PDA

View Full Version : TEV capacity related to pressure drop







DaBit
04-12-2002, 01:31 PM
How is the capacity of a thermostatic expansion valve (Danfoss TEN2, orifice 0X in my case) affected when the pressure drop over the valve decreases?

In my small liquid chiller, measurements showed that with a pressure drop of about 9 bars (temp measured in the center of the condenser: 48 degrees C -> 12.5 bar pressure, evaporating at 3.2 bar measured) , the full open capacity of the valve is about 400W. This fits the datasheet, which says 0.5 kW capacity at 8 bar difference.

But, when the pressure drop over the valve is only approximately 4 bars (temp measured in the center of the condenser: 22 degrees C -> 6.1 bar pressure, evap pressure 1.78 bar measured), full open valve capacity drops below 150W. This does not match the datasheet, which says 0.43kW capacity at a pressure drop of 4 bars.

The full open capacity is measured by adding a known amount of heat to the system using a variable voltage source and a resistive heater element, and adding that to the amount of heat leakage determined earlier. The TEV was set to full open by heating the bulb a bit with a hair dryer.

I have about 15K subcooling at the TEV inlet, so flash gas seems not to be a problem. Is the datasheet wrong? (only 0.07kW less capacity when halving the pressure drop seems a bit odd to me). Do I have another problem somewhere?

herefishy
04-12-2002, 03:12 PM
Hi DaBit :)

When you reference your data sheet, are you referencing the valve capacity at the same evaporating temperature as that at the higher PD, or are you adjusting the reference evaporating temperature (if you are using a table)?

The other thing that you must take into consideration is that when the evaporating temperature drops, your compressor capacity will drop accordingly.

The valve capacity rating will decrease with decreased PD.

What is your superheat? That would be my primary criteria for valve operation. I have never heard of such a procedure as you have illustrated in (TEV) troubleshooting.

I think that if the valve is functioning properly and there actually is a complaint about the machine's performance, that an inefficient compressor could result in how I interpret the symptoms that you are describing.

It is the compressor that does the work, not the valve. ;)

Prof Sporlan
04-12-2002, 03:36 PM
Liquid flow thru a TEV, and for that matter, an orifice and most solenoid valves, can be described by the orifice equation:

m = C<sub>f</sub>A<sub>o</sub>[2g<sub>c</sub><FONT FACE=Symbol>rD</font>p]<sup>1/2</sup>

where:
m = flow rate, lb<sub>m</sub>/sec
C<sub>f</sub> = flow coefficient
A<sub>o</sub> = orifice area, ft<sup>2</sup>
g<sub>c</sub> = gravitational conversion factor, 32.174 lb<sub>m</sub>-ft/(lb<sub>f</sub>-sec<sup>2</sup>)
<FONT FACE=Symbol>r</font> = density, lb<sub>m</sub>/ft<sup>3</sup>
<FONT FACE=Symbol>D</font>p = pressure drop, lb<sub>f</sub>/ft<sup>2</sup>

If we simply change pressure drop, and leave everything else alone, then the effect we would expect on flow rate is the square root of the pressure drop ratio, i.e.:

m<sub>2</sub> = m<sub>1</sub> [<FONT FACE=Symbol>D</font>p<sub>2</sub> / <FONT FACE=Symbol>D</font>p<sub>1</sub>]<sup>1/2</sup>

So if we reduced pressure drop across the TEV from 100 psi to 50 psi, we would expect:

m<sub>2</sub> = 1 [50 /100]<sup>1/2</sup> = 0.707

or TEV capacity at 50 psi pressure drop to be 70.7 percent of its capacity at 100 psi pressure drop.

Therefore one would expect a 0.50 kW rating at 8 bar to be reduced to about 0.35 kW at 4 bar, assuming everything else remains the same.

DaBit
05-12-2002, 09:46 AM
Originally posted by herefishy
Hi DaBit :)

When you reference your data sheet, are you referencing the valve capacity at the same evaporating temperature as that at the higher PD, or are you adjusting the reference evaporating temperature (if you are using a table)?

Hi! :)

I use the evaporating temperature according to real evaporator temperature. Danfoss provides a table of capacities for different evaporator temperatures. But there is also a low difference between capacities at different evap temperatures, so it does not change the big picture.



The other thing that you must take into consideration is that when the evaporating temperature drops, your compressor capacity will drop accordingly.

I know, but the compressor's rated capacity at an evaporator temperature of ~ -13 &deg;C (corresponds to 1.78 bar) is mucht higher than the 150W I am getting.

[quote]What is your superheat? That would be my primary criteria for valve operation. I have never heard of such a procedure as you have illustrated in (TEV) troubleshooting.

I thought superheat would also be my problem, so I tried to open up the valve by heating it. I had the hair drier nearby, so it was a quick test. Since the (liquid chiller) evaporator can handle the maximal load possible in the system, I do not risk liquid floodback.

Superheat at the 4 bar drop scenario is 4.6K (calculated by taking evap pressure, cross-referencing this to temperature, and evap outlet temp)
This fits the TEV superheat setting, which is (factory)set at 4K.


I think that if the valve is functioning properly and there actually is a complaint about the machine's performance, that an inefficient compressor could result in how I interpret the symptoms that you are describing.

The compressor is about 6 months old, and has been handled correctly. I do not expect problems with the compressor itself.


It is the compressor that does the work, not the valve. ;)

I know, but everythings works together. Change one thing, and be prepared to adjust the rest.



Originally posted by Prof Sporlan
Liquid flow thru a TEV, and for that matter, an orifice and most solenoid valves, can be described by the orifice equation:

Thanks Prof, this is how I like to be teached :)

herefishy
05-12-2002, 05:00 PM
Danfoss TEN2-OX - R-134A

I need to look at this in (other) units to see the picture -

.44kW = 1,494BTUh

SCENARIO 1

PD = 130psig

condensing temperature = 118degF :(
discharge pressure = 181psig

evaporating pressure = 46psig
evaporating temperature (SST)= 41.1degF :(
_____________________

SCENARIO 2

PD = 58psi :(

condensing temperature = 80degF
discharge pressure = 88psig :(


evaporating pressure = 25psig
evaporating temperature = 29degF :(
_________________________________________________


the datasheet, which says 0.5 kW capacity at 8 bar difference

...at 10C evaporating temperature (50degF)

at 0C = .47
at -10C = .44
at -20C = .41
at -30C = .37
at -40C = .33

...at 4bar

10C = .43
0C = .42
-10C = .38
-20C = .35
-30C = .32
-40C = .28

at 15K subcooling, a correction factor of 1.2 would be applied.
____________________________________________________

I assume design SST should be about -10C to -6C (14 to 20degF), huh? The machine doesn't seem to be able to reach such a condition. I rule out short-charge due to the low pressure ratio at the low-load condition. Running a 90psi head, I would expect to see a lower evaporating pressure/temperature than you have indicated, assuming proper charging and technique.

I'm still leaning towards compressor failure. :)

I would like to know compressor information.......

DaBit
06-12-2002, 09:32 AM
Originally posted by herefishy
Danfoss TEN2-OX - R-134A

I need to look at this in (other) units to see the picture -


Heheh, next time I post a problem here, I will post values in both SI and 'your' (Imperial??) units. :)


...at 10C evaporating temperature (50degF)

OK, got the wrong column. But it doesn't change the picture.


at 15K subcooling, a correction factor of 1.2 would be applied.

For evaporator capacity selection......



I assume design SST should be about -10C to -6C (14 to 20degF), huh? The machine doesn't seem to be able to reach such a condition.

Design SST is -20 &deg;C (-4F) @ 150W (510 BTUh) load (In fact: design SST is 'as low as possible'). It is able to reach design SST, but it just takes a long time to reach -20 &deg;C, at least longer than expected based on the calculation I made.


I rule out short-charge due to the low pressure ratio at the low-load condition. Running a 90psi head, I would expect to see a lower evaporating pressure/temperature than you have indicated, assuming proper charging and technique.

System is leak-free, and has been pulled vacuum for about 4 hours. But I do not have a high-side access point. Charging was straightforward; just adding refrigerant until temps stabilized, and then a bit more. I calculated the minimal amount of refrigerant needed (105g) and maximal amount of refrigerant assuming 80% receiver fill (175g). I did this by calculating the weight of refrigerant in the liquid line and a bit in the receiver, and I added the weight of refrigerant in the compressor, evaporator and suction piping at the highest design evaporator pressure (at 38 &deg;C). But I am not able to weight in such a small amount by putting the cylinder on a scale. Filling was trial and error.



I'm still leaning towards compressor failure. :)
I would like to know compressor information.......

Hmm. I don't want to agree with you :)
The compressor is the most expensive part of the system.

About the compressor: this is a Danfoss NL11F small hermetic reciprocal R134a unit, designed for LBP use.

Capacities at Tc = 35 &deg;C (95F), according to datasheet:
To = -30: capacity = 210W (715 BTUh)
To = -25: capacity = 288W (980 BTUh)
To = -20: capacity = 381W (1300 BTUh)
To = -15: capacity = 487W (1650 BTUh)
To = -10: capacity = 608W (2070 BTUh)

I took the conversion factor 1W = 3.4 BTUh. I don't know if it is the official factor.

herefishy
06-12-2002, 03:45 PM
Hi DaBit :)

Just the fact that the motor even turns the crank at a +10C SST is an indication of failure, to me !

http://www.danfoss.com/compressors/pdf/datasheets/r134a_220v_50_60hz/n-series/NL11F_R134a_220V_50Hz_08-01_Cg43j202.pdf

According to Danfoss information, at a -10C SST (14psi) you should be drawing 1.89 Amps (325watts). @ 55C condensing, 32C ambient, 55C liquid.

If you could somehow get the pressures in and out of that (defective) compressor into the test range, and check your amp draw, I think that you might be convinced. :)

I couldn't find a "performance curve" chart. If you can get your suction to within the (suction) operating range of the compressor and note the suction pressure, discharge pressure, and the exact voltage and current (watts) at the time the noted pressures exist, you may be able to call tech support who may have a curve to reference at your operating conditions.

DaBit
06-12-2002, 04:33 PM
Originally posted by herefishy
Hi DaBit :)

Just the fact that the motor even turns the crank at a +10C SST is an indication of failure, to me !

Not to me. Many people in the PC overclocking realm run this compressor with R404a and R290, without problems. I did not dare to, so I used R134a. Danfoss compressors are pretty well built.

Besides: the 3.2 bar I mentioned corresponds to approximately 2.5 &deg;C evaporation (36F), which matches the MOP point of the valve (0 &deg;C / 32F)

But I will check the compressor. I really hope it is not broken since it is an expensive part, and getting a second hand one which has not been abused by leaving it on the shelf for months with the ports open is not easy.


If you could somehow get the pressures in and out of that (defective) compressor into the test range, and check your amp draw, I think that you might be convinced. :)

Hmm. As an electronic engineer (my profession is designing custom chips, not refrigeration :) ), I did not even check the amp draw. I am ashamed.

But I must admit that I might have a mental block about a broken compressor :)


I couldn't find a "performance curve" chart.

You can obtain those charts out of the Danfoss RS+ program; a piece of software provided by Danfoss which allows you to select a compressor for the job and draw the required graphs.

herefishy
06-12-2002, 05:11 PM
You can obtain those charts out of the Danfoss RS+ program

yea... I started to download it off of the website.... but it looked like it was going to take awhile. So I decided that I'd let you do any further research..... :)

If you've got the program... check it out.