superheat mesurement is helpful to hand adjust expansion valve. Is there a general method for it? what about your idea?
rgds
LC
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superheat mesurement is helpful to hand adjust expansion valve. Is there a general method for it? what about your idea?
rgds
LC
Hi LC,
What are you using the hand expansion valves for in your business?
In most industrial applications, hand expansion valves are used for two basic reason.
1) Liquid flow into to an evaporator (liquid overfeed system). In this example, the valve is used as a hydraulic balancing valve to control the flow of liquid refrigerant to the evaporator. They are like circuit setters for hydronic systems.
In the refrigeration application you will desire no superheat from the evaporator. You just need to make sure the coil has sufficient liquid flowing into it to make capacity.
2) Maintain liquid level in vessel for a specific purpose. Here, the valve is adjusted to provide a sufficient volume of liquid refrigerant into the vessel. The flow is controlled by a level monitoring device and solenoid valve (or some other means to stop and start the flow when required). In this case superheat is not important.
I presume you mean a thermastitic expansion valve, and by hand adjust you mean unscrew the cap and adjust by the spindle with a screw driver.
First, you should never have to adjust an expansion valve, On the ocations I have adjusted them, i have found it has worked for afew days then you are back again to fiddle about untill you end up changing the valve. The old one would have been defective. There are the occations where you need to adjust the super heat to acomadate a certain type of application like the flower cold store that was discussed last week, it was sugested you should have a lower superheat. To do that you would have to open the valve a wee bit.
Frankly I'd prefer to buty kit made for the job so i dont need to re-engineer it too much, but thats the spoilt ac engineer in me and not the inventive refrigeration engineer. Fridge is constantly about re engineering things to make them work. Anyhoo..
Read the suction presure , (which should be the evaporator pressure assuming pipe size is correct and no epr's or other metering device is in the suction line) and take the suction temperature as it leaves the evaporator. It would usualy be btween 6 degrees c (cabinate or displac case) and 1 or 2 degrees on a cold store. The lower the super heat the more eficient the system but the higher risk of flooding back to the compressor. Efectivly your super heat is your saftey margin.
On systems where you have intelegent control systems such as RMS or Elm or JTL, Trend you name it, using electronic expansion valves, alot of these maintain very low super heats because they can control so acuratly.
Things to watch out for:
Make sure the phial of the tev is in extreamly good contact with the suction pipe. Very well clamped, and idealy wraped in insulation so it is in good contact with the pipe but insulated from other heat sources.
If unsure of the correct setting of the valve super heat, Usualy if you turn it all the way in , then turn it three and a half turns out you should be somewhare near 4 degrees super heat. Ie. If in doubt get back to default.
Make sure someone hasent replaced the orifice inside the valve for the wrong size. Ive actualy had this happen to me and its a devil of a game to recognise this without recalculating the system design from scratch, Or ringing NRS technical like i do :)
My take on this. Most valves are adjusted needlessly. They should not be adjusted on a service visit in any case as they were adjusted if need be on the commisioning, If they need readjustment after it is generaly because they are failing. Or because the tex phile (sensor) is not in good enough contact with the suction pipe.
Hope that helps
Depending also on what you are adjusting the valve on there are certain precautions when adjusting a valve, for example on a supermarket or rack system, making sure compressors are on manual, condensor fans are not cycling which when adjusting the valve can give headaches, anyone have anything to add as im sure everyone out there has different views on the subject....
Forgive me that I say thermal expansion valve,i seldom use hand adjust expansion valve,but i still thanks Iceman anyway for his good opinion on hand expansion valve in industrial application.
What Chilly says is quite right. In theory,we need not adjust the thermal EXV.But in many times we should do some adjust to keep it better matched with evaporator.
rgds
LC
Suction Temp at the compressor is maybe another worthwhile consideration when setting a tx valve. Particularly for sealed units. Although with the old three and half turns out you usually can't go wrong if the design is right.
Wouldnt it better to take the evaporator temp rather than the suction temp roo boy?????? You would have to allow for pressure drop in suction line when taking temp back at the compressor...... Just another question why is 3 and a half turns out okay is that applicable to all makes of valves????
Hi LC,
Now that I understand the question I will make another attempt.;)
In my opinion suction superheat has two areas that you want to verify:
1) At the evaporator coil outlet
This is important, if the TXV superheat is adjusted too high, the evaporator capacity can decrease. The superheat is being developed by the heat transfer surface. You are using the heat transfer surface for sensible heating rather than a phase change (liquid to vapor) of the refrigerant.
2) At the compressor suction valve
If the suction lines have long piping lengths, the suction line insulation is the only barrier from heat gain into the refrigerant vapor. Supermarkets are good examples of this.
If the TXV (or EXV) is set to maintain evaporator superheat at the manufacturers setting, this implies you will get the maximum performance from the evaporator coil.
As the vapor flows through the long suction lines, the vapor absorbs sensible heat from the surrounding ambient environment.
This has two negative impacts to the system:
1) Higher superheat increases the specific volume of the refrigerant vapor. This decreases the gas density and lowers the mass flow. A compressor is a constant volume device (I'm assuming no capacity control for this statement). The compressors pumps only cubic feet per minute, or cubic meters per second, or any other volume flow measurement we use. As the suction vapor density decreases, the mass flow also decreases.
2) Higher suction temperatures (actual gas temperature, not the saturation temperature) increases the discharge temperature of the compressor. If the compressor suction superheat is too high, a higher discharge temperature can also cause problems with increased wear, oil breakdown, increase run time, etc..
The last area of a "short" discussion on superheat is:
Is the superheat useful or non-useful?
Useful superheat occurs in the evaporator only. Since the evaporator is superheating the refrigerant vapor by absorbing heat from the process (air, water, etc. being cooled), the heat gain is useful and provides a net increase in the NRE (net refrigerating effect) of the refrigerant.:rolleyes:
If the superheat is being developed by parasitic heat gain in the suction line, this is non-useful. It does not increase the NRE and only affects the compressor performance.:rolleyes:
Now back to the original question. For my input, only adjust the expansion valves when you are measuring superheat. Do not just crank the valve stem open or closed.
Any adjustment needs time to stabilize the new setting with the valve performance. Do not expect to make a change and leave the job-site. Close monitoring is advisable.
The factory setting works well for most applications. In my experience, the longer piping installations may require adjustment.
I hope this version is more useful to you LC.
Regards,
US Iceman
Sorry I am a bit slow to reply Tonto. I hope this is still relevant.Quote:
Originally Posted by tonto
You are quite right you have to take temp at the evaporator, but it is still necessary to maintain reasonable suction temp at the compressor, which may not be the case if their is considerable heat gain in the suction line.
I should have clarified the valve. Three and half turns out on a DANFOSS TE series valve is what I was referring to and seems to be fairly universal.
Correct me if I'm wrong
Cheers
Rooboy
You're completely right rooboy, the same for Alco valves. I do exactly the same.Quote:
Originally Posted by rooboy
lc shi
I assume you want some values to plug in a how to determine superheat.
If you have a pressure tap at outlet of the coil you can take pressure convert to temp. You also need coil outlet temp. Subtract Coil outlet temp from converted temp difference is superheat.
If you don't have a pressure tap at the outlet of the coil then you can take a coil temp reading midway in coil on a return bend, make sure of coil circuiting. Take temp at outlet. Subtract mid coil temp from outlet coil temp difference is superheat.
Some superheat set points we use are; high temp 10*-12*F, medium temp 6*-8*F, low temp 2*-4*F. If the valve is sized right there will be about a 2*F fluctuation. If the system is close coupled you would want to go to the upper end of the range.
Steve