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Friobernal
16-03-2003, 08:33 PM
Hi,

Iīve got 3 questions about ammonia evaporators:

1. Number of re-circulations. Iīve read is about 4 in flooded systems but... I need somebody explain it to me

2. Top pump / bottom pump. The connections change, but ... I donīt know more. What / why top or bottom pump?

3. Iīm attaching a drawing of an ammonia evaporator. I donīt understand the reason for circuits in vertical and not in diagonal. I mean in the last circuit (from the air inlet in the coil) the air temperature is lower than in the first one (on the left in the drawing) so, I think that ammonia will have more difficulties in evaporating (smaller DT). With collectors in vertical (as for R404A) and circuit in diagonal we would get less difficulties.... so why?

Bye

Friobernal
16-03-2003, 08:34 PM
Wrong attachment Sorry!!!

Andy
16-03-2003, 11:15 PM
Hi Friobernal,
Pumped nh3 is just that, you have no fancy distributor headers. Simply pump 2 to 4 times the mass of refrigerant around than can be evaporated in the evaporator and it works.
Simple coil designs, fully flooded operation and a refrigerant with a good specific heat capacity (less mass flow for the same duty compared with all common refrigerants)
Normally the liquid is in the bottom, out the top, the extra connections at the bottom are for hot gas during defrost.
Anyone else like to go a little further with this one?
Regards. Andy:D

Prof Sporlan
17-03-2003, 03:43 AM
Anyone else like to go a little further with this one?
Of course... :D

An explanation of the different types of evaporators might be useful here.

The type of evaporator being questioned by Friobernal is often referred to as a "recirc coil". With this type of coil, refrigerant is pumped thru the coil, and the unevaporated refrigerant is captured in a low side receiver. The refrigerant is often pumped via a mechanical pump, but discharge gas can be used for this purpose.

One might think that pumping more refrigerant thru the coil than it can evaporate is a waste of energy. But as turns out, getting recirc ratios up to 4X on ammonia applications improves the evaporator's heat exchange effectiveness.

A DX evaporator, of course, uses an expansion device. Invariably, it will be a TEV for ammonia service. Under normal operation, no liquid refrigerant leaves the coil, so the evaporator boils off all the refrigerant that enter the coil.

A gravity flooded evaporator coil is essentially a recirc coil without the pump. Here, the coil is fed from a reservoir controlled by a low side float. The refrigerant recirculates thru the coil and back to the reservoir due to variation in refrigerant density as the refrigerant evaporates.

Finally, we have flooded chillers where we have a shell and tube arrangement where refrigerant is on the shell side, and the refrigerant level is controlled via a low side float.

Andy
17-03-2003, 10:34 AM
Hi Professor,
thanks for the extra input, this has got me thinking. I have yet to see a top entry coil used on pumped circ, but I have seen this method employed on flooded plants. We (the company I work for) use a form of DX evaporator which is over-fed with refrigerant (up to 2:1), excess refrigerant is boiled off in a suction accumulator by addtion of heat indirectly from the warm liq line from the condenser passing thru the suction accumulator.
I have worked on "Torry Tunnel" Freezers, which have been flooded R22, in a similar manner to our system, they use what is essentially a pump recirc coil circuited so that the liquid enters the top and comes out the bottom. This is the only time I have seen this type of flooded coil arrangement.
Regards. Andy

bersaga
25-03-2003, 11:36 AM
Hi Friobernal !

Just to add on the Circulation Rate - you could refer to the ASHRAE Refrigeration Handbook because it differs from system type and refrigerant .

Circulation Rate(no of circulations) : Mass Ratio of liquid pumped/liquid evaporated

Recommended :

Ammonia - Downfeed , use 6 to 7
Upfeed , use 2 to 4
R22 Upfeed , use 3
R-134 a , use 2

It'll be good to hear from those with extensive experience on these type of system, the practicality of the above numbers.

Thanks