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ROCKY123
01-07-2015, 02:21 PM
Hi everyone,
I have an ammonia overfeed system @-30C and i am pumping it to 4 set of coolers. 6@-30,2@-20 and 4@0 .So while selecting the pump,Do i need to consider the pressure drop in all these coolers(ie,from -30 to -20 and -30 to 0)...or i need to consider only the larger pressure drop (ie -30 to 0)..please help....!

Segei
04-07-2015, 03:25 AM
You should have 3 vessels (one per each suction temperature). Each vessel should have at least one pump. Pumps should deliver liquid to the evaporators. Liquid pressure at the inlet of the evaporator should be at least 15 psig (1 bar) above the suction pressure of this evaporator. After hand expansion valve liquid pressure should be 5-7 psig above the suction pressure.

ROCKY123
04-07-2015, 08:26 AM
Thanks Segei......
But my sales engineer sold the project like this. Already its a multi stage system with the booster working for blast freezers at -40c. So he want to feed all the evaporators from this -30 vessel

Segei
04-07-2015, 05:41 PM
This is bad idea. First it is very inefficient and you will waste a lot energy. Second, to supply liquid to 0C evaporators liquid pump should have discharge pressure 5-6 bars. Two stage pump can do that.

RANGER1
04-07-2015, 08:29 PM
depending on 0 deg room load, it could be side load (economiser) with high pressure liquid supply.
It also depends on load of 0 deg room as to how inefficient plant could be.
What type of compressor/s are being used, screw single stage economised, or 2 stage reciprocating?

sandybapat
06-07-2015, 07:54 AM
In many Asian countries, especially in the food processing industries, I have come across many refrigeration plants which are supplied by road side contractors. The business is in unorganized sector. The contractors probably are of opinion, that once pump vessel is provided it serves the purpose of all the evaporators irrespective of their temperature duty. These people does not know (rather do not understand at all) about the specific energy requirements at various evaporation temperatures. The owners of such plants are commerce graduations with money to invest and want the cheapest working plant. They do not know the ABCD of refrigeration.
Minus 20, minus 30 and minus 40 deg C evaporation temperatures are mixed. They use the back pressure regulating valves to maintain the evaporation pressure. The Ammonia pumps are used to raise the pressure to highest temperature evaporator. The lower temperature evaporators are fed with undue subcooled liquid which wastes the heat transfer area of those evaporators. Overall the energy consumption in these industries is really a criminal wastage.
ROCKY123, for your information:
Considering Ammonia refrigerant, two stage economized system and 40 deg C condensing temperature the typical power consumed at compressor shaft for different evaporating temperature is as under:
For Minus 40 deg C evaporation temperature: 0.64 kW/kW
For Minus 30 deg C evaporation temperature: 0.49 kW/kW

ROCKY123
07-07-2015, 02:29 PM
Thanks sandy..........Our system is two stage booster with recip compressors..My doubt is If the pump puts 2.5 bar into the system,will my saturation temp goes up??because in this case i'm feeding -30 coolers first and then to the 0 deg.If so how can i bring it to -30?

RANGER1
07-07-2015, 10:20 PM
Thanks sandy..........Our system is two stage booster with recip compressors..My doubt is If the pump puts 2.5 bar into the system,will my saturation temp goes up??because in this case i'm feeding -30 coolers first and then to the 0 deg.If so how can i bring it to -30?

Any reason you can't use electronic TX valves with high pressure liquid from liquid receiver, return to interstage for 0 deg rooms.
Size compressors accordingly.
Intercooler also designed to suit as well.

sandybapat
08-07-2015, 05:30 AM
Rocky123: Using same pump vessel you will not be able to feed 0 deg C evaporator. Minus 30 deg C sat pressure is 0.181 barg. After raising pressure by 2.5 bar the pressure will be 2.681 barg and corresponding sat temp is minus 4 deg C.
With booster high stage system your inter stage sat temperature may be around minus 5 deg C. You can try and maintain inter stage temperature of 0 deg C and feed the 0 deg C evaporator from your inter stage cooler.

sterl
16-07-2015, 07:50 PM
The fun topic here is not the pump requirement: you need to be able to circulate to the highest temperature evaporator while at the same time manage the excess liquid pressure at the lowest T evap; that takes evap pressure regulators and balancing valve arrangements which will collectively be more expensive than your pumps. So the pump's discharge pressure requirement is essentially the lift to the coil location; plus the distribution requirement for the highest evap temperature coil (about .4 bar, consult the manufacturer's data...) plus the required pressure difference for the liquid side control devices, solenoids, strainers, hand valves, etc. plus the dynamic loss of the pipe run to the evaporator, usually the furthest one, plus the evaporating pressure. Deduct the vessel's suction pressure and you will have the differential for the pump...And you need to check into NPSHa and NPSHr to ensure the pump does not cavitate. The net volume flow remains simply the total of the flow requirement of all the evaps....

Irrespective of all that, the application may not tolerate that the O-Deg Evaps are receiving Minus 30-deg C liquid...There is going to be some portion of the coil pulling frost always because the liquid delivery is still minus 30; or very slightly higher. In a room with a high moisture load this will block a finned coil with frost in fairly short order....Defrost may be required; and the room will want to operate at a low dewpoint which may not be right if this is some sort of commodity storage.