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08-24-2012, 08:03 AM #14Regular Guest
- Join Date
- Jun 2010
I have to admit to not being sure what I should do. Too bad work isn't interested in me adding a CPR and check valve.
I think I'll put it back in service, order a TXV, and install it later on.
08-24-2012, 10:23 AM #15
Going back to my earlier post where I mentioned the system balance, I think it needs an example to explain just what I was trying to explain.
First, imagine a two walk-in coolers applied to a single condensing unit. Both are the same size (let's say 10 x 10 x 10), both are designed to hold 35ºF and both have a design load of 10,000 Btuh. Both have evaporators selected for a 10ºF TD and so, the condensing unit is selected for 20,000 Btuh at a 25ºF SST. For this example, say this works out to a 2 HP R404A MT unit.
When both coolers are calling, the compressor will run at its design evap temp of 25ºF. If one cooler cycles off, the system must re-balance to the new load condition...so what would we expect see?
First we know the specific capacity of the evaporator is 1000 Btuh/ºF TD (10,000 Btuh / 10ºF TD) and from the condensing unit performance data sheet we find the following capacities at various SST's:
25ºF 20,000 Btuh 20ºF 17,500 Btuh 15ºF 15,000 Btuh 10ºF 12,500 Btuh
If we plot this as a graph or do some basic interpolation, we find that at 17.5º the capacity of the evap and the unit match at 16,250 Btuh...so I would expect to see the system running at a 17.5ºF saturated suction temp with one cooler calling.
This is essentially what's happening with your prep table with the cold rail.
With a cooler/freezer dual temp application, we'd do the same thing, except we would have to select the compressor capacity at a much lower design evap temperature. If the freezer is designed for 0ºF with a 10ºF TD for the evap the SST would need to be -10ºF and as is often the case, the design Btuh load of the freezer is about the same as the cooler. (See p.43 of the Heatcraft Engineering Manual).
So now we would need a much larger condensing unit, like a 5 HP LT R404A to do 20,000 Btuh at -10ºF SST with both evaps calling. With only the freezer evap calling, the SST would be even lower, but with only the cooler calling (and no EPR), what would we expect to see?
Like we did before, with the condensing unit capacity data:
10ºF 28,000 Btuh 0ºF 24,000 Btuh -10ºF 20,000 Btuh -20ºF 15,000 Btuh -30ºF 9000 Btuh
Theoretically, the cooler evap and the unit would balance at around +8ºF SST at about 27,000 Btuh capacity with a 27ºF TD.
Now...if you were to install an EPR on the cooler evap to hold it at a normal 10ºF TD, its capacity would then be limited to its specific design capacity of 10,000 Btuh...and that would be the new load on the compressor. How would the system balance then?
Since the TD and thus the load is now fixed by the EPR, the evap capacity and the load on the compressor remains constant at 10,000 Btuh, we simply need to determine where the unit capacity equals 10,000 Btuh...which would be just above -30ºF SST.
So you now can see that it's all about system balance. Clear as mud?
Last edited by Educational Committee; 08-24-2012 at 03:23 PM. Reason: Corrected second capacity table
08-24-2012, 10:47 AM #16
As I said before, I think it's doing what it always did and you simply never noticed it until you had to fix that leak.
08-24-2012, 11:03 AM #17Regular Guest
- Join Date
- Jun 2010
Yes, I did mean an EPR valve. BTW, thanks for the education above. I don't normally work on stuff quite like this.
08-24-2012, 08:50 PM #18Professional Member
- Join Date
- Jun 2009
- Garland, TX
Thanks Ice for the explanation....
I guess the only thing I'm still not sure about is this:
I understand the low SST temp. What I don't understand is the high superheat. Now, if the condensing unit is grossly oversized, that would cause the low SST, but can it also explain the high superheat?
You know what, I JUST NOW remembered a store we have with 2 little remote Glastender bar coolers, on like a 1 or 1.5HP condensing unit. No matter what, the TXV's always freeze up, super low suction pressure, and suction line coming back to condensing unit is not even cold. Yet the boxes can get below 40*f. I guess the TXV is incapable of opening up any more to flow enough refrigerant as fast as the compressor's suction is pulling it out. Does that sound about right?
08-25-2012, 08:53 AM #19
The TXV does have its flow limitations with increasing pressure drop across it, and as such it will reach a point to where it acts more like a fixed orifice than a TXV. So with this in mind, my example of an evaporator doing 10,000 Btuh at one condition and over 20,000 Btuh at another would be highly unlikely simply because the valve can't pass that much refrigerant. The result would be an even lower suction pressure and yes...much higher superheat.