PDA

View Full Version : Hot Water Defrost for Aluminum Coil - Bad Idea?







RamAir95TA
13-02-2013, 11:45 PM
Hi guys. Long time reader, first time poster.

I am an operating engineer for a produce freezing plant. We have a very large Frigoscandia blast tunnel freezer. The system is a bottom-feed recirculating system using large Evapco aluminum coils (10 total, stacks of 2 in 5 individual compartments). Temp is -40 F. Defrost is hybrid consisting of hot gas defrost (90psig) and water defrost (water is at 55 F). Total cycle is 22 minutes. Each pair of coils is defrosted independently of eachother throughout the day with the first coil defrosting 4 times per day, the second pair 3 times per day, third pair 2 times per day, and the fourth and fifth coils once per day, PLUS a complete end-of-shift defrost for ALL coils simultaneously.

The plant manager wants to heat our washdown water supply for sanitation purposes, however the vessel that contains this water is also used for freezer water defrost. He would like to heat the water to 120 F. Safeties on the recirculator are set at 150psig.

How will aluminum coils at -40 F fair being sprayed for 7-8 minutes of 120 F water? We have had 3 coils crack over 10 years due to rapid pull-downs in the beginning of the shift so this is certainly a concern.

How about coil steaming and/or excess snow build-up? And what about LPR pressure on the final defrost? Really don't need safeties popping and the National Guard showing up...

Thoughts? Thanks! :)

-Joe

RamAir95TA
13-02-2013, 11:50 PM
Forgot to add - NH3, of course!

Segei
14-02-2013, 11:43 PM
I think that you can start defrost by using hot gas for 10 min. Then use warm water. You can install mixing valve to get cooler water temperature. 90 psig. Is this setting of back pressure regulator?

RamAir95TA
15-02-2013, 12:42 AM
I think that you can start defrost by using hot gas for 10 min. Then use warm water. You can install mixing valve to get cooler water temperature. 90 psig. Is this setting of back pressure regulator?

Thanks for the Segei.

The pressure regulator for the hot gas is set at 90psig.

When you say warm what is a good range to target?

The mixing valve is a good idea for sure. I'll have to look into that!

Thanks again!

Segei
15-02-2013, 03:17 AM
Warm I mentioned your 120F. Definitely, at this water temperature you will have more steam than at 55F. However, if these sections are separated and steam will be kept in defrosted section and it will not be an issue. Air can contain certain maximum amount of moisture.

sterl
15-02-2013, 05:27 PM
Ask Evapco about cracking aluminum coils....They have seen it before in similar applications.

Definitely your coils are going to suffer, not from the total temperature swing so much as from the rapid temperature changes. This is usually more aggravated going down in temperature than it is going up. The bond (nominally contact only with Al fins on Al tubes) will degrade; the return bends and coil end plates will get stressed and distorted. We have addressed a number of these concerns by mixing water through a 3-way valve driven by a motor such that it starts out with cool water and ends up warm, then goes back to cool...the cool water will actually absorb some of the steam unless it is clinging to a surface. We tried a surfactant in the HW tank which did do some good in terms of residual water on the coil.

We have also put strain gauges on U-Bends with a Data Acq arrangement driven through a similar cycle but on a different freezer. Frankly the results were scary.

The refrigerant pressure is manageable; its a matter of putting a second regulator parallel to your defrost relief in most circuits to ensure that you get all the refrigerant (liquid) out of the coils before you apply any water.

Is the hot water rinse a sanitary or debris requirement? are you dealing with cooking oil carryover or something similar?