Original message
| youngie
|  "TXv position"
, posted Thu 9 Oct 19:39  
I am working on a drill ship and I have just been checking out the refrigeration system on board. The TX valves for the all the coldrooms are on a common suction and are controlled by seperate thermostats. The TXV are all outside the rooms and are insulated about 10 - 15 ft away from the evaporators. I have seen this type of installation before and I have not been happy with the performance of the equipment. The plant arranged this way was always freezing up at the evaporator and because the TX was easy to get to it was always being adjusted.[not good practice] All the marine engineers say this is the correct place for the valves. I disagree, as I have always been under the assumption you need the TX close to the evaporator and the bulb placement on the horizontal at the 4O'clock position. Can you please advise me if it is OK to remote mount the TXV as has been done on this installation. Many Thanks
Ian Young
If at first you don't succeed- suck eggs they taste better
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Replies:
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| Jojo98
| "Re(1):TXv position" , posted Thu 9 Oct 23:03
10 ft from the evaporator is a long distance for a metering device. That's 10 ft of pipe that the refrigerant flashes to vapor prior to entering the coil. Where is the bulb in reference to distance from the coil?
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| | youngie
| "Re(2):TXv position" , posted Fri 10 Oct 00:10
The bulb is at the same distance, maybe 1 foot closer. This has ben my contention all along. The engineers will then say that the gas will not flash because of the pipe insulation. I am not convinced and I need to be able to prove it by quoting facts and figures. I think the valve should be in the room near the evaporator with the valve in the horizontal plane and firmly affixed to the suction line. This also will make the equalization tube shorter as well.
Ian Young
If at first you don't succeed- suck eggs they taste better
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| | Jojo98
| "Re(3):TXv position" , posted Fri 10 Oct 23:31
The liquid refrigerant flashes to vapor because of the pressuse differential between the high side of the TXV and the low side of the TXV. For the refrigerant to stay in a state of a saturated liquid, refrigerant pressure would have to increase once it passed through the TXV; becauce the the distributor piping is much larger in diameter than the liquid line.
Also, the further away the TXV bulb is from the evaporator, the greater amount of time it will take for the TXV to respond to the temperature changes at the evaporator.
I can not recall EVER seeing a TXV more than a foot from the distibutor, let alone ten feet.
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| | Schock Therapy
| "Re(4):TXv position" , posted Sat 11 Oct 00:51:
I can't say that I have ever seen the TXV that far from the coil, but at the same time it probably doesn't matter that much. The engineers are correct, as long as the pipe between the TXV and the distributor is well insulated, the low pressure liquid will not flash excessively. Remember, the state change requires heat as well as pressure drop. Many ductless split a/c units have their metering devices in the outdoor unit, and run a low pressure liquid line for the whole length of the line-set, and they work just fine. The TX bulb location doesn't matter that much either, as long as it is on the suction line for the coil served by the valve. It's only function is to control superheat in the refrigerant vapor leaving the coil. The bulb can be positioned anywhere but 6 o'clock or 12 o'clock. I prefer 9 or 3 myself. If it is outside the conditioned space, it should be insulated. It should also be mounted close to the equalizer connection.
I would say the biggest problem with the TXVs in that system is that people are playing with them like they are operating controls. They should be set up for 10-15 degrees superheat, and then left alone!
It's only a shock if your not expecting it!
[this message was edited by Schock Therapy on Sat 11 Oct 00:54] |
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