Quick question....before I make a mistake
I have a two door reach in I'm working on. Freezer on the top, refrig on the bottom. Tstats, Txv's, LL solenoids for each. Unit runs R404a. It is made by Gem and designed for pharmacy/lab applications.
It had a leak. I found one on a soft soldered joint on the refrig Txv. Told my boss we should buy a new valve....not a good chance on being able to resolder due to oil in the solder cup. He wanted me to go for it so I unsolderd the valve and used a acid brush with liquid flux inside the solder cup. Oil residue was noticed on the outside of the valve but not where the leak was at.
Got her resoldered/evacuated/recharged/etc. Freezer works great. put some switches on the LL solenoids so I could look at each section. Suction pressure on the refrig seems way too low. Txv body has frost on it. No difference in suction pressure as I adjust the valve and/or torch body/sensing bulb. I'm going to order a new Txv. Maybe I got solder in it....maybe the power head went and the body froze causing the solder joint to fail...I dunno.
No my question is....should I expect to see more or less the same suction pressures as I would for a straight refrig? After all the compressor is sized for both coils to be running and when the freezer is satisfied maybe I can't possibly expect to push enough gas through the one refrig evap to get a normal suction pressure????
Head pressure was about 90 degree satutrated....about -15 suction with the freezer alone. Refrig alone was way low....like -30 degree coil temp.
Sorry I don't have superheat numbers....suffice to say suction line is well below freezing with the freezer going....about beer can cold with just the refrig.
From paperwork valve is supposed to be sporlan EFS-1/4-C 2X3 ODF straight thru solder body. Best I can find is an emerson HFS1/4RZ ....not too many 1/4" by 3/8" choices. Sound like I'm on the right track guys? Missing anything?
Why not use Sporlan EFS-1/4-C
Originally Posted by Russ57
It's standard 1/4" by 1/2"
But you can order it 1/4" by 3/8"
You can't use the emerson valve because the Z charge is low temp. What if you used the 1/2" outlet valve and a 1/2" x 3/8" fitting bushing?
Damn, you are right....that is a low temp valve. Very little room. I guess I can get a bushing in there.
Any thoughts on what sort of suction pressure I should see when on the refrig solenoid is open?
The paper work calls for a C charge element?
Originally Posted by Russ57
If you wanted, you could go EFS-1/4-Z
That would make it a low temp...still have to order it for 1/4 by 1/2.
If you need this right now ASAP, then you will need reducers.
I need medium temp...for frig section. And yes, right now.
Unless the cooler evaporator has an EPR valve to keep its suction pressure up, the cooler evap will run at whatever pressure balances out the load on the compressor...ie, with both the freezer and cooler calling, the suction pressure would tend to run at a normal freezer suction pressure. With only one of them calling (eith the freezer or the cooler), the suction pressure will be even lower.
Originally Posted by Russ57
The fact that the factory paperwork indicates a C-charged TXV for the cooler section is puzzling to me. While the Sporlan C-charge will work OK over a pretty wide range of evap temperatures, if there's no EPR, the valve will always see freezer-like evap temps, and so I would expect the Z-Charge to work better for this particular (and rather strange) application.
So essentially, with the system the way it is, the cooler section basically operates like a freezer with the thermostat cranked way up.
This is why I ask you guys questions. Blows my mind to see a higher suction pressure with just the freezer running than just the frig running.
Still have to think the frig TXV is bad. It runs about a -30 saturation temp. Doesn't change as I adjust the valve. Doesn't change as I hit the valve or sensing bulb with a turbo torch. Valve body is frosted. Sure doesn't appear to feed enough freon. Suction line is barely cool. Box does reach setpoint just fine though.
Freezer section rocks. Suction line well below freezing. Evap saturation temp a little low...around -15. Setpoint is -20 Centigrade....think that is -4 F.
Now to find a med temp R404A 1/4 ton valve in straight thru solder. Rather limited on who I can order from. So far Emerson AA1/4SC is about the best I can do from Johnstone. Will have to bush to 3/8" outlet. Think I got it right now. Thanks for noticing I had picked a low temp valve.
Or maybe I don't have this right as the suction pressure is going to be very low? But does that matter? The TXV bulb is seeing temp, not pressure. And not matter what the pressure is, it isn't that cold. Valve is internally equalized.
I suspect the only possibility of the TXV being able to actually maintain control would be when both the freezer and the cooler are calling...and even then I wonder if it would really work all that well.
When the freezer is off, the cooler evap temp drops from -15 to -30. This would tend to dramatically increse tat evap's TD and hence its cpacity. If the TXV has the flow capacity to feed properly at the higher evap temp of -15, it may not have enough flow capacity at the lower -30 condition...ie, it would be running wide open and so no superheat control or adustability would be possible.
Does the cooler section work OK in either mode as it is right now? If so, I wouldn't get to deep into getting the TXV to work as you'd like...because I doubt it can.
The frig section whichs setpoint just fine. Maybe I don't really have a problem? If I understand you right I'm basically running a -30 evap with 80 degrees of superheat. TXV body is very cold but the coil isn't cold enough to have frost on it....even with the evap fan off while adjusting valve.
I suppose I'm spoiled by the straight lab refrigs that pull down really fast. This one takes awhile to pul down from room temp.
From what I can see here, the box is performing as designed...as MIckey Mouse as that design may be.
Originally Posted by Russ57
If you check around with different manufacturers of dual temp boxes, you'll see that the majority use two separate systems...one for the freezer and another for the cooler. This approach is more $$$ of course, but they'll perform better and probably last a heck of a lot longer.
I don't understand your thinking on this one. And I want to learn, that's why I'm questioning you, so please don't take any offense!
I relate this to my experience with self-contained prep tables that have a base refrigerator and a wrapped cold rail on top. When the base is calling, you will have a SST of 15-20*f, and the top cold rail will not frost up very well, it try to frost but not much. But once the base cycles off, the SST drops to around 0*f and the top rail starts frosting really good.
I realize a freezer is different, but still it seems that when that med temp TXV is being fed refrigerant, that cooler has a good load and it is going to raise the suction pressure because of all the heat it's absorbing (if TXV is feeding properly). In my mind, I think it should work like this: while cooler is calling, it will run higher suction pressures and freezer will suffer, but once the cooler cycles off, the freezer will work good.
When I look at BTU sizing charts, the higher the SST you want, the more BTU's required. This leads me to believe that the cooler is the greater load, and the freezer is the smaller load. Is my thinking on this flawed?
Why will it not work like I think it will in my mind? Please explain to me because you have my mind all twisted now!
I'd try a new TXV for the cooler, too, just FYI!
No offense taken as I think I understand your confusion.
Originally Posted by trippintl0
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:
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:
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