little superheat on txv controlled system

[resolutetech]

yesterday on a carrier heat pump i had delta t of 12. so i open ahu and see wheather i have a piston or txv. i see txv so i know im charging using sub cooling. i get my fluke temp meter and 2 clamp on thermometers hook up guages and see no sub cooling. i add 3 lb of 22. and get my subcooling to 11 degrees(marked as correct subcooling on odu data plate) and then i notice superheat of about 3 degrees. i know this should be higher if the valve is controlling superheat. im in boonies and cell wont work so cant call boss even if he would pick up on fri evening. i stop charging and can only think the txv is open. for some reason. the valve is closed by spring pressure and the external equaliser. it is opened by bulb pressure. could the bulb be off the suction line or stuck open. what else could cause this.

[HVAC Pro]

It could be sticking open or there could be a stuck check valve if has an external one.

[resolutetech]

can a stuck check valve be unstuck by energizing and deenergizing the reversing valve to sort of shake it?

[dnt]

How do we know that the ambient dry bulb and evap inlet wet bulb do not require this amount of superheat you know that TXV's adjust for those conditions if you have three you are probably good what is your delta T after charging to subcooling? Also go back and check your temp probe connection sand the line clean/dry it and reclamp your probe you may not have a problem at all, other than not getting a accurate reading.

[resolutetech]

delta t after charging was 20 and wet bulb about 70 ambient about 72

[davidr]

Don't mean to ask a stupid question but was the TXV's sensing bulb insulated & clamped tightly to the suction line?

[sadlier]

Originally posted by dnt
How do we know that the ambient dry bulb and evap inlet wet bulb do not require this amount of superheat you know that TXV's adjust for those conditions...

Please expound on this. How do they sense or adjust for the ambient dry bulb or wet bulb when their operation is based on: (1) the suction line temperature converted to a pressure; (2) the suction pressure acting opposite to (1); and (3) a spring?

[Shophound]

Please expound on this. How do they sense or adjust for the ambient dry bulb or wet bulb when their operation is based on: (1) the suction line temperature converted to a pressure; (2) the suction pressure acting opposite to (1); and (3) a spring?

Have you noticed that when the conditioned space has a high humidity level, the delta T across the evap is lower (especially with piston systems)? This is due to latent heat. While not a form of heat that causes a change in temperature, it's still just as real and is absorbed by the refrigerant when it boils.

A coil with a high latent heat load will likely run at a higher pressure with normal airflow across the coil. A high latent load means the coil isn't removing as much sensible, so the entering air temperature remains higher until the conditioned space's humidity levels are lowered to where the latent/sensible ratio tips toward sensible.
It is true the bulb on the suction line for the TXV is reacting to sensible heat. However, the higher latent load translates to higher operating pressure, which means a warmer coil with warmer refrigerant.

With a piston, warm ambient outdoors combined with high heat and humidity load indoors translates to a higher mass flow rate through the evap. This is why piston superheat can vary much more than with a TXV with variant conditions.
With a TXV, which has two forces to oppose the opening force of bulb pressure, the evap superheat is more consistent, but a high latent and sensible load would tend to open the valve and allow more refrigerant in, thus a lower superheat.

It could also be, that if the charge is correct and all other factors are normal, someone before you jacked with the TXV spring pressure and now you've inherited it.

[HVAC Pro]

Very nice explanation shophound.

[davidr]

Originally posted by shophound
With a TXV, which has two forces to oppose the opening force of bulb pressure, the evap superheat is more consistent, but a high latent and sensible load would tend to open the valve and allow more refrigerant in, thus a lower superheat.

I was always under the impression that a TXV under a high latent/sensible load would open the needle all the way open.
Then it would operate at a higher superheat because the refrigerant was boiling off so fast in the evaporator due to load conditions being high.

Am I wrong on this?
If so please correct me.

BTW,congratulations on the new job shophound!

[tinknocker service tech]

shop hound very imformative and well explained

[Shophound]

Originally posted by davidr

Originally posted by shophound
With a TXV, which has two forces to oppose the opening force of bulb pressure, the evap superheat is more consistent, but a high latent and sensible load would tend to open the valve and allow more refrigerant in, thus a lower superheat.

I was always under the impression that a TXV under a high latent/sensible load would open the needle all the way open.
Then it would operate at a higher superheat because the refrigerant was boiling off so fast in the evaporator due to load conditions being high.

Am I wrong on this?
If so please correct me.

BTW,congratulations on the new job shophound!

Davidr, you make a good point. Perhaps when the system with a TXV is started and the indoor conditions are very high both sensible and latent, you'd see a TXV wide open and a corresponding higher superheat.
I was thinking more along the lines of a system that's had time to stabilize and is maintaining design conditions (hot/humid day in many areas) indoors.

As the TXV opens and admits more refrigerant, the mass flow rate through the coil increases, resulting in less time the refrigerant is in the coil to pick up heat. This would tend to result in a lower evaporator superheat. Keep in mind we're discussing superheat measured at the sensing bulb (as you likely know) vs. superheat measured at the compressor.
As the TXV throttles closed, the mass flow rate decreases, allowing the refrigerant to have more contact time in the coil to pick up heat, plus more superheat once the vapor is completely boiled off.

Thanks for the congrats! This job will be a big change from where I am now but I'm eagerly looking forward to it. I'm about to get fully submerged into the world of 21st Century HVAC big time!

Thanks to all for the kind words.

[bananaboy]

"...As the TXV opens and admits more refrigerant, the mass flow rate through the coil increases, resulting in less time the refrigerant is in the coil to pick up heat. ..."

I thought that the speed (of velocity) of the air flow accross the evap. coil would determine the absorbtion of the latent/sensible heat. Not the mass flow rate of refrigerant.

Example: if the blower is at lower speed - it'll have more time to pick up latent and condense it on the fins, whereis the highest speed will not have as much time to do it, resulting in a cooler (sooner) but still humid environment.

Or may be I am missing something.