Help needed with coolroom evaporator diagnosis please.

How do i use a temp probe and service gauges to diagnose the gas charge in a coolroom evaporator .
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If my room temp is plus 10 degrese and my thermometer probe in the coil is reading 0 degrees and my service gauges say 125 kpa or minus 27 degrees on R404 then what am i supposed to understand about the system . Is it under charged ?
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If my coolroom is running correctly and the room is at 1 degree then what should my thermometer in the coil read and what should my service gauges read?
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thankyou

Szarn

And here we go........

im an apprentice. im sorry this question is insulting to you so will go and ask on another site.

thanks anyway.
szarn

Hi szarn.
is your question from a test paper or have you been thrown in the deep end to sort out a problem?

Be a test or not, you can not consider only one part of a system to solve a problem

Its not a test. I am aware the pressure relates to temperature. I am aware that i need 6K superheat and 3 to 5K subcooling.*

I just seem to struggle some times when i have to attend a service call and i need to diagnose a coolroom for example.

I have seen people using probes in the evap coil and reading their service gauges and these people seem to diagnose in seconds what takes me an hour.

I had the feeling that i was missing something. I do not have much to do with airconditioning. I mainly do coolrooms and freezers and the odd domestic freezer for one only client.

Szarn

Your room is at + 10
Your coil is at 0
You are evaporating at -27
And you are running R404

That's all you can say about your system, the service gauges would also give you the condensing temperature but this wouldn't prove whether it's over or under charged either.

Thinking about the temperatures instead of pressure is a good idea but remember that the p-t relationship only applies to a saturated liquid / vapour mix, if its 100% liquid (sub cooled) or 100% vapor (superheated) then you need to measure the pipe temperature instead.

You mentioned sub-cooling and superheat, these readings will really help you figure out what's going on, maybe the people you've seen working have been calculating the superheat, this will tell you whether there's enough refrigerant in the evaporator and might help you work out if it's also undercharged.

You're nearly there, it's a process of elimination.

yes undercharged or the expansion valve is too small, or not set correct.

From these measurements we could only conclude that:
Evaporator approach is 0°C to 27°C = 27K (high)
Evaporator Δt is 10°C to 0°C = 10K (high)
TD is -27°C to 10°C = 37K (high)
Evaporator is at high load from high room temperature.
TD is to high, but we also could have MOP TXV or suction pressure regulator.
It gives to me indication of low air flow.
First you should check correct air flow direction and that fan s are rotating at correct speed and that air flow is not restricted by ice formation.

Evaporator probe could be placed where air is not flowing and therefore gives you false indication of temparature at that place.

Always first check conditions of heat exchangers, air flows and cleanliness and then proceed with measurements.

Check this.

Quote:

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Originally Posted by szarn

If my coolroom is running correctly and the room is at 1 degree then what should my thermometer in the coil read and what should my service gauges read?

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Refrigeration (evaporator) :

Tai-Tao = 3 to 5 K	Evaporator Δt
Tai-Te=5 to 10 K	Evaporator TD. take low TD for high relative humidity
Tao-Te=3 to 5 K	Evaporator approach

Tai	- air entering temperature
Tao	-air outlet temperature
Te	-Evaporating temperature
Twi	-water entering temperature
Two	-water outlet temperature
Tc	-condensing temperature

If cold-room temperature is 1°C then:
Coil sensor temperature should be 3-5K lower if its placement in coil represent coil air-out temperature.
Evaporation temperature should be 5-10K below evaporator air in (room) temperature.

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