System Balance

[georgedvf]

Hello!!!

In a system (HeatCraft Consending Unit - Evaporator) that was installed many years ago the Condensing Unit is no longer working, so the client wants to replace the complete Unit. The thing here is that for reasons I don't undersantd it is cheaper to get a 10HP Scroll Unit than the 7.5HP Semi-Hermetic Unit that was installed before; so if I install the 10HP Condensing Unit, in BTUH capacity relation it will be about 35% above the evaporator capacity. The question is: ¿Is there any problem when the compresor-condenser has big difference of capacity from evaporator(txv and pipes will stay the same)?

[mad fridgie]

Hi, Hp is a nominal term that has been used for years, and in my opinion means absolutely nothing in todays refrigeration.
You will find (I suspect) that the 7.5Hp semi will have a similar displacement as the 10Hp scroll, refrigeration duty will be similar, do not worry about the balance (if it was already balanced before)
Cheers
Mad

[georgedvf]

Thanks mad fridgie.

But I think I didn't explain myself well. The 30% above I'm talking about is in BTUH (refrigeration capacity).

Do you use a standar limit to balance a system? How bigger/smaller than Evaporator can be the Condensing Unit? (Talking about BTUH)

[powell]

Without model numbers, refrigerant and design temperature we can't help you.

Yes, scrolls are less expensive than semi-discus. Who picked a 10HP scroll? Sounds like your just comparing prices and not btuh's

[georgedvf]

Thanks powell.

Yes, the thing is the scroll is cheaper and is in stock, so the client wants to buy it, no matter if it is 30% "bigger" talking about BTUH's than the one installed before.

Please tell me if I don't explain myself. I'm not sure if my english is completely understandable, jajaja...., it's not my native language.

[Gary]

The saturated suction temp will run a little lower, which will cause the humidity to run a little lower. Other than that, it shouldn't make much difference.

[powell]

georgedvf,

Without knowing the existing equipment, which could be mis-matched, the new 10 HP could or could not cause troubles.

It's best to determine the design criteria of the old system before making assumptions that a 10HP will work.

[georgedvf]

Thanks Gary.

Powell:

You said the words I was looking for: "mis-matched". I need to know what is the criteria to determine if a system is "mis-matched". And what could happen if it is mis-matched (oversized Condensig Unit).

In this case, the evaporator = 72,995 BTUH, "broken" condensing unit = 80,776 BTUH. There was a 10% capacity differential.

If they keep the evaporator (72,995 BTUH) and buy the 10 HP Condensing unit (106,890 BTUH), there will be a 30% capacity differential.

[Gary]

What products are you cooling/freezing?

[georgedvf]

Tomatoes, Potatoes, and other vegetables. It is cooling...

[Gary]

The lowered humidity may be an issue for produce.

The system can be balanced and the humidity held at a high level by installing an EPR valve in the suction line.

[mad fridgie]

Not sure of exactly how you rate, but will take some basic rules.
Evap 73000btu/h/10F or 7300Btu/h/F,
So 14F temp diff evap would then rated at102000 Btuhr, suction pressure would drop on refrig unit, so duty would drop, so balance would be around 95000, which would TD back up marginally. So no problem refrigeration side.
The air will be drier, which in turn dries out product. If product is only in for a very short period, not reaaly a problem, but if long term storeage, then it could be, if this is the case install an EPR, check enverlope of compressor (as suction would drop)

[mad fridgie]

The lowered humidity may be an issue for produce.

The system can be balanced and the humidity held at a high level by installing an EPR valve in the suction line.

Hi gary, beat me to it!

[georgedvf]

... so balance would be around 95000, which would TD back up marginally. So no problem refrigeration side.

Could you explain why do you say balance is 95000?

I did understand the system will balance increasing TD, but one more question :

When the evaporator is to small and condensin unit too big and there's no balance even increasing TD, is there a problem? Just wondering...

[mad fridgie]

If your air temp is constant and you want to increase evap duty, then you must reduce evaporating temp, by reducing the evap temp, you reduce refrigerant mass flow to the compressor, thus reducing compressor capacity. at some point the 2 parts balance, in this case based upon limited info 95000BTU.
No problem with refrigeration

[oldesky]

George, One of the main reasons for the price difference is volume sold. Scrolls are sold worldwide in their millions. I believe that you will be able to use the scroll with little problems as the overall system balance should not really compromise the humidity for short term storage,although long term storage may be a problem.

[Mark II]

George,

I agree with Gary's 1st comment..
All that will happen is the system will have a shorter duty cycle.. It's debatable if this actually effect the product..
I would only fit an EPR if the system has other work to do..
If your customer wants to design his own system & won't take your advice..Check his wallet & carry on regardless