Frosting in an evaporator

[Friobernal]

hi;

Which could be the main reason for a fast frosting in the lowest part of the coil of an evaporator (hot gas defrost R404A)?

Regards

[Friobernal]

I´ve been "touching" everything and i have realised that in this evaporator the hot gas defrost is done through a collector (or manifold, i don´t know the right word) this manifold is for avoiding pressure drop of hot gas passing through distributor, but i´ve seen that the lowest part of this manifold (the one for hot gas) is full of liquid, that is flooding the lower tubes, so the bulb is detecting liquid in suction line and closes TXV, but the tubes in the high part of the evap are not working. This is my theory.

Does anybody have a similar experience, or any comment?

[harbertor]

Hi, Marc

Which could be the reasons?

Is a problem of the design of the evaporators or a wrong installation?

Which would be the best solution from your point of view?

Bye

[Dan]

I´ve been "touching" everything and i have realised that in this evaporator the hot gas defrost is done through a collector (or manifold, i don´t know the right word) this manifold is for avoiding pressure drop of hot gas passing through distributor,

I am having difficulty picturing this manifold. What I am used to seeing is a bypass around the TEV (through a check valve) but not around the distributor. I feel sure that Marc is correct that you have a distribution problem, but I am hung up on picturing this manifold and how it affects refrigerant distribution during refrigeration.. as opposed to during defrost.

[harbertor]

Hi Marc

I´m Friobernal, writting from a friend´s nick.

As I been told, the situation is the following right now:

* The installer has cut the manifold of hot gas defrost, and has only a suction manifold, so the problem is other.

* The installer says that he begins a refrigeration cycle with the coil completely clean, and he sees a quickly formation of frost in the lower tubes, there is a maldistribution (or uneven distribution)
obviously.

* There are two evaporators in the room with the same situation, so it shouldn´t be a problem of distributors of the evaporators.

* The TXV has been changed, so it isn´t sub-dimensioned, and there is enough refrigerant

* The compressor has been operating with just one evaporator and it hasn´t change the situation

* The installer insists, that it isn´t a problem of hot gas defrost, because he does a "pump down" after and before the defrosting.


Have you got an idea of possible reasons?

[Peter_1]

It should be a lot easier if you could draw a schematic of the real situation (manifold, complete defrost cycle tubing including valves) and perhaps some pictures of the manifold and the frost.
Just a suggestion: there is maybe a lot of liquid and oil remaining in the evaporator after a defrost, which causes the evaporator to freeze for a short time after defrosting. Is it in a cabinet or a welk-in freezer?

[harbertor]

Hi,

About this problem i have more data and a theory, let me know what do you think?

* The distance between the fan motors (diameter 560mm), and the front wall is 3,8 m. and the air throw of this evaporators is 40 m.

* Volume of the room = 230 m3

* Air flow (each evaporator 22900 m3/h), so 200 air recirculations!!

* I think that air is recirculating on the top of the room and is too much air that is not exchanging heat to the refrigerating

* Finally the evaporators, has been changed for others with 13000 m3/h (each one) and the problem is solving

What do you think? What had happened?