Internal V's External Expansion Valves

[Latte]

Hi Guys,

Can some explain (In simple terms) the benifits betweer internal and external equallised expansion valves.

I have on a few occasions, to get myself out of trouble fitted an internal valve and soldered up the equaliser pipe to the suction and have not noticed any difference to the performance of the unit.

Surely, there is a technical reason for using external or internals but what is is

Regards

Fatboy

[rbartlett]

Where's andy when you need him?

this will give you all you need..

http://www.sporlan.com/10-9.pdf




The external line compensates for the pressure drop through the evap which is why it's on the outlet when if left out will effect superheat (P & T remember) and can lead to erratic metering and/or loss of performance esp on larger evaps

this is a subject in itself and more will follow for sure..correcting me as it goes...

cheers

richard

[wambat]

The major difference is the pressure drop in the evaporator, normally
If it exceeds 2# you would want to use an external equalized valve
An externally equalized TEV may be used for all
refrigeration applications. It provides no operational disad-
vantages with respect to an internally equalized valve other
than requiring an external equalizer line be connected.
With both internal and external evaporators, pressure is
transmitted to the underside of the
valve diaphragm by one of two methods. If the valve is
internally equalized, the evaporator pressure at the valve
outlet is transmitted to the diaphragm via a passageway
within the valve body or through a clearance around the
pushrods. If the valve is externally equalized, the under-
side of the valve diaphragm is isolated from the valve outlet
pressure by the use of packing material around the
pushrods or with pushrods which are closely fitted.
Evaporator pressure is transmitted to the diaphragm by a
tube connecting the suction line near the evaporator outlet
to an external fitting on the valve. The external fitting is
connected to a passageway which leads to the underside of
the valve diaphragm. For a full understanding go here:

http://www.sporlan.com/10-9.pdf

[Deejey]

Don't ever cap off an ext equalised tx valve and use it as an internal valve. With the internal equalised TX valve the pressure at the valve outlet (or evap inlet) is transmitted to the TX valve diaphragm via a passage way within the valve or through clearance around the push rods. On an externally equalised TX this pressure is isolated from the diaphragm. Instead the suction pressure is transmitted to the diaphragm via the external equaliser line. Cap off the external port and the valve does not sense the evap pressure.
Whenever an internal TX is used on an evap in which there is a pressure drop exceeding 20kPa then the pressure at the evap outlet will be less than the pressure operating under the diaphragm resulting in increased superheat with "starving of the evap." This is particularly severe when a distributor is fitted to the TX. Fitting an external TX prevents the evap being "starved"

[rbartlett]

many thanks

you are of course right

cheers

richard

[Deejey]

I knew you knew that Richard..you just probably had a hard day directing services there......

[rbartlett]

we just had a mini heatwave and the workload has gone through the roof..plus ..well you know the usual....oh and I just got baned from a forum and was trying to find out why :-/

anyway many thanks for highlighting my stupidity ;-)

cheers

richard

[Deejey]

well you corrected your post so I did mine too...what forum you been banned from....

[rbartlett]

I'm well into the old conspiracy stuff and david icke banned me ;-))

cheers

richard

[Temprite]

Once saw a problem concerning this.

Coolroom in a bakery had an old R12 condensing unit connected to a Buffalo Trident evaporator. TX valve was internally equalized and working fine for many years.

Compressor burnt out and was replaced with a new hermitique R22 condensing unit and the installing engineer replaced the TX with an internally equalized R22 type.Unit was cooling but not properly

No matter what he did he could not get it to feed properly.Even went to the extreme of putting a much larger than normal orifice in it.I came down and checked the super heat, room temp about 5 degrees C.Super heat was about 14 degrees at bulb of TX.

After consulting the suppliers of these evaporators discovered that upon commisioning you have a choice of two orifices R12 and R22. These extra orifices are installed into the distributor and because of originally being R12 this was causing the R22 internal valve not to feed correctly.

Installed external R22 valve and correct TX valve orifice.No problems superheat perfect.