heat exchanger

[COBRAKAI]

All right boys thought I would put this one out there and see if any one could shed some light on it for me.

Had a call to a walk in freezer system tripping. Found and rectified the issue. But the system was having weird pressures. It was operating on r404 and the back pressure was 0.2bar head pressure was 150psi. the system reached temp no problem -18 within an hour but found these pressures strange. I noticed there was a heat exchanger installed there was a degree of ice on the suction up to this point. Has anyone had this also and is the pressures accurate due to the heat exchanger.

Any advise would be great cheers lads!!

[Magoo]

You talk in psig and bar, your discharge is too low at 10.3 bar gauge. Should maintain 35'C minimum SDT. Cold ambients is the problem. System SST is -40'C ish, you could have oil carry over problems

[ironmen]

Its all depend on the ambient temperature.Its seems to me your ambient is around 19 to 21 degrees celcius.Thats normal.Maybe you've got a fan speed controler on the condenser its been set wrong.Refer to your temperature chart.By the way how did you go with the domestic fridge,we will appriciate some feedback.Cheers.

[COBRAKAI]

UPDATE:

Went back to site and had look over system. Found no fan speed controller was fitted. Also discharge line was supplying the water heat exchanger. This in turn was giving me the low head pressure. Found expansion valve opened all the way and superheating at around 20k. System also operating slightly short of refrigerant. Also diff was set wrong on lp and compressor jumping in and out while controller diff cut out.

I fixed these issues and also found the compressor clunking to a halt. Poss comp damage? Also still cant explain the rouge heat exchanger that no one seem to know about. Got a feeling that install company re-used old pipe work and never examined the pipe work. How ever my next question is this. when the compressor enters a pump down mode it has a staggered rise to about 1.5bar. could this in turn be in conjunction the clunking? there is a small capillary on top of the compressor which I never examined fully could this be some sort of pressure relief?

[taz24]

UPDATE:

Went back to site and had look over system. Found no fan speed controller was fitted. Also discharge line was supplying the water heat exchanger. This in turn was giving me the low head pressure. Found expansion valve opened all the way and superheating at around 20k. System also operating slightly short of refrigerant. Also diff was set wrong on lp and compressor jumping in and out while controller diff cut out.

I fixed these issues and also found the compressor clunking to a halt. Poss comp damage? Also still cant explain the rouge heat exchanger that no one seem to know about. Got a feeling that install company re-used old pipe work and never examined the pipe work. How ever my next question is this. when the compressor enters a pump down mode it has a staggered rise to about 1.5bar. could this in turn be in conjunction the clunking? there is a small capillary on top of the compressor which I never examined fully could this be some sort of pressure relief?

You need to take all the issues in order and elliminate them one by one.
You seem to have resolved most of the problems and are not far off.

It seems to me that somone in the past has had low discarge pressure and poor liquid flow
through the expansion valve (due to the heat exchanger and low ambients) they have then adjusted
the valve in an atempt to get liquid into the evap.

They also have had cycling issues so felt the need to adjust the lp switch.

So you have resolved those issues and hopfully are onto the reasons why????

The heat exchanger is to recover heat for pre heating hot water presumably and
will have been done at some time in the past.
You could either fit a water control valve, condenser fan speed reglator or disconect
the heat exchanger all together.

Your call on that one, two of the above options will be costly.

As for the comp "clunking". I don't recall you mentioning what type it is so I will gues.
It could be mechanical, clunking is normally mechanical and it could be the result of a
previous incident or fault.

But the biggest thing you state is the back pressure rises on pumpdown. Depending on the time
this takes, it could be good or bad. If the pressure rises after an hour or so its fine if it does it
over a few mins then it could mean somthing is wrong inside the comp.

All the best

taz

.

[COBRAKAI]

Cheers for your input taz.

Yea I installed a fan speed in my previous visit and set expansion valve. Overall the system is running allot better. I do share the same thoughts with you on the compressor the comp is a scroll type and i think it may be entering into mechanical failure. The pressure has a sharp rise over a 15 second period. ultimately I think the lack of refrigerant flow returning to the compressor will have caused this. Just a waiting game now lol...

[COBRAKAI]

Shutoff noise.
Scroll compressors may run backwards for a brief period at shutoff as the internal pressures equalize. This momentary reversal of direction of the scrolls has no effect on the compressor durability and is entirely normal. Single phase compressors built since May 1995 have a device which prevents the reverse rotation and eliminates the noise. Three phase models have a device which reduces but does not eliminate the noise.
Shutoff sound.
The reversing valve should be wired so that the valve does not reverse when the system is shut off by the operating thermostat in the heating or cooling mode. If the valve is allowed to do this, suction and discharge pressures are reversed to the compressor. This results in a condition of system pressures equalizing through the compressor which can cause the compressor to slowly rotate until the pressure equalizes. This condition does not affect the compressor durability.


JUST FOUND THIS IN TROUBLE SHOOT PAGE THIS MIGHT EXPLAIN IT WE WILL WAIT AN SEE

[taz24]

Cheers for your input taz.

Yea I installed a fan speed in my previous visit and set expansion valve. Overall the system is running allot better. I do share the same thoughts with you on the compressor the comp is a scroll type and i think it may be entering into mechanical failure. The pressure has a sharp rise over a 15 second period. ultimately I think the lack of refrigerant flow returning to the compressor will have caused this. Just a waiting game now lol...

Ok then......

Scrolls should have a one way valve inside them on the discharge to prevent this.
If they don't they can run backwards untill the pressure equalises. It also happens
when more than one comp is fitted.

Back pressure rising on shutdown with a scroll might not be a problem and the cluncking
could be the machine taking up the slack somwhere inside so we'll never know.

In my opinion if you have it somthing like ok then leave it.
Explain to the customer your findings and prepare them for a large bill in the future, the
trouble is it might run for 1 day, 1 week, 1 month or 1 year. No way of knowing realy.

But well done and thanks for the feed back.

All the best

taz

.