TEV range

[Lc_shi]

Hi sir

VFD compressor frequency form 30 to 90Hz,capacity from 10 to 27kW, i'm not sure if it can match well with thermal expansion valve which need the 30%~100% capacity range . pls help to confirm it.

rgds
LC

[The Saint]

There is no effect on expansion valves during modulation. You should select expansion valve at 27KW rating.

[US Iceman]

Hi LC,

I normally recommend a minimum turn-down capacity of 50% for TXV's. If the full capacity requirement is 27 kW, then the minimum allowable capacity (for good control of the superheat) would be about 13.5 kW.

At 10 kW, you are at about 40% of the valve capacity assuming nothing else changes. The liquid feed pressure may decrease due to the excess condenser surface available at part load though.

An electronic expansion valve may produce better results, but all of my experience is with mechanical TXV's.

[ernestlin]

Hi, Mr sze.
I agree with Saint. The TXV should fit to the max capacity. During the modulation, TXV could regulate the turn-down rating by the pressure of temperature sensor. As Iceman said, the EXV will enhance the precision of regualtion.

[US Iceman]

The TXV should fit to the max capacity.


I agree, as long as the capacity of the valve is based on the normal operating conditions. Depending on the actual operating conditions, the nominal valve capacity could also be 30 kW to achieve a 27 kW performance value if the differential pressure across the valve can be reduced.

[BESC5240]

One should remember 2 things:
- TXV are able to handle more then their nominal capacity. Mostly 20 % more, some valve even 30%. So I don't agree to select the valve on the maximum capacity.
- the capacity of a certain TXV is (almost) linear with the superheat. (large capacity = large superheat; small capacity = small superheat). So if you need a constant (or an as low as possible) superheat over the full capacity range, an electronic expansion valve is the best choice.

[ernestlin]

One should remember 2 things:
- TXV are able to handle more then their nominal capacity. Mostly 20 % more, some valve even 30%. So I don't agree to select the valve on the maximum capacity.
- the capacity of a certain TXV is (almost) linear with the superheat. (large capacity = large superheat; small capacity = small superheat). So if you need a constant (or an as low as possible) superheat over the full capacity range, an electronic expansion valve is the best choice.
yup, BESC. when you select the valve, the overmeasure should be considered, but it's based on the max capacity.

[monkey spanners]

Could you not fit two TEV's one rated at 15kw and one at 28kw and switch between them with a couple of solenoid valves to match compressor speed/capacity? Could solenoid control be tied in to the variable speed control system? An old alfa laval glycol/slush maker i saw on a dairy farm had a load of capillaries run in parallel with a TEV, did'nt switch between them though.

[Lc_shi]

Could you not fit two TEV's one rated at 15kw and one at 28kw and switch between them with a couple of solenoid valves to match compressor speed/capacity? Could solenoid control be tied in to the variable speed control system? An old alfa laval glycol/slush maker i saw on a dairy farm had a load of capillaries run in parallel with a TEV, did'nt switch between them though.

I think you're right. If the capacity changes largely,the parellel TEV with solenoid valves should be considered.

[Renato RR]

I wote for 2 TEV.But wich kind of machine is this?

Renato

[knighty]

instead of 1 at 27kw and one at 15kw and solenoides to change between them....

why not just 2 at 15kw and a solenoide to turn one and off ? ;)

(p.s. Lc_shi... wait till someone who knows what there doing posts before you go out buying bits!)

[Lc_shi]

Hi knighty
yeah, totally agree with you:)
But i prefer to use 27kw and it can cover 15kw capacity point. I'll test it and the fact will have the say.

regards
LC

[monkey spanners]

What i was thinking was that at the point of switching between 15Kw and 27Kw both valves would be sensing the same evaporator pressure and superheat, would the 27Kw valve have shut down to about 15Kw to match the load, and hence cause a seemless transision from the 7.5Kw/15Kw range to the 13.5Kw/27Kw range? Would this work equally going down in capacity? If we had two valves of 15Kw would the second valve be at 15Kw as its brought on line? Giving 30Kw until both valves can adjust? I'm asking not telling, this needs someone brighter than me on it:D

Cheers Jon