks to you guy's.It was one thing that i was struggling with.So as long as superheat is 15-20 at compressor no need to worry about floodback?If not how do I check for floodback?

On a hermetic compressor, if the shell gets cold or even frosty around the suction inlet, that is a sign of liquid floodback. On a semi hermetic the same applies at the compressor body next to the suction inlet. However this is not a precise diagnosis. A very small amount of liquid floodback isn't always easy to detect in other way than lack of superheat. You will of course not have any superheat at the compressor inlet if you have liquid floodback.

Edit:
On a freezer with, lets say -25°C suction, the area near the suction can be cold or frosty without liquid floodback, especially on a semihermetic. But the rest of the compressor should be warm, and the discharge shall have substantial superheat. The best is to measure suction superheat. It shall normally be at least as much as the evaporator superheat. If you have a noticeable suction superheat, then you have no liquid floodback.

[US Iceman]

On a freezer with, lets say -25°C suction, the area near the suction can be cold or frosty without liquid floodback, especially on a semihermetic.


I agree. This is an important concept to remember. Just because the compressor is sweating or has some frost on it, does not mean liquid is entering the compressor. It only shows the gas is cold!:cool:

[cretan]

So on a walk-in freezer and walk-in cooler running on one compressor with a evaporator pressure regulator in the freezer,superheat is adjusted at the evaporators only?If so 8-10 degrees superheat also in this situation?

[scruffy]

Thanks Nike123

[US Iceman]

So on a walk-in freezer and walk-in cooler running on one compressor with a evaporator pressure regulator in the freezer,superheat is adjusted at the evaporators only?


That is a rather unusual arrangement. When one evaporator shuts off the suction pressure must decrease to a very low pressure, unless the compressor has capacity reduction. It sounds like a brute force type of system which would cost more to operate.

But yes, the evaporator superheat on this type of system should be set for 8-10 degrees of superheat for each evaporator.

Warning: you want to adjust the superheat when both evaporators cooling and the system is stable. If one evaporator is off or just recently started the system is trying to stabilize. Adjusting TXV's with the suction pressure with an unstable operation can contribute to the valve hunting.

When you make an adjustment it can take at least 20-30 minutes for the valve to stabilize, so don't plan on doing this quickly.

[cretan]

That is a rather unusual arrangementYes it is.It has a hermetic compressor,L.L solenoids controlled by thermostats in both the cooler and freezer.pumpdown.404 gas.compressor runs constantly.I adjusted the low pressure control so the system would shut down when thermostats were satisfied,then the system would not start again when the stats were calling.Problem was the low side pressure would not increase to tell the pressure control to power-up the system.So I ended up putting the settings back the way they were.One thing I thought of doing was putting a check valve on the high side,close to the compressor to keep pressure in the line when system is off.Then when the L.L solenoid opens there is pressure at the txv,which should raise suction pressure and turn on the system.The only thing I am worried about is when it is in defrost and the txv opens and the refrigerant in the line enters the evaporator.Don't know if it would overload the compressor at start-up.