Frozen TXV

[jwasir]

We installed packaged chiller on the roof. Ambient was -30C and suction dropped. It was found that after heating the power element of the TXV, the pressure rose.

Is anyone aware of this kind of siuation?

How can we keep TXV element and capillary warm enough that the charge inside won't liquify?

[Swampy]

What are you trying to chill at an ambient of -30?

[Brian_UK]

Build an insulated box around the TXV - but - as the man asked --

Why do you have a chiller running with those ambients - or - are you just commissioning ?

[BESC5240]

Hi,
since this is a chiller, I presume the TXV is one with MOP.
MOP valves have a limited charge. If the charge has migrated from the bulb to the valve top, a raise in temperature at the bulb (superheat raise) will not open the valve, because the is no liquid left in the bulb who could produce a pressure increase above the diaphragm. Result : the valve will stay closed.
Heating up the thermostatic element is the right thing to do, to let the charge re-migrate towards the bulb.

An insulated box around the TXV will prevent this charge migration to the valve top. But be careful: during very warm periods it could result in a the same effect. If the valve is better insulated then the bulb, it will be colder in the valve then at the bulb (at standstill) ... resulting in a charge migration ...
It all depends on the situation. A selfregulating flexibel heating (combined with some insulation?) could be a solution.

[jwasir]

Thanks BESC5240,

Yes, it has MOP valve.
We'll change the power element.
Do we still need insualtion with the regular charge bulb?

This is a application of packaged medical chiller sitting on the roof and chilling liquid for MRI cooling.

Thanks to all.

[nike123]

Thanks BESC5240,

Yes, it has MOP valve.
We'll change the power element.
Do we still need insualtion with the regular charge bulb?

This is a application of packaged medical chiller sitting on the roof and chilling liquid for MRI cooling.

Thanks to all.

Than it is better to install some free cooling coil. It could save your customer up to 75% of energy bill and prolong machine life. Also, you could earn additional money on upgrade.

Check this pdf!

P.S. I don't advertise this company products, I only illustrating what I mean.

[BESC5240]

Yes, it has MOP valve.
We'll change the power element.

Be careful. The chiller manufacturer did install the MOPvalves for a reason (compressor protection at high suction pressure). If you replace them by standard thermostatic elements, the compressor(s) is (are) no longer protected ...

Do we still need insualtion with the regular charge bulb?

Sorry, but I have no experience on ambient temperatures of -30°C ..

[Gary]

We installed packaged chiller on the roof. Ambient was -30C and suction dropped.

So what? Why is this a problem?

[Gary]

This is a application of packaged medical chiller sitting on the roof and chilling liquid for MRI cooling.

What temperature are you chilling the liquid to?

[Gary]

Thanks BESC5240,

Yes, it has MOP valve.
We'll change the power element.

MOP has nothing to do with it. Do not change the power element.

[jwasir]

Gary,

This is a Medical Chiller used to cool MRI magnet.

The Glycol temp. used is cooled @ 48F.

So you think changing the power element will not change the scenario? Any other suggestions?

Thanks,

[Gary]

Running the chiller in low ambients just doesn't make sense. A free cooler loop is the answer.

If you absolutely must run the chiller, make sure the TXV bulb is mounted with the cap tube up and heavily insulate both the TXV and the bulb.

[wambat]

Wrap it with prestite tape as Gary said

[Marc O'Brien]

In -30 degree conditions with an insulated evaporator assembly being warmed by 48 degree fluid there will be TEV charge migration and especially with a MOP system.

Sporlan produced a TEV assembly where the TEV-side power assembly was oil driven being connected by an oil filled capillary to a gas driven power element mounted right on top of the sensing bulb. This would be ideal in this circumstance.

Wrapping the TEV power assembly with a self regulating heater tape would be a quick and effective solution.

Better yet, install a hysave liquid pump in the liquid line, install a bypass system round the compressor and turn the system into a 100/1 COP pump assisted thermosyphon package - during low ambients.

[Slatts]

if you do decide to replace the MOP valve, you might want to look at fitting a crankcase pressure regulating valve. I'm not a big fan of MOPs and would go for a CPR valve by preference.
That free cooling loop sounds like a great idea if properly implemented. It's not something I see round here with our cold winter nights rarely getting below 5C.

[Gary]

Hi Marc

In -30 degree conditions with an insulated evaporator assembly being warmed by 48 degree fluid there will be TEV charge migration and especially with a MOP system.

Sporlan produced a TEV assembly where the TEV-side power assembly was oil driven being connected by an oil filled capillary to a gas driven power element mounted right on top of the sensing bulb. This would be ideal in this circumstance.

Wrapping the TEV power assembly with a self regulating heater tape would be a quick and effective solution.

Better yet, install a hysave liquid pump in the liquid line, install a bypass system round the compressor and turn the system into a 100/1 COP pump assisted thermosyphon package - during low ambients.

If the power head is colder than the bulb, then the charge will migrate to the power head, whether the charge is MOP or not.

But yes... a small heater on the power head would ensure that it is never colder than the bulb.

When using the liquid pump, would the compressor bypass be needed? Wouldn't the pressure difference be sufficient to lift the suction and discharge reeds?

[US Iceman]

When using the liquid pump, would the compressor bypass be needed? Wouldn't the pressure difference be sufficient to lift the suction and discharge reeds?

You need the bypass to reduce the pressure difference. By having a low pressure loss you allow the refrigerant to move from the evaporator to the condenser with minimal impacts.

If the pressure in the evaporator rises too much to overcome the valves, etc you limit the system capacity at the ambient conditions.

[Gary]

You need the bypass to reduce the pressure difference. By having a low pressure loss you allow the refrigerant to move from the evaporator to the condenser with minimal impacts.

If the pressure in the evaporator rises too much to overcome the valves, etc you limit the system capacity at the ambient conditions.

The bypass then, would consist of a pipe from suction to discharge with a check valve in it?

Would the liquid pump be necessary for it to work?

[US Iceman]

Would the liquid pump be necessary for it to work?

Yes, if the evaporator is above the condenser. You need some way to get the liquid up to the evaporator, so that it can boil.

Otherwise, you need to be very careful in planning the piping for adequate liquid delivery to the evaporator if it is located below the condenser.

Thermosyphon system design is not similar to normal refrigeration piping design. You need to minimize pressure loss!

You can use a check valve or an electrically powered high performance butterfly valve.

[Marc O'Brien]

Hi Marc



If the power head is colder than the bulb, then the charge will migrate to the power head, whether the charge is MOP or not.

But yes... a small heater on the power head would ensure that it is never colder than the bulb.

When using the liquid pump, would the compressor bypass be needed? Wouldn't the pressure difference be sufficient to lift the suction and discharge reeds?

Often with a non mop valve at least some charge remains within the bulb.

You do need a bypass round the compressor because you're especially using the pump to create a liquid over-feed effect. The concept is more like pump assisted heat pipes than thermosiphon. Heat Pipes involve phase change and Thermosiphon does not. The pump creates an overfeed in order that heat exchanger U values can be raised by about 30%. With this increase in U value the heat exchangers can perform the designed load with just 76% of the design TD.

Chilled water temperatures are regulated by cycling the condenser fans.

Ordinarily condensers in low ambient with all their fans off can still achieve 30% capacity just on natural convection up through the coil. In this case the temperature difference between condensing liquid and air is so slight that cycling the fans almost entirely unloads the system without any massive pressure fluctuations.

I have attached a design I have been working on - the illustration is the most basic of the idea.

There are only two non return valves and no solenoid or motorised valves. The accumulator sits above the condenser. The compressor can start at anytime it wishes without any preparation cycles.

You saw it here first :-)

[Marc O'Brien]

Keep in mind too that my design concept would require a receiver for use in warmer ambients to store the excess refrigerant only used during free cooling over-feed operation and cool ambient floating head operation.

[SteinarN]

Interesting system Marc. How do you avoid the compressor filling with some liqiud when the low ambient mode is active? By placing the compressor as the highest component and cranckcase heater?

[Gary]

What's a Kevin Pieterson?

[US Iceman]

You saw it here first :-)

Maybe.... I have a design like this going in right now, so you are late and second.