Proper mounting of txv bulb

[Larry2]

I'm looking for some discussion of correct connection of the txv bulb to the suction line. I believe this should be achieved with copper straps and fully insulated. I found nylon cable ties fastening mine down. It seems like if you are interested in regulating refrigerant flow, one would make an intimate thermal connection from the bulb to the suction pipe. Many don't seem to think it is to important. What is the effect on superheat and system life for a poor installation? TIA.

Larry

[wambat]

poor preformance like wet return, low SH or flooded compressor, oil washout.

[Blackbird]

What he said &

Copper straps would be ideal, but in saying the nylon straps are still providing contact for the bulb as long as they are tight. I'm not sure how much heat (or lack of) is actually conducted by the copper straps. As long as the connection is tight and insulated and in the correct position, (ie not at the bottom of the suction line where oil in the line can insulate the refrigerant from the bulb) I would say it's OK.

Still it sounds a bit cheap since copper straps ARE provided with TX valves!!
Probably also more chance of nylon straps snapping - how many times i've had them break on me on cold mornings!

[Larry2]

Thanks! I have lower than expected superheat with the liquid line pressure on the mark for the chart table. It's a mix-match coil and condenser, so bets are off. But the coil should be large enough for the new gear. Air flow should be adequate too, but I guess that's the place to start.

I went to check things tonight and I was astonished to find the txv bulb tied lose with nylon ties. I know these come with thermally conductive copper straps and hardware, so this connection with nylon ties is only lazy conduct. They came with copper strap when I was twelve years old, so forty-five years later.... and last txv I bought two years ago came with brand new copper strap and hardware. I think many times, technicians think "good enough" when it's not.

Maybe it helped my system die early... Except this one died during winter. I ran a hose clamp down around things tonight. I'll replace it with copper strap pretty soon. It was fascinating to watch the thermocouple readings wander as I fastened the bulb down with a solid clamp. I hate to hook up gauges and lose any refrigerant but I need to measure superheat again.... Compressor discharge line felt a bit cool to the hand so I am concerned, but I have no experience on these higher efficiancy units.

[star882]

I also put a little thermal compound between the bulb and line.

[taz24]

When instaling the TEV I often wondered why somthing simpler is not used, instead of straps and clamps.
I often thought that if I installed a section of oversized pipe in the suction line directly after the evap (dependant on suction pipe size) with a short 7/8 fridge pipe brazed into and along the inside of the pipe. This would be a gas tight hole for the TEV phile to be slid inside and then total control would be achieved with out finicky clips and straps to use. Also TEV replacement would be easy because you would need to melt any ice in the plug but the phile would just slide in and out.

taz.

[taz24]

When instaling the TEV I often wondered why somthing simpler is not used, instead of straps and clamps.
I often thought that if I installed a section of oversized pipe in the suction line directly after the evap (dependant on suction pipe size) with a short 7/8 fridge pipe brazed into and along the inside of the pipe. This would be a gas tight hole for the TEV phile to be slid inside and then total control would be achieved with out finicky clips and straps to use. Also TEV replacement would be easy because you would need to melt any ice in the plug but the phile would just slide in and out.

taz.

Just thought on..
It would also work if the 7/8 pipe was brazed along the outside of the suction pipe thus removeing the need to increase the suction pipe to alow constant gas flow. but this would need to be insulated afterwards.

taz

[Larry2]

The bulb and the pipe are both round, or nearly so. Well the bulb is not wonderfully precise after the drawing process and brazing it went through in manufacturing. When you place two round items against each other, geometry implies there is very little surface area shared between the parts. What is the contact surface area shared between two cylindrical objects? Nil. Heat conduction from one item to another requires significant shared contact area.

That is where the copper straps come in. The straps are not just to hold the bulb in place. They made of are copper because copper is wonderfully conductive for heat. Copper straps contact the pipe and bulb with a large surface area. The straps provide a good heat path from the suction line to the bulb. They don't provide a perfect path because they are thin and there is some length associated with the connection. They provide a much better thermal connection that tangential touching and two nylon ties.

I measured a superheat of slightly over 6 degrees F with the nylon twist ties. They had not broken and they weren't hanging loose. They just weren't holding the part with much pressure and they provide no thermal path of their own.

With the bulb properly mounted with copper straps, I measured a superheat of 16.8 F.

Lets say the heat pump was set to heating mode and refrigerant was flowing through the indoor air handler at 100F/38C. The txv bulb reaches this temperature because it is against the pipe and warpped with insulation. It heat soaks long enough to reach this temperature after a run time.

Now lets say the system goes into defrost. The reversing valve sends liquid refrigerant to the indoor txv. The txv is wide open, metering gas at full throttle. Now, lets say the suction line temperature drops rapidly because expanded refrigerant reached the evaporator outlet pipe. The bulb doesn't receive that same temperature. It remains warm for a while because with tie wraps holding it on, it has a very poor thermal path. I is thermally isolated from the evaporator outlet. The wide open orifice floods the compressor. With enough cycles of this abuse, even a Copeland Scroll will throw in the towel.

The manufacturer packages the mounting straps with the TXV. It's part of their design and with thought out reason. How much time did the installer save when he threw out the copper strap and used a nylon tie?

Superheat is important to keeping liquid out of the suction port of the compressor.

I think tie-wrapped TXV belongs in the hall of shame.