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lana
20-11-2005, 06:58 AM
Hi there,

Has anybody designed or worked on a DX system without a receiver?
I have once worked on one but I had difficulty charging the system properly because the sight glass was full a minute then it was empty. The condenser acts as a receiver and I think it takes time for the liquid to accumulate in the condenser and reach the TEV.
I would appreciate any suggestion.
Cheers:)

Rfcont
20-11-2005, 09:17 AM
Hi Lana!
It can be without reciver. We use a resiver with remote condenser as a rule.
I think you have problem with TEV.
Do you have air or water condenser?

Temprite
20-11-2005, 11:20 AM
G'day lana

You could always put an oversize dryer on it. That will hold some liquid.

Peter_1
20-11-2005, 10:14 PM
Carrier has standard chillers of +/- 400 kW and even bigger (don't have the right figures here with me), air cooled condensers, EEV's and without receiver.

The theory behind it is when at full load, maximum refrigerant is flowing and almost no liquid is on that moment in the condenser.
As soon you start at part load condition, you don't need the full condenser area anymore (compressor at part load) and you can use the remaining condenser area for storing the liquid and even subcool it this way.

US Iceman
21-11-2005, 01:24 AM
The theory behind it is when at full load, maximum refrigerant is flowing and almost no liquid is on that moment in the condenser.
As soon you start at part load condition, you don't need the full condenser area anymore (compressor at part load) and you can use the remaining condenser area for storing the liquid and even subcool it this way.

I have seen the same thing occur on DX water chillers with water-cooled condensers and no receivers. The water-cooled condenser provides the subcooling and pump down volume for the system. This is approaching a critical charge system in my opinion, so the refrigerant charge should be evaluated carefully.

lana
21-11-2005, 06:09 AM
Hi everybody,
Thanks for your replies.
If the system has a water-cooled condenser then it can act as a receiver and as the size goes up then it will have more volume available to act as a receiver.
In the Zurich University there is a huge heat pump system with water-cooled condenser. The condenser is so big that it can act as 100 receivers.:D
But in the air-cooled systems the condenser may not have enough volume available and also if there is a pump-down-control then the problem becomes more complicated. As US Iceman mentioned, the refrigerant charge will be very critical.
Cheers.:)

Peter_1
21-11-2005, 10:29 AM
You have to charge it almost as carefully as a system with a capillary.
Monitor HP and LP, SC and SH very close and cover the condenser with plastic foil if outside temperatures are low.
We once had troubles with a Carrier chiller with the EEV's standard mounted on it because there was flashgas and the controller had problems with it.

We throw them out and replaced it with TEV's and problems were gone.

TXiceman
23-11-2005, 08:10 PM
If you are using an air cooled condeser with low ambient flod abck controls, you will have to utilize a receiver to to handle the flood charge for the condenser.

If you want to get all of the subcooling and not loose it, you need to use what is termed a "surge receiver". Typically, a system will use a "through-flow receiver".

Your home A/C is a critically charged sytem and has no receiver. The critically charge system can be either A/C or water cooled and has it's place in the right application. In a larger system you will use either a high side control and dump all of the liquid to a flooded evap or use a low side float control. Thes systems generally work on a steady state load and do not react well to a sudden load swing.

Ken

Peter_1
27-11-2005, 10:54 PM
The condenser carries more liquid with greater subcool during higher loads and/or higher ambients. At lower loads and/or lower ambients the evaporator carries more liquid while subcool drops to a minimum. This is the principle from which the SPR valve in an Enviroguard system is derived.

This need some more explantion for me - even a lot more: lower evaporator load means more liquid in the evaporator. :confused:

US Iceman
28-11-2005, 04:55 PM
Flooded or liquid overfeed coils do not have the inherent flow control ability to adjust to varying heat fluxes. For this reason, the percentage of liquid volume will increase at low loads in these coils.

The mass flow rate through a flooded coil is dependent on the heat flux applied to the coil. More refrigerant circulates during high loads, than does during low loads.

A liquid overfeed coil has no "adjustment" for the mass flow rate at all. The hand expansion valve is set for some flow rate, which adjusts the liquid feed pressure to the coil. Mass flow through these coils does not change due to different heat fluxes.

The volume of liquid refrigerant in the evaporators is simply a function of the amount of heat applied to the heat transfer surface (heat flux). When higher heat fluxes occur, the boiling action increases, therefore a greater volume of vapor is produced.

A flooded or liquid overfeed coil will simply fill up with liquid at part load, or no load conditions.

A DX evaporator can also tend to carry more liquid in the coil, if the coil has been operating at no load conditions for a long time. This could also mean a severely frosted coil.

The TXV is only controlling evaporator superheat. During low load conditions liquid can "stack up" in the coil. This normally occurs when large evaporators use a single TXV in my experience.

With no receiver and at part load conditions (with capacity control), the liquid will back up into the condenser. The subcooling may be greater, but the discharge pressure will not increase drastically, since the heat rejection requirements have decreased.


The condenser carries more liquid with greater subcool during higher loads and/or higher ambients.

This implies the discharge pressure will increase and that this is acceptable???