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bernard
26-08-2004, 07:38 PM
Hello all

Can you use an externally equalised exp valve as an internally equalised one if you blank off the 1/4 port with cap ?

regards

Bernard

chemi-cool
26-08-2004, 07:48 PM
NO!


Chemi

bernard
26-08-2004, 07:52 PM
What do you think would happen if it was fitted

bernard

Dan
26-08-2004, 10:54 PM
You can, however use an externally equalized TEV to replace an internally equalized TEV. Simply connect the equalizing line to the evaporator outlet and it will operate fine.

shogun7
27-08-2004, 02:11 AM
The constant pressure or automatic expansion valve responds to outlet pressure. It meters liquid refrigerant to the evaporator, maintaining a constant evaporator pressure. Refrigerant flows at a rate that exactly matches compressor capacity, and the valve incorporates a diaphragm that separates atmospheric pressure and system pressure with a range spring with an adjustable pressure setting is located above the diaphragm.
Beneath the diaphragm is a stainless steel push rod and ball assembly backed by a closing spring with an o-ring located on the push rod creates a balanced port. This feature balances out the effect of increasing or decreasing inlet pressure, and aids in maintaining a constant outlet pressure. IMPORTANT..An equalizer passageway is incorporated into the valve body and this is what you donít have on a TXV. So if you insist on using a TXV you need to drill an equilizing passageway. This can be done but you need to talk to the mfg for the right size drill ;)

Blueboy
27-08-2004, 07:26 AM
I'm with Chemi

NO

Just get the right valve I mean £20 !!!???

There are three main forces in an ext equ. TEV the spring and the equalising line pressure to shut it and the bulb pressure to open it. The main closing force is the equalising line pressure.

GET THE RIGHT VALVE

frank
27-08-2004, 09:14 PM
Inspired by Dan's post, you'd be convenienced to remember that if the evaporator consists of multiple parallel circuits then it'd be fed by a distributor which can have its own pressure drop normally much greater than the evaporator's pressure drop, with this application type you MUST use an externally equalised valve.

I remember posting a reply in one thread stating that I was taught that a "rule of thumb" way to remember about TEV's was that if the system had a distributor then you should fit an externally equalised TEV. I also remember that this remark was not widely accepted as true :D

Peter_1
27-08-2004, 09:27 PM
I remember posting a reply in one thread stating that I was taught that a "rule of thumb" way to remember about TEV's was that if the system had a distributor then you should fit an externally equalised TEV. I also remember that this remark was not widely accepted as true :D
I do the same Frank, anyway installing a TEV with an eq. line can't harm never. It just cost a little bit more.

bernard
27-08-2004, 09:28 PM
hello blue boy

Its not a question of money,it was curiosity and to confirm something I,ve seen on site.

regards bernard

shogun7
28-08-2004, 07:57 AM
I remember posting a reply in one thread stating that I was taught that a "rule of thumb" way to remember about TEV's was that if the system had a distributor then you should fit an externally equalised TEV. I also remember that this remark was not widely accepted as true :D

Frank you are correct! In fact on an R-22 system distributer the pressure drop can be an average of 35 psi

Blueboy
28-08-2004, 08:03 AM
I remember posting a reply in one thread stating that I was taught that a "rule of thumb" way to remember about TEV's was that if the system had a distributor then you should fit an externally equalised TEV. I also remember that this remark was not widely accepted as true :D

I personally think it is a pretty good rule of thumb and one I have always found to be true!!

Blueboy
28-08-2004, 08:05 AM
OH I see Bernard I did not mean to apear bolshy. It does not change my answer however

Dan
28-08-2004, 01:07 PM
I remember posting a reply in one thread stating that I was taught that a "rule of thumb" way to remember about TEV's was that if the system had a distributor then you should fit an externally equalised TEV. I also remember that this remark was not widely accepted as true

Hmmm. Don't know why that would be so, Frank. It's actually a little more than a reule of thumb. Sporlan Bulletin on distributors:


When selecting a thermostatic expansion valve (TEV) for
use with a Sporlan type distributor, an externally equalized valve must be used to compensate for the distributor
pressure drop.
BULLETIN 20-10

Dan
28-08-2004, 01:28 PM
I think I understand, Marc. Your observations regarding hunting, and possibly flooding are very interesting and plausible, whereas, I would normally expect only evaporator starving to occur with an improperly applied internally equalized valve. I believe your friend's scenario is what happens when we try to compensate for that starving with TEV adjustments.

Prof Sporlan
28-08-2004, 02:57 PM
None the less, "we" (in the trade) often come across evaporators being fed by just 2, 3 or 4 distributor tubes which are fed off of a plain short straight length of pipe fitted without a distributor nozzle. These system sometimes come from the factory without externaly equalised TEV's.

These header arrangements generally do not create much pressure drop, and as a result, an internally equalized TEV can be expected to control a reasonable superheat with them. But that assumes the header is distributing refrigerant well, which will often not be the case. A hunting TEV can be expected.

If you have more than the equivalent of a 2įF pressure drop across the evaporator coil, your best bet is to use an externally equalized TEV. A multi-circuited evaporator with a conventional distributor will require an externally equalized

As the Prof recalls, Sporlan played around with internally equalized TEVs having thermostatic charges that were compensated for distributor pressure drop for Carrier in the 1960s. The problem with this approach is pressure drop across the distributor varies as the load varies, causing the TEV to flood at light loads and starve at high loads, making it not much more effective than a capillary tube.

Prof Sporlan
28-08-2004, 03:16 PM
See, even the distributor tubes themselves require to be fed equally and so a distributor nozzle is employed to establish even distribution to the distributor tubes. In the tested application mentioned above, where the total distributor assembly's pressure drop is 25psi we are likely to find that the distributor nozzle's pressure drop is 18psi while the remaining pressure drop through the tube(s) is 7psi. Now, if the internally equalised TEV is closing because it see's a low "apparent" superheat on account of distributor and evaporator pressure drops, it is likely to affect a system capacity loss well in excess of 15%. The resulting reduced nozzle pressure drop, responding in proportion to the square of the remaining capacity factor, can very easily result in uneven distribution to the distributors and so to the evaporator, making valve hunt likely and floodback possible.


If the distributor is sized for a 35 psi pressure drop at nominal conditions, the internally equalized TEV will invariably need to be adjusted all the way out to attempt to compensate for this pressure drop.

Starving can be expected. Since distributor pressure drop is roughly proportional to the square of the refrigerant mass flow as noted above, disrtibutor pressure drop can fall well below expected values which can lead to distribution problems, hunting, and possibly some flooding.

Peter_1
29-08-2004, 07:11 AM
Funny thing is, he turns over nearly as much as we do, the man has balls :)


Turning over a lot is not that difficult, it's what remains in the wallet that counts if I made myself clear.