TEV selection

[Lc_shi]

Hi Sir,
may be a silly quesion?:o
TEV selection may be a easy work. The general procedure is select TEV according to capacity at specified Te and calculate pressure drop along it and leave 15~20% buffer. I'm not sure if select capacity is 20% more than nominal ,e.g. 40% ,how it will affect the refrigeration system,could it cause liquid slug?

rgds
LC:)

[Cofreth]

I guess it will cause excessive flash gas at the inlet of the evaporator coil thus rendering an efficiency drop on the evaporator.You may have difficulty to achieve a clear liquid sight glass.

[Peter_1]

What you will see Lichuan - but you probably know this already - is hunting of the valve.

You will never reach a stable situation.
Liquid slugging... I think it all depends how fast the valve is reacting (large or a small sensing bulb), how big it was oversized (20% by 50%), working conditions (a higher condensing pressure starts hunting faster), state of the liquid fed to the TEV...

You still have the suction line to vaporise the rest of the not vaporised liquid or to superheat it.

You can compare it to a TEV with a too large orifice mounted in it.

If you search on hunting TEV, it will help you already a lot further.

One of them I found after Googling
http://www.cjche.ca/journal/issues/aug98/pg722.pdf

[US Iceman]

Hi LC,

The valve capacity is determined by the orifice size, pressure difference across the orifice, and the amount of liquid subcooling entering the TXV.

From my experience I have had better results with the TXV sized for the exact load, or slightly smaller. I have found that a slightly smaller TXV capacity provides better control (less hunting). This is true with most control valves.

The valves also have a certain turn-down range. Turn-down is the capacity reduction of a control valve.

For most TXV's I would consider the minimum capacity of the valves to be about 50% of the rated condition. Balanced port TXV's are a little better I believe, but I normally use 50% for all valves to be safe.

If the refrigeration system has capacity control increments below 50%, two expansion valves should be used to provide better control (less hunting).

Valves that are over sized will simply not work as well.

Most refrigeration systems have to operate in varying conditions,.i.e., hot and cold ambient. The TXV should be selected to provide the capacity at the varying conditions.

One design point is not sufficient. What I like to do is look at the range of operating conditions and then select the TXV that works the best.

Also verify the amount of subcooling. If a mechanical subcooling system is used, it may be easy to generate liquid refrigerant temperatures below the evaporating temperature.

If the liquid refrigerant temperature is below the evaporating temperature, the liquid will not flash.

I would not suggest the TXV capacity be over sized by more than 10%. An exact match is not normally possible, so we have to find the closest capacity to meet the load. Again, smaller valves tends to work better.

If the capacity and system operating conditions are variable, I would recommend a balanced port TXV in almost all cases.

Sporlan has a very nice bulletin on this.

Regards,
US Iceman

PS. This is not a silly question!;)

[chemi-cool]

Hi Marc,

I liked the idea of the of the cap tube bypass.

Have you done that before?

Can you be more specific about it and explain exactly how you choose the cap tube and whats happening with suction temp and refrigerant flow to the compressor when TEV is closing.

Thanks

Chemi:)

[TXiceman]

One of the biggest problems I see is correctly sized TXV's. Often it is due to missing or lack of information provided by the field. One of the hardest things to get accurately is subcoling of the liquid which has a drastic effect on the valve capacity. On a DX coil with a distributor and nozzle, you can adjust the sizing (ie pressure drop) and have some effect on the vlave capacity. You do need to lok at a valve capacity and selection at both full load conditions and off load and low ambient.

Subcooling to the valve can trop drastically if you have condenser flood back controls and operata in colder weather. The amount of subcooling can be as much as 40 dF or more to the valve. Tjis will really make the valve oversized.

I would rather have a valve sized a bit on the tight side and starve a bit at high loads and have the ability to control better at low loads. A balanced port valve will generally have a wider operating range than a non-balanced port valve.

Sporalnd has an easy to use TXV selection program for both thermal and electronic valves.

Ken

[US Iceman]

I would rather have a valve sized a bit on the tight side and starve a bit at high loads and have the ability to control better at low loads. A balanced port valve will generally have a wider operating range than a non-balanced port valve.

Amen. I completely agree with you Ken.

If I remember correctly, the Sporlan valves have some reserve capacity to handle the minor increase in load requirements that we can occasionally see.

I would rather see the valve selected to meet an average of all conditions (or a best fit for the majority) and not use a cap tube in addition to the TXV.

Cap tubes break easy and I would rather have a bullet-proof design rather than adding things that may make the system more prone to leakage.

Refrigeration is as much an art as it is a science. TXV's certainly fall into this category.

[chemi-cool]

Thanks Marc.

Chemi:)

[tonto]

I have been told on a number of occassions that on danfoss valves that there capacity has a reserve of about 20 percent??

[Lc_shi]

Hi sirs,
It's really a good discussion. I learned much from it-:)
I'll compare TXV and EXV in my new heat pump system by the test. And will post something while I get some result.

thanks

Rgds
LC