Superheat and subcooling.
Hello everyone,
My background is that I have NVQ level 2 in refrigeration and have been working as a service engineer for 12 years.
What is unusual is that I have only ever worked on environmental chambers. Never A/C never on refrigeration.
I work on R23 systems every day.
Other than touching on superheat and subcooling during college or FGAS, I never have to use them.
Diagnostics are different in my field. For example if the chamber has tripped it's low stage HP switch that means there's a problem with the high stage not working properly.
If the high stage compressor has frosted up, the low stage has lost all it's gas.
R23 is usually charged to a standing pressure and the high stage is charged to the sight glass.
I became a bit unsure of myself when I had to charge up a single stage R404a chamber...
Don't worry, I do know what I am doing it's just more common to work on cascade!
I cleared the sight glass and was looking for a box temp of -40'C but could get no lower than -32'C.
I just closed down the expansion valve to evaporate at a lower temperature and soon got -40'C.
What I'd like to know is what would be a more scientific way of doing what I did? what superheat/subcooling should I have been looking for?
It's all very well measuring these variables but unless you know what they should ideally be then the readings can be a bit meaningless.
cheers,
Cascademan.
Re: Superheat and subcooling.
.
Hi Cascademan.
As a rough rule of thumb we work to 4k to 7k subcooling and superheat,
when working on air cooled condensers.
For control and setting up we check the liquid pipe temp out of the condenser
and compare the measured temperature to the condensing temperature.
For subcooling we can only control the temp out of the condenser (without additional external subcooling).
If the airflows are good and without fan speed control normal subcooling is about 4k to 7k.
The suction superheat is again measured with a thermometer and compared to the evaporating
temperature, normal temps are about 4k to 7k, with normal forced air heat exchange.
Adjusting expansion valve settings is bad practice on an established working system,why would
a TEV need adjusting. Normal superheat cannot be achieved until the room is down to temp
because until the room is at correct temps and cycling, the valve will almost certainly
be fully open and therefor false readings will be given..
Sight glasses, receivers and existing systems would have a label indicating charge weight,
but if it did not, the receiver is a good indication of the total charge weight.
With a TEV and receiver the regulations require the receiver to be 10% larger than the total
charge weight, to allow for expansion and prevent liquid hydraulicing. The receiver at a glance
could indicate the maximum charge weight, but not all systems have the correct sized receiver
on them, because they are stock and tend to be oversized.
But at a glance if it had a 4kg, 5kg, 6kg, 10kg or whatever size receiver, you could be confident
it holds less refrigerant than that. In that case after repairing the leak, pressure testing and vaccing
out you put a few kg's of liquid refrigerant in and run the machine up. Then charge until the sightglass
is clear.
After the sightglass is clear, let it run and pull down the temperatures, only then can you set up
and check the superheats and subcooling. Normal forced air would be about 4k to 7k superheat and subcooling.
With the sightglass clear and with the thing at temp the superheat and subcooling should be about 4k to 7k.
Why would a valve on a system that was working ok need adjusting?
If it has failed it would need replacing but they do not lose calibration and they do not need to be adjusted
if they were ok.
How long had the system run before it had developed a leak? Had the valve been adjusted prior to the leak.
Regards
Rob
.
Re: Superheat and subcooling.
Hi,
Thanks for your reply. The expansion valve was new because I did a compressor change and the system previously had R69L with an R502 expansion valve.
There was no service history or charge weight on the machine.
The information you have given is helpful. I hope to have a more scientific approach next time.
At least I got it working. This was the beginning of 2015 and I haven't been back there so it must be OK.
Thanks again,
Cheers,
Cascade man.
Re: Superheat and subcooling.
Quote:
Originally Posted by
Cascademan
Hi,
Thanks for your reply. The expansion valve was new because I did a compressor change and the system previously had R69L with an R502 expansion valve.
There was no service history or charge weight on the machine.
The information you have given is helpful. I hope to have a more scientific approach next time.
At least I got it working. This was the beginning of 2015 and I haven't been back there so it must be OK.
Thanks again,
Cheers,
Cascade man.
In that case then, Yep I agree, you did the best thing.
New valves might need to be adjusted a little to achieve
optimum performance.
Sounds like you did what most of us would have done :)
Rob
.
Re: Superheat and subcooling.
Quote:
Originally Posted by
Cascademan
Diagnostics are different in my field. For example if the chamber has tripped it's low stage HP switch that means there's a problem with the high stage not working properly.
If the high stage compressor has frosted up, the low stage has lost all it's gas.
cheers,
Cascademan.
For trouble shooting purposes:
From the low stage's point of view, the high stage is it's condenser fan motor.
From the high stage's point of view, the low stage is it's evaporator fan motor.
Re: Superheat and subcooling.
Quote:
Originally Posted by
Cascademan
Diagnostics are different in my field. For example if the chamber has tripped it's low stage HP switch that means there's a problem with the high stage not working properly.
If the high stage compressor has frosted up, the low stage has lost all it's gas.
cheers,
Cascademan.
For trouble shooting purposes:
From the low stage's point of view, the high stage is it's condenser fan motor.
From the high stage's point of view, the low stage is it's evaporator fan motor.
Re: Superheat and subcooling.
TX's usually come from the manufacturer set at about 8°C superheat. This is a safe temperature and can usually be slipped in and turned on without risk of liquid floodback which is often what happens. A tech will slip it in, turn it on, fill out the paperwork and shoot through. However, 5 - 6°C of superheat realizes more evaporator power so the TX should be adjusted (tuned) for the more efficient operation. Anything below 5°C is a danger area for TX's as they can't respond quickly enough if liquid is threatening to fill up the evaporator and overflow to the compressor.
Re: Superheat and subcooling.
Superheat of TXV should be adjusted at commissioning or replacement of valve.
Superheat should be adjusted at about 65% of evaporator design TD when ambient temperature of cooling equipment is 1-2 K near design temperature.
http://maleki-hvac.ir/Portals/0/Kuba...ructions_2.pdf
Subcooling should be adjusted with refrigerant charge when ambient temperature is near design temperature. Condenser could be partially covered to get that design condensation temperature. Than when unit is at near inside design temperature and outside design temperature, charge should be adjusted to about 7K subcooling for R404A. Do not rely on sight-glass, since it sometimes could still have bubbles at that subcooling.
At -40°C ambient, evaporator (dynamic) TD is about 4-5K and superheat should be around 3K.
