electric defrost on medium temp

[dave1234]

Im not involved with this job in any way shape or form but caught wind of the situation by way of conversation. Newley installed unit and evaps on a huge warehouse keg cooler (1 unit 2 staged evaps 404 i believe sharing a common stat)- one of the evaps keeps frezzing over. My info is limited on the particualrs since as I said it just came up in conversion. The off-cycle defrost for said coil is "maxed out" (I guess the other guy said to add anymore would overlap the other evap and cause to great of swing in box temp). If they keep having freeze up problems he said they were going to add electric heat elements including fan delay and defrost termination. Apparently the fins have the recess to install a heater already. My question is aren't fins on a low temp evap further apart and wouldn't that in theroy decrease the dehumidification effect? Can any problems be expected from electric defrost on medium temp app? In theroy it should be ok and the termination stat should cut it out if it gets to warm. Also I never understood the "film factor" theroy in regards to fin spacing and de-humidification. I know theres a difference and must be considered for a specific product because my old employer told me once to grab "that" evap in the shop and when we got to the job it had electrtic heat and was a freezer coil. It was just a small reach in and he opted to use it any way but told the customer to only use it to store... cant remember, beverages I think because the lack of de-humidification would make for soggy produce or maybe thats backwards. Sorry for the long winded post. Thanks

[VTP99]

My thinking is the fins per inch on freezers is less to allow for the frost build up you naturally get on a freezer coil.

[dave1234]

My thinking is the fins per inch on freezers is less to allow for the frost build up you naturally get on a freezer coil.

Right Ive always understood that but wouldnt having less fins (assuming it is adequetley defrosting) =less surface area=less condensing surface=less dehumidification?

[VTP99]

Right Ive always understood that but wouldnt having less fins (assuming it is adequetley defrosting) =less surface area=less condensing surface=less dehumidification?
Changing your TD across the coil will change your humidification.

[dave1234]

Typically 10 for low temp and 15 for medium- why is low temp lower again?

[Phase Loss]

The tubes are the primary coil surface and the fins are the secondary coil surface. Less fins = Less coil. So you are correct by saying there will be less surface area for cooling and de-humidification.

You can run electric defrost for medium temp, just gotta make sure termination is functional and fail safe times are not way out of range.

But I am willing to bet that your co-workers are having a system problem that is causing that one coil to ice up. And installing heaters could be the wrong action to take.

[dave1234]

But I am willing to bet that your co-workers are having a system problem that is causing that one coil to ice up. And installing heaters could be the wrong action to take.

I totally agree and that was my thought exactly. But then who am I- a lowley apprentice- to question the journeymen :)

[icemeister]

...Newley installed unit and evaps on a huge warehouse keg cooler (1 unit 2 staged evaps 404 i believe sharing a common stat)- one of the evaps keeps frezzing over.

If the evaporators are staged, does the condensing unit have 50% unloading capability...ie, either unloaders or dual compressors? If not, the 1st stage evap will see double the TD and build up a lot more frost, and so the evaps should run together as one stage.

What box temperature are they trying to maintain? I try to run 35ºF-36ºF for keg beer. Anything below 34ºF will require either electric or hot gas defrost.

[dave1234]

If the evaporators are staged, does the condensing unit have 50% unloading capability...ie, either unloaders or dual compressors? If not, the 1st stage evap will see double the TD and build up a lot more frost, and so the evaps should run together as one stage.

What box temperature are they trying to maintain? I try to run 35ºF-36ºF for keg beer. Anything below 34ºF will require either electric or hot gas defrost.

There is just one compressor- looks to be a big scroll so no means of unloading. I think they're trying to keep it around 33-35, not really sure. That is an excellent point. the 1st stg sees more load across it simply because it will refrigerate at a higher box temp before the 2nd comes on right?

[icemeister]

There is just one compressor- looks to be a big scroll so no means of unloading. I think they're trying to keep it around 33-35, not really sure. That is an excellent point. the 1st stg sees more load across it simply because it will refrigerate at a higher box temp before the 2nd comes on right?

No, the box temp should remain about the same. It's the fact that the evaporator temperature will drop when the second stage shuts down to defrost and vice versa.

I said the TD would double running on just one evaporator. That's an oversimplified view.

Actually, since the compressor will then running at a lower saturated suction temperature (SST), its capacity at that condition will be somewhat less tha when its running at the normal, higher SST with both evaps on line.

For example, take a 10 HP medium temp R404A scroll unit running at +25ºF SST/ 95º Ambient. It will have a capacity of about 108,000 Btuh. If the design TD for the evaps is 10ºF TD then each evaporator should have a capacity of half that, or 54,000 Btuh. Therefore if only one evap is running the compressor will drive the SST down until the compressor capacity matches the evap capacity. You can determine this point by plotting the evap capacity vs the compressor capacity on a graph.

Here's a past thread on balance curves:

http://hvac-talk.com/vbb/showthread.php?t=377632

In this case, that balance point for a 35ºF box temp would be somewhere around 17ºF SST for an 18ºF TD for one evap on that 10 HP unit. You can see that you would be building frost a lot faster at +17ºF than at +25ºF (the balance point for both evaporators).

[crackertech]

Anytime beer is in danger of being warm it's icemeister to the rescue.:D

[VTP99]

Anytime beer is in danger of being warm it's icemeister to the rescue.:D

He is definitely a fountain of information. :yes:

[dave1234]

Turns out i was mistaken- he specifically said the evaps where "staged" but was referring to the defrost cycle- one then the other to minimize temp swing. That makes more sense.

[codgy]

I would do away w/ staged evap defrost. Explain to customer that air temp swing and product temp swing are to totally different things. I run into this w/ some customers who get hung;up on 1 deg.diff, it can take some time but once they see the results, less service, better reliability and less worry they come around quickly. I would defrost 6 per day x 25 min in this application for a start. I have some units with off cycle thermostats to defrost coil each time tstat cycles, kind of a homemade constant cut-in control. Another thing to check is solenoid valve closing every time t-stat satisfies.