Charging system with TEV

[ahl]

Hi :)

I have been looking on this site for information about correct charging and it has been very interesting.

I have been working on a Trane Chiller with two TEV’s feeding two coils in the same AHU. I have only the charge weight of the chiller and the system is split so need to add more for pipe work and coils, what is the best way to check that I have not under charged this unit?

Is it subcooling and superheat method if so what is the best way to check this?

Thanks

[GXMPLX]

I don't know your chiller so can give only general information.

If you don’t understand some of these terms, get a book and read it first.

If you have information on the charge and you split the unit, just add the refrigerant that fits in the volume of the liquid line. If lines are too long check how much fits in suction and add this amount. Use densities of refrigerants at saturation at high and low pressure.

If for some reason you modified the system, you can calculate add all the volume where there should be liquid in and multiply by density at saturated expected conditions then add something less than 10% for the suction. This gives you an approximation of the amount you should charge.

TEV require more charge at maximum load conditions, but this amount could be too much at minimum load so you have to keep a close watch because it is hard to predict when maximum and minimum occur.

Either

Calculate how much pressure drop and temperature raise you have between condenser receiver-TEV don’t forget to add for valves and vena contracta. Convert that into a saturated temperature difference. You may use the method of equivalent lengths which is easier.

Then charge measuring at the outlet of the receiver until you get the subcooling you need.

OR

Install temporary sight glasses at the inlet of the TEV until you get full hause. The pipe must go down to the TEV not up. The sight glass must be as close to the entrance as possible.
Charging at one condition does not mean you are done you have to check system at other load conditions and verify closely superheat at compressor during low load.

If your thermal load is too high in some situations check you never charge more than what fits in the sum of receiver and condenser volume. Your charge must be less.

[US Iceman]

I have been working on a Trane Chiller with two TEV’s feeding two coils in the same AHU.



I'm confused.:confused:

How can you have 2 TEV's feeding coils in the same AHU and this be a chiller?

Water chillers usually have the TEV's on the chiler itself and the water is pumped to the cooling coils in the AHU.

[ahl]

Sorry its a Trane DX split system not chilled water.

[US Iceman]

Sorry its a Trane DX split system not chilled water.


OK, that makes a lot more sense.

You don't charge by superheat. The expansion valves control that, not the refrigerant charge. Once the valves are properly adjusted, that's it.

Hopefully no one has adjusted these yet from the factory setting!:rolleyes:

Charging a system depends on many factors. GXMPLX provides some good introductory info on these.

[nevgee]

I don't know your chiller so can give only general information.

If you don’t understand some of these terms, get a book and read it first................
...........Calculate how much pressure drop and temperature raise you have between condenser receiver-TEV don’t forget to add for valves and vena contracta. Convert that into a saturated temperature difference. You may use the method of equivalent lengths which is easier.:confused:

Then charge measuring at the outlet of the receiver until you get the subcooling you need...................

................If your thermal load is too high in some situations check you never charge more than what fits in the sum of receiver and condenser volume. :eek: Your charge must be less.

:confused:
Maybe something got lost in the translation :D

[GXMPLX]

Maybe something got lost in the translation

Could be, but I think time constraints were more important! Anyway you are absolutely right to point this out. TKS!

What I wanted to say is that you also have a maximum charge you should not surpass.

Receiver should be sized so that ALL the refrigerant fits in 80% of the volume. (And we may not agree from here on because it is based on experience, not rules)…

…But I rarely find this sized well in the field, manufacturers tend to undersize equipment and PEOPLE tend to overcharge systems.

Thermal load too is usually larger than what originally the system was designed for. ( One bad excuse for overcharging).

In a perfect world none of this should happen!

What can you do then?

It’s hard to give a quantity but you should never charge more that what fits in the sum of the volume of the liquid receiver plus the condenser. It must be less, not equal!

Since the use of recovery equipment became widespread, charging more than what fits in the liquid receiver became less important, IF YOU DON’T HAVE INLET AND OUTLET VALVES. Specially don’t close both while the receiver is warm and full!

I find more systems with only one outlet valve in the receiver.

So now you can hit me with reason … ready!