Large Walk-in Cooler Redundant system electrical issues

[icemeister]

This looks baller and pretty much plug 'n play but it's so expensive
http://www.emersonclimate.com/Docume...A-00319%29.pdf


So the ZP is designed to maximize evaporator capacity below -10F SST and ignore temperatures about about 20F at the bulb. The regular Z runs high high SH and lose capacity at MT where you probably don't need the capacity if the system is already sized to be adequate for 0F box. The Z runs high SH at MT which sort of serves as a CPR as well at refrigerator temp.

The C loses capacity below -10F SST while it maximizes evaporator output all the way to 55F SST. I think it will make the system unstable and short cycle when it's pretty close to box temp outdoor at night. Icemeister... thoughts?

That's a graph with plots of SH vs Evap Temp for various thermostatic element charges. I chose the C-charge because of all of them it has the flattest curve between the two Evap temps we'll be running at (-10 and +20). All that means is if we set 10 Deg F SH at one condition it will still be close to the same at the other...nothing more than that really.

I don't follow what you getting at with the unstable system, short-cycling on a cold night thing. ???

[ICanHas]

That's a graph with plots of SH vs Evap Temp for various thermostatic element charges. I chose the C-charge because of all of them it has the flattest curve between the two Evap temps we'll be running at (-10 and +20). All that means is if we set 10 Deg F SH at one condition it will still be close to the same at the other...nothing more than that really.

I don't follow what you getting at with the unstable system, short-cycling on a cold night thing. ???

Well, if the walk-in is designed to handle hold 0F box in the summer with traffic with 27,400 BTU output, 41,100 BTU available at 35-45F holding temp when the box is only 10-15F below ambient, you'd be oversized.

The Z charge gains SH towards MT, and loses SH towards lower end, I am thinking it would moderate the capacity a bit better.

[icemeister]

Well, if the walk-in is designed to handle hold 0F box in the summer with traffic with 27,400 BTU output, 41,100 BTU available at 35-45F holding temp when the box is only 10-15F below ambient, you'd be oversized.

The Z charge gains SH towards MT, and loses SH towards lower end, I am thinking it would moderate the capacity a bit better.

Let's ignore the TXV's, element charges and superheat, just for a moment.

These systems are not variable capacity. They don't unload. They match the available load, which is variable, solely by cycling on and off. The run time decreases with decreasing load and vice versa.

Now back to SH...

Normally, you will set up a system to cycle at a fixed temperature, like 0 Deg F. At this condition you want to set up your TXV to run a fairly low superheat to maximize the evaporator's and hence, the system's performance. On the charge plots you posted above, the almost parallel dotted lines represent the normal operating range of superheat control for a TXV and it seems to run about +/- 2 to 3 Deg F from midrange. This tells me the minimum SH I would ever expect to control to would be just over 3 Deg F to avoid any undue liquid floodback. This I think is why you'll rarely hear anyone setting a TXV lower than 4 Deg F SH.

Now with the Z, ZP or even the C charge at -10 Deg F SST you can set up the valve for 4 Deg F SH and it'll work fine at tha condition. We also know however for pulldown from a hot box the ZP thottles way back at its MOP while the Z would continue to feed, but this is where I believe these plots are telling us something interesting.

I think what it's saying is as you move up in evap temp. from an initial condition like -10 Deg F evap temp to say 0 Deg, the Z/ZP plot says if you set a 9 Deg F SH at the lower condition it would increase to about 12 Deg F SH when your evap temp went up to the higher condition. This says to me my initial 4 Deg SH would increase by about the same amount. So with the Z/ZP the SH would tend to increase/decrease dramatically when the ET changes.

The C-charge shows a much flatter curve with very little change in the resulting SH as the ET goes up or down. The C will hold the SH steadier and tighter over a much wider range.

While it's true that a high SH will degrade a system's capacity to some extent, this isn't something you ever want to design to. More importantly than maintaining SH for performance, you want to maintain it for proper compressor protection.

[UmmScott]

update on this....both evaps are going to be replaced and have been ordered.
2 week lead time...good thing its not an emergency. We have confirmed that we're getting coils with 480V rated heaters and will ensure they're wired up properly