Evaporator feed

[Herschmidt]

Hi all

Is there any diference in evaporator capacity if I feed the evapòrator from the top or the bottom in a overfeed system.

Im asking these because I have cold room with two evaporator fed from the top and they dont work properly, and in the next cold room the evaporators are fed from the bottom and I dont have any problem with them.

thank in advance.

[Sandro Baptista]

Both system should give more or less the same capacity albeit my feeling says that feeding by bottom it will give a better "natural" circulation and more wetted coil.
Please send a schematic of the connections of the top system. Maybe is something wrong related with the connections...

[charlie n]

The coils have to be designed either for top or bottom feed & should be fed according to the manufacturer's design. I agree with Sandro that bottom feed can be better however properly designed top fed can work quite well. Defrost is often way faster on a top fed coil since the refrigerant is easily drained out during "pump down" & the hot gas enters an empty coil.

[That's too cold]

Make sure all the controls valves are working properly. Are there pressure regulators that need re-adjusting? You might have a Defrost Hot Gas valve leaking through into the coil.

[sterl]

Depending on where your pipe mains are located, and whether or not you have a heavy frost load on an HG arrangement: a top feed can be a blessing or a real pain in the neck. So these things accumulate frost or?

Actually Top Fed on a coil with horizontal headers and a low count of rows in the direction of air flow works quite well.

Its the location of the suction nozzle with respect to the mains that really counts...If defrosting is not involved, it makes no difference on a vertical liquid header whether the nozzle is at the top or at the bottom. In fact, for a so called Top Feed where the mains are lower than the suction nozzle and HG defrost is involved: it makes no difference whether the liquid nozzle is low or high except at the first seconds of hot gas injection.

BUT....A top fed with the mains higher than the outlet: the suction riser has to maintain enough velocity to keep the liquid headed up the hill.....so with frost on them and a reduced rate of vapor generation, they can become "logged" with liquid; and once they get that way it can take considerable time for them to clear even with the pipe detailing done right....Which would nominally include traps and a 4th pipe for the defrost condensate. In worst cases there will be need for managing the liquid delivery pressure as well.

And with the suction riser filled with liquid: The coil is working at a pressure considerably higher than suction, so it just gives up transferring heat...The temperature difference having reduced due to the high sat pressure in the coil.

With the mains lower than the suction: nozzles: both cases should be much the same while refrigerating. As indicated, the pump down for the Top Fed coils should be shorter, the coil volumes being equal.