Bad Evap?

[Telecaster1]

I have a question that I think I know the answer to but I would like the advise of fellow members of the trade.

I have a walk in cooler that runs on 12 with a evaporator that is looks pretty beat up on the coil. If I try to straighten the fins out, they just come apart. With a 100 degree ambient I have about 185 on the head with about 11 degrees of subcool. I had a super heat of about 21 degrees at the evaporator and adj the TXV to about 10 so there would be some cool vapor going to the compressor (25' line set that had about 70 degrees at the compressor previously). After the TXV was adjusted, I went and put my probe on the suction at the compressor and noticed a fluctuating SH between 10 to 30 degrees. Have a delta T at the evap of about 3 degrees, 48 in and 45 out.

This all started when I was setting the thermostat to get the box below 40 degrees. This caused the evap to frost at the top after a couple of days. After I adj the TXV it started to frost even worse. I looks like a lack of heat transfer and I'm attributing it to the beat up evap.
Thanks

[taz24]

I have a question that I think I know the answer to but I would like the advise of fellow members of the trade.

I have a walk in cooler that runs on 12 with a evaporator that is looks pretty beat up on the coil. If I try to straighten the fins out, they just come apart. With a 100 degree ambient I have about 185 on the head with about 11 degrees of subcool. I had a super heat of about 21 degrees at the evaporator and adj the TXV to about 10 so there would be some cool vapor going to the compressor (25' line set that had about 70 degrees at the compressor previously). After the TXV was adjusted, I went and put my probe on the suction at the compressor and noticed a fluctuating SH between 10 to 30 degrees. Have a delta T at the evap of about 3 degrees, 48 in and 45 out.

This all started when I was setting the thermostat to get the box below 40 degrees. This caused the evap to frost at the top after a couple of days. After I adj the TXV it started to frost even worse. I looks like a lack of heat transfer and I'm attributing it to the beat up evap.
Thanks

Adjusting the TEV (TXV) is fatal in most normaly working systems.

If the thing was ok why would the supperheat alter??

You need to look at other possible faults.

Short of refrigerant springs to mind initialy but the system
needs checking thouroughly. Unfortunatly you will have caused
more problems by adjusting the valve than you know how to deal with..

Go back to basics. Why did it freeze up if you adjusted the stat??

All the best

taz

.

[chemi-cool]

Get a new evaporator!

It sounds that the one you got is partially clogged.

[Telecaster1]

Thanks for the reply guys. Taz, as stated in my original post the cooler was not working normally. I had a cooler that was not getting below 40 degrees (or below +4 celsius) and evap superheat of 21 degrees ( or 10 celsius) with a 10 degree F subcool and a clear sight glass. The coil froze up only after I adjusted the thermostat to get the box below 40. First, with that high of a superheat at the evaporator, wouldn't that starve the coil? Most TXVs, depending on application are factory set between 7 to 12 degrees F. How long would a compressor last with 70 degrees going to the compressor (as stated in my previous post)? Carlyle and Copeland recommend no less than 65 degrees at the compressor inlet. Can you have a low charge with a full sight glass and 11 degrees f of subcool? What would you consider a situation that a TXV would need to be adjusted? The fact that I had a full receiver would be an indication that there is enough refrigerant. I adjusted the TXV only to achieve the proper superheat at the evaporator and to insure a proper cooling for the compressor. With the coil still freezing up and the box not getting down to temp, wouldn't that point to a heat transfer issue? Thanks

[Gary]

R12
185psi = 132F
132 - 100 = 32F TD

This would indicate a heavy heat load being rejected by the condenser (if we can assume good condenser airflow), therefore a heavy heat load is being absorbed by the evaporator.

When the evap air in temp is high, the TXV outlet superheat is supposed to be high. That 7-12F SH is when the box is down to temp.

We know the cond air in temp is 100F, but what is the cond air out temp? Air out minus air in (cond dT) tells you if you have good cond airflow.

Also, I don't see the suction pressure anywhere in your post.

[Telecaster1]

Isn't a TXV a proportional control. Bulb vs evap outlet press. The suction was about 24# at the final reading. I'll have to get back with an exact condenser outlet temp but I can tell you it was at least a twenty degree difference.

[Gary]

The advantage of the TXV is that it can open up and flow more refrigerant when the heat load is high. It can only do so if the SH is high. High SH causes it to open wide and stay open until the load drops.

And yes... all proportional controls have an offset which gradually decreases as the controlled medium approaches setpoint.