Expansion valve question

[juicetavo]

Hello again everyone,

I come to you this time with a question regarding expansion valve operation behaviour. I have a data log of temperatures measured every 2 minutes at the cooler inlet and at the compressor inlet, with which i can calculate the superheat. After plotting this data in a linegraph i was able to observe that the superheat increased and decreased alternatively and dramatically (from around 5°C to 20°C) every 10 minutes or so. Glycol flow was kept constant through the cooler although ECW and LCW temperatures varied throught the measurements. My question is: is this superheat variation caused by the expansion valve? if so, is this alternate superheat temperature normal?

I've consulted the manual and it states that the total amount of discrete steps for the expansion valve is 1500, which would allow very accurate refrigerant flow, which in turn would suggest that maintaining a constant superheat wouldn't be too much of a problem.

I apologise in advance for any thechnical atrocities i might've said, I'm just starting to work with refrigeration systems so i'm still learning. Any help or correction would be greatly appreciated

thanks

[AlexG]

The expansion valve is electronic? Who is manufacturer?

[juicetavo]

The expansion valve is electronic? Who is manufacturer?

yes it is an EXV, the chiller manufacturer is carrier, i would don't know the EXV's manufacturer but i will investigate.

[stufus]

You need to take a temperature reading at the evap outlet, and take a pressure reading at the compressor suction (inlet) and convert this pressure to temperature on a PT chart to calculate your superheat.
Cheers
Stu

[juicetavo]

You need to take a temperature reading at the evap outlet, and take a pressure reading at the compressor suction (inlet) and convert this pressure to temperature on a PT chart to calculate your superheat.
Cheers
Stu

I have the temp reading at the compressor suction, and i have the temperature at the evaporator inlet (which i assume to be the same or very close to the temperature at the outlet). That's how i calculated the superheat, my question is why does it vary so much.

thanks

[The Viking]

Jucetavo,
I recently posted this in another thread about superheat:


Ok here goes,
In the thread "Overcharging of refrigerant", HERE (http://www.refrigeration-engineer.com/forums/showthread.php?34089-Overcharging-of-Refrigerant&p=249791#post249791) I recently explained subcooling, superheat is exactly the same but in reverse.

If you imagine the evaporator as a long pipe submerged in hot water, in one end we will inject liquid refrigerant. This liquid refrigerant will then heat up as it travel along the pipe and slowly "boil" or turn in to vapour, at some point there will be no more liquid left, only vapour.

If we at this point let our refrigerant enter a compressor then there is a chance that some liquid droplets will be carried over, sucked along with the vapour, and enter the compressor. As liquid can't be compressed this will damage the compressor.

We therefore need a safety buffer. This is created by letting the refrigerant travel further in our pipe, past the point of where all the liquid has evaporated. It will then pick up more heat to ensure that there is no chance of liquid to be carried along.

The difference in temperature between when all the liquid should have boiled off and the refrigerant when it leaves the evaporator is what's commonly referred to as superheat.

(yes, there are other places in a refrigeration circuit where you will find superheated vapour but this is what we were talking about)

The temperature difference between the refrigerant when it enters and exits the evaporator is not superheat as we (or your expansion valve) knows it.

To find the true superheat as calculated by your expansion valve you have to get a pressure reading and to convert that to your refrigerant's dew point/temp at that given pressure.
The differential between that dew point and the temperature leaving the evaporator, that is your superheat. (as we know it and more importantly, as the expansion valve measures it)

Having cleared that out,
To answer your question "is it normal".
Yes, the temperature differential you have measured will / must vary as the expansion device operates.
To answer the implied question "is my chiller working as it should".
It is a bit like if you looking at a car and takes readings of the rev counter, sometimes it shows 2000rpm other times it shows 8000rpm. Normal or not you can't really tell from the information given.

If you have close monitoring of your chiller the best you can do is to look at the water's delta T across the chiller. If that, or the overall water temp, increases outside set parameters then it is time to call your service guy out...

Hope this helped clarify things and good luck..

:cool:

[chillerman2006]

Glycol flow was kept constant through the cooler although ECW and LCW temperatures varied throught the measurements. My question is: is this superheat variation caused by the expansion valve? if so, is this alternate superheat temperature normal?



I am with viking on this and agree it sounds normal

you have a variable water temp entering the evap, so the system needs time to stabilise

try and supply a constant water temp and retake your measurements after 10/15 minutes of run time

[frank]

High superheats can be measured during pull down or periods of high heat loads. Providing you have around 6K superheat at design conditions, you can be confident that you are not getting liquid flood back.

[stufus]

Assumption !!!is the mother of all F**k ups
As the lad's have pointed out there are many variables to consider and influencing factors to be accounted for.
Cheers
Stu

(which i assume to be the same or very close to the temperature at the outlet). That's how i calculated the superheat, my question is why does it vary so much.

thanks

[Raisins78]

Assuming the Chiller was commissioned correctly, and not too long ago (a few years, not 15 years ago) then watching the Delta T (temp difference between inlet and outlet of the water/glycol) should give you a 99% indication of the chillers health. (assuming no fault codes keep popping up etc)
Ive always worked on 2-4k across the vessel, on commissioning, if its higher than 4k, check water flow (blocked strainers, worn out pumps, incorrectly working bypass valves etc) if its lower than 2-3k, look for refrigeration issues (gas leaks, head pressure issues like filthy coils, EXV problems, condensor tubes in need of cleaning etc.)

The EXV/TX valve etc will adjust as required, so if you really want to check superheat, take the refrigerant outlet temp off the evap vessel, compare it to the PT conversion of the evap pressure and see what you get.
On a Carrier, you should be able to do this through the ProDialog controller, so no tools required.
press the evap pressure button, note down the pressure, then press the same button again, and it will give you suction temp.
Not as accurate as real gauges and temp probes in the correct spot, but enough to make a judgement call on (should be around 4-6k on a Aquasnap style chiller, 3-5k on a twin rotor comp machine from memory)