imi expansion valve problem

[fridgemandan]

Hi there all, i was given a job to replace a small copeland scroll on an imi mcu50 condenser, after replacing and carrying out relevant pressure tests i started to gas the system but noticed that the pressures were no where near what i would be expecting on r22, after checking the system i discovered that there was no expansion valve on the evaporator and after investigating i found no expnasion valve anywhere on the system.

this system is ten years old and looks like it has never had one fitted, also the fan speed controller and sensers had been removed. After speaking to imi there told me that an expansion valve should be fitted just after the service port on the condenser, is it just me or is this a strange place to locate the valve. please let me know your views on this.

from a very puzzled brummie

sorry its on an above ground cellar system :confused:

[richardb14]

well, its strange right enough but they do work well, its a mechanical in line fitting that has a strainer and an injector fitted to it, unless you know what you are looking for it can be pretty hard to spot.

I have also came across this fault, I fitted a new compressor to a system that was 'inefficient', the pressures were also not quite right - the customer had 6 compressors fitted to it over the last ten years and all with the same fault.

there was NO expansion valve fitted to it . . . .

[nike123]

Obviously, you have replaced compressor but cause of dead of old one is remained.
System has to have some expansion device, that doesn't necessary need to be TEV. It could be also capillary tube or orifice.

[Andy AC]

It's not an expansion valve, it has a fixed orifice which is within the extra bit of brass directly after the liquid shut off valve. Does the same as a cap tube, which on your system gives you an expansion line, just like a standard split air con.
Did you make sure the coils were clean? The evaps on those old things get extremely dirty inside, which would give lower suction pressure and freezing up.

Andy

[coolbone69]

:rolleyes:IMI!!!!! 10 yr old you say,box of matches and sum 4star.ha

[Gary]

Hi there all, i was given a job to replace a small copeland scroll on an imi mcu50 condenser, after replacing and carrying out relevant pressure tests i started to gas the system but noticed that the pressures were no where near what i would be expecting on r22...


That's because refrigeration is not about pressures... it's about temperatures. The reason we use gauges is to find out the temperatures of the refrigerant.

You say the pressures are not what you expected, but you failed to tell us what those pressures are. If you had told us the pressures, we could get out our P/T charts and find out what the temperatures are... because that's what is relevant here.

You seem to have already decided that the expansion device is at fault, which is in fact the least likely fault. On what criteria do you base your decision?

[monkey spanners]

Also if the fan speed controller is faulty it needs replacing as without it you will have a low head pressure and hence a low suction pressure.

Jon

[Lincoln]

I have found that with this type of unit charge until you achieve 10 deg across evap approx and a reasonable suction superheat the compressor will be fine.

[Willem V]

thats correct, it's a so called accurator expansion device, located in de liquid line just outside the unit (and with a strainer in it !!), Carrier used the same in the pasted, mostly on heat pumps because of the bi flow possibility

[fridgemandan]

i have based this on speaking to imi technical who have given me the information on how the system whould have been installed. fan speed controllers were fitted as standard but apparently there has been a long running problem with installers not fitted components to the system as they have specified. All coils were spotless, as i had already cleaned and blown through, the part will arrive on friday so i will be sure to let you all know how it goes

[BritCit_Juve]

The most likely 'Fault' here is the strainer on the inlet to the expansion device if the problem is high head and low suction.
If the problem is high suction and more or less equal head then either the compressor is going backwards (3phase scroll) and noisy or the bullet in the expansion device is not sealing over.
Head pressure control is standard on MCU+ models but not on MCU although always recommended for UK use.
Without knowing more on pressures/temperatures can't help you much more
Brit

[chillyblue]

Hi

i have had the same problem as you with low suction pressures, i have reclaimed and recharged to factory spec, but still had a low suction pressure and high superheat. the syptoms found indicate a undersized orifice or blocked liquid line, i have checked filters, strainers, pipe sizes, i checked the orifice size with IMI and it was correct. i thought the problem may be due to undersized equipment. Am i correct in thinking that under high evaporator loads the amount of refrigerant in the system and the quantity that can get through the orifice is not enough, or the quantity of refrigerant in the system (bear in mind the refrigerant charge is critical, there is no reciever for excess liquid)is not enough for the high demand. The system i have problems with works fine until summer then starts freezing the first couple of runs.
I have spent hours on this system and can only assume that it's due to starvation of refrigerant in the evaporator, these system do not accomodate varying loads well due to the fixed orifice and critical refrigerant charge. Under normal operating loads the evaporator is fully utilized but under high loads the evaporator i believe is starved, which would be ok for short periods, as eventually the room temp would fall and the system would rebalance.
Be careful if you overcharge/ charge the system under high load conditions, the system it will work fine during high loads but will flood liquid into the evap during an off cycle, when the comp restarts the excess liquid will enter the compressor, causing damage or failure, the same will happen under low load conditions, the superheat will fall and flood back to the compressor.

Hope that makes sense

CB

[Andy]

Hi Guys,
just a sugestion.

If there was hot gas in the liquid line the amount of liquid passing into the evaporator would be reduced and a low suction pressure would exist.

As the unit only causes problems in the summer, it could quite possibly be a condenser issue.

Kind Regards Andy:)

[chillyblue]

Hi Andy

That may be correct, but the system i worked on had good subcooling.

CB

[Gary]

Hi Andy

That may be correct, but the system i worked on had good subcooling.

CB

How much subcooling is "good subcooling"? How much superheat is "high superheat"? How much suction pressure is "low suction pressure"?

Give us data, not conclusions. These are the same conclusions by which you have not figured out the problem.

Give us temperatures/pressures and let us draw our own conclusions.

[coolments]

Hi all

Worked on a lot of these in my time, as stated earlier the charge on these is critical and if right it will run for ever, if you weighed it in I bet it aint right. One thing about these units is that they can run fairly short of gas quite happy during low load conditions as the fan speed controller slows down the condenser motor and holds the back pressure up, I am assuming your issue with abnormal pressure is high discharge and low suction which makes you suggest the orifice at fault.

I reccomend the following, when charging these systems.

Note two adjustments on the FSC, temp & min speed. the meathod of head pressure control is by measuring the temp of the liquid line as it leaves the condenser coil via a sensor tyraped to it.

1. turn the fan speed controller temperature setting so that the fan stalls.
2. turn the min speed to maximum.
3. Charge with gas using superheat measurements, you know your getting close when the frost on the orifice & expansion starts to go and sweats instead.
4. throttle back min speed setting on fan speed controller to just before 275psi or the fan stalling, which ever comes sooner.
5. turn up the temperature set point on FSC until the head pressure settles at 250psi.

The unit should now work fine what ever load its under (in its kw range of course).

FYI. the min fan speed is there to keep the fan running in the event of sensor error, it has to be set at a higher than normal running pressure or the unit would not control & over condence when running on cold days.
If the FSC fails then pull the crimp wire of terminal No2 and put it straight to Neutral, this wires out the FSC and the fan will run on its own.

If the condenser fan is not spinning fast enough to condense properly change the fans black and red wires over in the terminal block, this will turn the fan into high fan speed, not many people knowe these fans have two speeds.

Thats all with it, good luck with

[mwatts]

hi there just a suggestion that the wire filter located on the liquid valve could be blocked with carbon due to the compressor changes. you can remove and clean these.

[coolments]

Just realised, you said it has had the fan speeder removed, doh...!! must read better.

It needs one to work effectivley, the head pressure has to be regulated around 250 psi.

[chillyblue]

Just realised, you said it has had the fan speeder removed, doh...!! must read better.

It needs one to work effectivley, the head pressure has to be regulated around 250 psi.


I'm not doubting you but thats seems high! why do they need to run at that high a discharge pressure?? most system would run fine with a much lower discharge pressure.

How much subcooling is "good subcooling"? How much superheat is "high superheat"? How much suction pressure is "low suction pressure"?

Give us data, not conclusions. These are the same conclusions by which you have not figured out the problem.

Give us temperatures/pressures and let us draw our own conclusions

Yes you are correct Gary, but i can't give you any data cause it was a while back and had to make my diagnosis when i was there.

[mwatts]

How much subcooling is "good subcooling"? How much superheat is "high superheat"? How much suction pressure is "low suction pressure"?

Give us data, not conclusions. These are the same conclusions by which you have not figured out the problem.

Give us temperatures/pressures and let us draw our own conclusions



you could always speak to imi for this infomation

[coolments]

Hi chillie

I not going to the whole subcooling superheat conversation there are plenty of threads for that, just trying to help by giving advice based on experience, the person posting the problem needs to produce data.

by the way way, there are no set head pressure pressures as every thing is relative to the aplication and that moment in time ie ambient conditions, 250psi is a giude line and good all round head pressure to be at for AC in uk conditions if you have to stick your neck out and state one, check your gauges and note the condensing temp of R22 & R407c at this pressure

[Gary]

you could always speak to imi for this infomation

I don't care what imi has to say.

I want to know what the pressure/temperature measurements are.

Why do people say things like "the pressure is low", instead of saying "the pressure is ___psi"?

[mwatts]

if you can gauge around a 10oc diff on air on and air off on the indoor and out door coil that will give you a clue or speak to imi and get thier sub cooling temps
if your air on is at 21oc then you could look for 11oc off then you can determine from that

[BritCit_Juve]

imi units are designed for a suction pressure of around 60psig, a head pressure of 250 to 280psi with the headpressure control to maintain those conditions throughout ther year.
Using a higher head pressure allows a smaller condenser coil or lower fan speed.
If the system has been installed to imi spec the suction line pressure drop is going to be around 2°C
Usually the cellar coolers will be evaporating between -2 and 4°c
Brit