Evap losing capacity

[chillyblue]

Hi

Hope you can help, i have a freezer which is operating with R404a and has two cappillaries both going into the liquid inlet of a distributor of a evaporator,there are 6 lines coming out of the distributor (not sure but can you use cappillaries feeding a liquid distributor???? i think not, but it has been operating for a number of years, but not very well!! always had problems, i'm tempted to change it to TEV, what do you think???)the cappillaries both enter the same pipe feeding the liquid distributor, the coil is only icing the bottom half of the coil, top halve totally clear, it is using hot gas defrost and when it defrost it clears the ice on the bottom, but will start frosting backup again as soon as it goes back onto refrigeration.
Because the evap is only operating at half cappacity the room temp is high and the suction pressure is low causing it to ice up quiker
I thought of cutting into the lines and blowing them through but because it has a liquid distributor you can't really do that.
Any ideas

Cheers

CB

[frank]

An externally equalised TEV should be fitted with a distributor.

All is explained here (http://www.sporlan.com/10-9.pdf). Happy reading

[chillyblue]

An externally equalised TEV should be fitted with a distributor.

All is explained here (http://www.sporlan.com/10-9.pdf). Happy reading

Hi Frank

thanks for the reponse and i do realise that an externally equilized expansion valve should be fitted to a system with a distributor, and i would also do that. someone seems to have modified this and put cappillaries on and taken the TEV off (don't know why??)
the main reason for my concern is that only the bottom half of the evaporator is freezing up. Why is there no distribution to the top section???:confused:.
is it because it is blocked?? ( there are 3 distributor lines going to the top section) is it poor distribution??
is it because of the cappillaries?? could one cappillary be blocked, but why the bottom half of the coil?

Cheers CB

[lana]

Hi there,

Is the distributor placed horizontally? If so then the reason is obvious. More liquid is going into the bottom tubes of the distributor.

If the distributor is placed vertically (feeding from top to bottom) then the problem may be from the cap tubes connected to the distributor. Again it is feeding more liquid into the some of the distributor tubes (which goes to the bottom of the coil).

Cheers

[frank]

Are the capillaries all the same length?

Could be that the charge is not correct now that capillaries are installed - remember that a system operating with a fixed orifice (capillary) metering device has to be critically charged as it is not able to adjust to varying evaporator loads.

Could also be that the cap tubes have been partially blocked during brazing

[chillyblue]

Cheers, thanks for the help

Lana, no i have already checked that the distributor is vertical.

Frank, i have charged the system as i think, a manufacture charge does not exist as it is not a standard system (been modified from original design)
I have been this morning and the temperature is right down, but i think because it is on cappillaries it needs nursing down because it can't adjust to varying load like a TEV can, and i assume that is why TEV's are used becasue they can adjust to system load, where as a cappillary is a fixed load??

Cheeers

CB

[lana]

Hi there,

If I may a little correction, I hope you don't mind.

TEV does not respond to load or better say does not do capacity control. TEV responds to super heat.
When TEV closes then less refrigerant goes into the evaporator and YES the capacity of the evaporator decreases BUT BUT the response is for super heat not the load.
System capacity control can be done only with the compressor.

As you mentioned cap tube systems need special attention. TEV systems are more simple (IMO!!). In TEV system we are in Charge but in cap tube system, CAP TUBE is!!!!.

Cheers

[John Hunter]

Hi. Fixed orfice set ups are a problem when installed in duty situation where the heat load has a high diversity, ie low/high load variation. The reason the evaporator is appearing to Ice up in the lower sections and the compressor cycling low is an indication of refrigerant starvation. If it was my job I would determine what heat load requirements are , check the evaporators design capacity to handle then select a TEV to suit. The instalation would be as practice for a pressure balanced TEV instalation , single liquid suppply with soln v;v distributer to coil circuits etc and the super heat sensor correctly located on the suction line. Set the superheat to be no more than 5K (10F) Cold store to 8K 15F Air Cond loading at high load.

[750 Valve]

Hi

Hope you can help, i have a freezer which is operating with R404a and has two cappillaries both going into the liquid inlet of a distributor of a evaporator,there are 6 lines coming out of the distributor (not sure but can you use cappillaries feeding a liquid distributor????



G'day mate, its amazing what people will try to do! :confused: The bottom distributor tube is filling with more liquid because of our little friend gravity ;) and not enough velocity, like you suspected - NO it won't work, yes it will get cold and yes it will clear on defrost but this does not mean its working properly. Whack that TXV on.

[Samarjit Sen]

Size the cooling load and based on the same get an Externally equiliser TXV. You will also have to change the Distributor. Get the matching distributor with the correct size of the orifice and also size properly the distributor tubes. You can all these details from the Sporlan Bulletin on distributor. I would further suggest that while you are changing the distributor and the tubes, please flush the system specially the coil for removing any oil that may be located there.

[chillyblue]

Thanks for all your help everybody, i am fitting a externally eqalized TEV tomorrow.
I have one more question, because the reciever has also been removed i am having to fit a new one and it looks as if the positon of the existing one was fitted above the condensor block (the liquid will have to go vertically up into the reciever), will this cause any problems with the condenser draining liquid into the reciever.??? and cause the condenser to fill and the head pressure to rise before flowing out.???

P.S.
Cheers 750 valve, your answer re the velocity was what i was after (the reason for the uneven dristribution).

Cheers

CB

[Pooh]

CB
slightly confused as to your initial comment that the plant has worked for several years OK but now is icing the bottom of the coil, my qusetion is what has changed?, was the system always on R404a or has it been retrofitted. From what you say the capillary lines are not ideal but it worked before. Just interested as I like mysteries. I had a similar situation a while back due to a R404a system that had been charged with vapour instead of liquid, although the glide is minimal with R404a the evaporator on iced at the bottom and the room struggled down to temp.

Ian

[John Hunter]

Hi
I will always locate a reciever below the condenser, it provides good fluid flow, refrigerant and oil . If the reciever is above the condenser the liquid line has to be trapped to create lift, the condenser is then not free draining. Note some times this is done to create subcooling but the condenser set up has to designed for this. If the reciever is above the condenser a balance line between the condenser out let to the reciever should be fitted.

[chillyblue]

Hi Ian

Basically the room has always coped!! and i mean coped!! but also has always be a pain in the rear. No it hasn't been retrofitted and has always been on R404A.
it's just one of those things that when you leave site you know that its working but you also have a little thought at the back of your mind that its not right.
No one else who has looked at it has had the Balls to stand up and say what is required, and i have decided that it is time i should, just wanted to get my facts right.

Cheers CB