Superheat

[cmac]

Hey folks

I know this has been covered alot but,

I was on a job today and there were schraeder connections at the evaporator inlet and outlet.

How accurate would it be to convert the pressure readings at both points for calculating superheat?

[nike123]

You should use port at evaporator outlet.
On inlet (if port is betwen TEV and distributor you have some pressure loss at distributor and evaporator piping) you have false reading.
If you have short piping, compressor suction is also acceptable!

[US Iceman]

I would not use both the inlet and outlet pressures, only the outlet pressure. If you use both pressures the pressure loss of the evaporator could be picked up as superheat, which could lead you to set the valve improperly.

[Argus]

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I agree with Iceman.


There is no point using the inlet test point for working out the superheat, you'll get all sorts of queer reading due to its closeness to the TXV, or what ever you have in place..

You can get a very accurate profile of the SH during running if you observe and record the simultaneous pressures and temperatures at the exit point - it's worth doing it over a period of time if you can, because the combination of TXV action, coil efficiency and cooling load is not static.

However, the pressure point at the inlet is not entirely useless. In combination with pressures taken at the exit point, it can give you a good idea of the evaporator pressure drop, if that is significant, because that can effect the efficiency of the TXV and ultimately your SH setting. It's all closely linked.

While you are at it, you can also record any significant suction line losses, with pressures taken at the unit. Suction losses are often overlooked.

One point that occurs to me is the issue of Schraeder valves in such a location on the evaporator that is constantly exposed to low temperature and probably frost. They are notoriously prone to leaking and it's very easy to get moisture in the system by attaching gauges to a wet connection.



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[cmac]

Okay thanks.
Was just wondering what the schraeder at the inlet would be used for, as they were factory fitted and I had never seen on other units before

[al]

Hi Cmac

The inlet could be used to charge liquid to the system?

al

[Argus]

Okay thanks.
Was just wondering what the schraeder at the inlet would be used for, as they were factory fitted and I had never seen on other units before


Yes. It's unusual to see a gauge point in that spot. It's also unusual to see one on the exit, but they are useful for setting the TXV, you normally have to get the pressures at the suction service valve and make allowances for the suction line pressure losses.




Hi Cmac

The inlet could be used to charge liquid to the system?

al


You can do that at the liquid line service valve.



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[nike123]

It's also unusual to see one on the exit, but they are useful for setting the TXV, you normally have to get the pressures at the suction service valve and make allowances for the suction line pressure losses.

It is not unusual in multiple evaporator/TXV systems.

[NoNickName]

It's used to measure the pressure drop across the evaporator and diagnose clog.

[cmac]

It's used to measure the pressure drop across the evaporator and diagnose clog.

Ok what would the expected pressure drop be if everything was working correctly. It was on a water chiller running on R407C

[NoNickName]

I have no idea. You shall ask the evaporator manufacturer. It very much depends on the inner geometry, which is unknown to me.

[icemakerman]

[quote=Argus;128599]Yes. It's unusual to see a gauge point in that spot. It's also unusual to see one on the exit, but they are useful for setting the TXV, you normally have to get the pressures at the suction service valve and make allowances for the suction line pressure losses.


I recently had system with subcooler (heat exchanger at outlet of evaporator) on 404a the increase on suction pressure from evap to compressor was 60KPa (aprox. 9PSI) found charge port at evap outlet was only way to set superheat

[wilsoncheung]

Hey folks

I know this has been covered alot but,

I was on a job today and there were schraeder connections at the evaporator inlet and outlet.

How accurate would it be to convert the pressure readings at both points for calculating superheat?

__________

schraeder? What's that?

[nike123]

__________

schraeder? What's that?

Access needle valve for connection of gauge manifolds etc.

[icecube51]

Yo Nike123,

was this an easy or dumb question ???

(lol)
Ice

[nike123]

Yo Nike123,

was this an easy or dumb question ???

(lol)
Ice

Easy one! I would also need to add one of signatures which have Google in it self! ;)

[Scramjetman]

Taking the pressure reading at the compressor can give you a false reading as pressure drop in the suction line distorts the actual value of the evaporator temperature and therefore the superheat.

Superheat really needs to be calculated at the evaporator outlet because this is where we are measuring the condition of the superheated vapour. Any other point in the system is subject to pressure differences and delivers distorted results.

[nike123]

Taking the pressure reading at the compressor can give you a false reading as pressure drop in the suction line distorts the actual value of the evaporator temperature and therefore the superheat.

Superheat really needs to be calculated at the evaporator outlet because this is where we are measuring the condition of the superheated vapour. Any other point in the system is subject to pressure differences and delivers distorted results.

Pressure drop from evaporator to compressor could be accurately calculated.

[gitster a/c]

Normally the extra screider is on to charge up R407c on evaps to allow some evaporation of the liquid . Best usage if weighing in correct charge but if using superheat if could affect the temp at the bulb and shut down tev and show full sightglass you then go away and it settles down and is short of gas. also it can be used for pressure drop . its mainly found in my experience on close control dx.

Gitster

[Argus]

Normally the extra screider is on to charge up R407c on evaps to allow some evaporation of the liquid . Best usage if weighing in correct charge but if using superheat if could affect the temp at the bulb and shut down tev and show full sightglass you then go away and it settles down and is short of gas. also it can be used for pressure drop . its mainly found in my experience on close control dx.

Gitster



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