Technical clarification on humidity controlled chamber

[Neela]

Dear all

I am new to this forum. Look forward to your support. We have a humidity controlled chamber where we are trying to maintain temperature at 15 DegC (at +/_ 0.5 DegC) & Humidity at 40RH (at +_ 5%). we have a hot gas bypass system installed. the solenoid on the liquid line has to be switched on & off at 2sec on/ off for maintaining the parameters at the reqd. accuracy. Is this frequency OK? I understand that all solenoid valves have max no. of on/off cycles. Is it generally done like this?

Please advise
Neela.S

[Tesla]

Hi Neela and welcome to our forum.
No it is not normal to switch a solenoid of/on in 2 second intervals - it will burn out prematurely. It will depend on what other control components are in the system like EPRV, EEV, VAV etc. Please tell us what other components are in the system for control. That is tight control and not easy to achieve - what is the application and are there multiple system components?

[Neela]

Dear Mr. Tesla
Thank you for the reply. The unit is 350L capacity. We are using KCJ467HAG compressor from Emerson. The is one solenoid valve (3/8") before the capillary. The hot gas bypass valve ADRIE1-1/4 from sporlan. The control logic is through the solenoid valve on/off only. The humidity is controlled through switching on/off the blower of a vapor generator. This is a water bath with a heater. both the heater & the blower will switch on for humidification.

[Neela]

sorry, I have not answered all the questions. The application is - the unit is a plant growth chamber. There are no other control elements in the circuit. No additional Evaporator coils, EPR. Not even a thermal expansion valve. We have a capillary in the system.

[Tesla]

Thank you Neela for your quick response and answering the questions well. I would suggest that you incorporate a fan for the evaporator with adjustable speed which Emerson can easily supply for your application and increase/slow the pulsing/duty cycle of the solenoid this should give you the control you require with a simple system ensuring reliability.
Rather than using a water bath heater I would also recommend using an atomiser - the type used for or as a garden fogger/mister or vege humidifier. This type of unit will atomise water with no added heat reducing required power and slowing the control process.
Please let us know what you decide and the result so we can all learn from this post.
Thanks Tesla

[Neela]

Thank you for the suggestion. We have taken up the control of the speed of the fan. The on/off time at high humidity settings is reduced. We tried this atomizer but the size of water droplets is large for the growth chamber where customers don't appreciate water droplets inside the chamber. This equipment seems to be a tough call. We are not able to stably maintain the parameters. We are using hot gas bypass provision for both defrosting & humidification. I will be thankful if you can advise us as on how to go about.

[lana]

Hi there,

Indeed this application is complicated.

In this kind of application you must use a very good (complicated) control algorithm. Your control system must be able to do Psychrometric calculations to determine what to do, i.e. cooling, heating, humidification,...

On/Off liquid line solenoid valve is out of the question. Hot gas by-pass system is for capacity control only.

I suggest that you incorporate psychrometric calculations in your control system. I know it is not easy but there is no other choice. I have seen this kind of systems which people tried to build, but majority of them had this same problem. One of them was a growth chamber for Genetic research. Two engineers tried to do that but they only build the cooling system without any knowledge of psychrometric and it was a desaster...

Good luck
Cheers

[Emmett]

Neela,
First of all if you must pulse the solenoid so frequently it might be best to use an X series valve they are rapid cycle duty valves available from sporlan.
Second you mention the fact that the system is fitted with an ADRIE1-1/4 valve this is an expansion valve in theory if it is fitted correctly it should maintain the evaporator pressure and therefore your solenoid should not need to cycle at all but be on at all times.
Third if everything is set correctly and fitted correctly then you should not need to defrost this system ever!!!
Fourth your requirements can be achieved, I would set the refrigeration system up to allow for maximum dehumidification and cooling without freezing the coil and then add reheat and humidification to your specs. It has been suggested that the use of psychrometrics in the control loop is the way to go, I do not disagree but we are achieving these same parameters without using them.
Perhaps you should provide all of us with all of the information you have on this system.
Good Luck

Dear Mr. Tesla
Thank you for the reply. The unit is 350L capacity. We are using KCJ467HAG compressor from Emerson. The is one solenoid valve (3/8") before the capillary. The hot gas bypass valve ADRIE1-1/4 from sporlan. The control logic is through the solenoid valve on/off only. The humidity is controlled through switching on/off the blower of a vapor generator. This is a water bath with a heater. both the heater & the blower will switch on for humidification.

[Gary]

Measure the air temperatures entering and leaving the coil. Adjust the fan speed until the difference between these two temperatures is 13K. This should give you about 40% RH when the temperature reaches 15C. Tweak the fan speed from there if needed (higher fan speed for higher humidity or lower fan speed for lower humidity).

Then its a matter of controlling the temperature to maintain 15C.

[Tesla]

Hi Neela
The posts above should point you in the right direction but after careful thought I thought I should mention - In the old days to achieve this type of close control of humidity and temp we would use two systems in series. The first one would get the conditions roughly then a second system would be for finer control of the parameters. Also I think an EEV should provide much better control than a capillary, I have seen them on very small systems like slurry/slush drink machines. Let us know how you are going with progress on this one. I installed a tight control system like this but bigger for a film archive many moons ago.

[Neela]

Thanks to all (from the bottom of my heart).

We are trying the following now:
1. use fan control as a supplement for humidity control (in addition to the Vap Gen, for higher humidity) by controlling the delta across evaporator around 13K. We will implement it & come back with the result.

I have some queries

The valve ARDIE 1-1/4 that we have used is a hot gas bypass valve & not an expansion valve. Am I wrong?
I too had the same doubt. When our technician installed the valve, I presumed it is automatic & will respond to pressure drop to ensure set pressure for the suction line. But he suggested pulsing of the solenoid valve. There was a panic since I was worried for the compressor. But I came across some literature where they have followed similar system with pressure switch, which I believe should be equivalent to the Hot gas bypass valve. One immediate solution can be to try the valve suggested from sporlan. however, I know we should find wiser way of controlling the unit.

We have bought a TEV. The conclusion we came to was that we have control deltaT across evaporator coil for chamber temperature control & pressure for humidity control . Does it make any sense? We are trying to lay the components with this logic. I will get back to you as soon as we progress. The psychometric chart incorporation into the control logic is above my head. let me understand it & get back to you.

Thanks once again with a Japanese bow

[Gary]

Thanks to all (from the bottom of my heart).

We are trying the following now:
1. use fan control as a supplement for humidity control (in addition to the Vap Gen, for higher humidity) by controlling the delta across evaporator around 13K. We will implement it & come back with the result.


Air at 15C@40%RH has a dewpoint temperature of 2C. Coil leaving air temperature gives us a rough approximation of coil dewpoint, the accuracy depending upon the bypass factor of the coil. Thus, if the chamber temp is 15C and the leaving air temp is 2C (13K dT), the RH should be close to 40%.

[Emmett]

http://www.parker.com/literature/Sporlan/Sporlan%20pdf%20files/Sporlan%20pdf%20090/SD-057.pdf

The valve ARDIE 1-1/4 that we have used is a hot gas bypass valve & not an expansion valve. Am I wrong?

Check out this link, it may be helpful in explaining operation of your valve.

[mad fridgie]

I have completed many such applications for uni's and the like, but not one so small at 350L. The size will it self cause a problem. Your instrument probles have to be very small and have little thermal mass, you may just find that you are getting deviation soley down to the reaction time of your probes. under and over shoot.
Being so small we need not look at efficiency?
On the mechanical side, i would tend to keep the evap pressure at a constant (as gary has suggested) I would use a false load (resistive element) in combination with with your hot gas valve, i would not modulate your liquid sol valve. Try and maintain a constant discharge pressure.
i would look at using a Piezo electric humdifier (cold fog). If I had a choice.
If not throttle the humidier nozzles, to the point where it seems to struggle to meet humidification rate.
Effectivily we are try to slow the whole system down (by reducing the mechinical effects), this will then allow your PID controller to stabalise (after mods you should see some deminishing swings, allow four cycles before you try any further adjustments)
Look at stabalizing your temp first (you can not have a stable humidity unless you temp is stable), then your humidity
Forgot to say run evap fan at a low speed speed, to give the air some dwell time through the evap

[Magoo]

I have done several deer velvet drying rooms that had two indentical coils of equal capacity, both with slow air flow to snow up coils, then hot gas one at a time, alternating. Cannot remember details but RH was nominal 30% at 20'C control. Eye drying inside, blink blink stuff, air balance ports required for pressure differentials.

[attitude11]

How to control TD(Difference bet entering air and evaporator temp) in cold room.? How to maintain TD in cold room

[attitude11]

How to control TD(Difference bet entering air and evaporator temp) in cold room.? Or more specifically how to maintain TD in cold room ?

[S.M.Gokhale]

Hi all
checked in after a long time so missed this and many other posts.

Maintaining 2 Deg. C Dew point is impossible unless ........ hot gas is fed continuously to the cooling coil.
Have designed such a chamber. Actually in a test chamber once the temperature is achieved no load is imposed. The chamber temperature and evaporating temperature will start dropping. Ice will form on the cooling coil and it won't be possible to maintain the TD. I have used an EEV to control the cooling and a hot gas bypass valve to add artificial heat to maintain the dew point ( air leaving temp.) With a sophisticated dual loop controller ( to control the temp. and RH). Works very well maintains temp. within +/- 0.1 Deg. C and Rh +/-0.5% (The controller is Eurotherm make - model 2604. It is actually a triple loop controller with a pressure control thrown in.)