PDA

View Full Version : How to controll Logarithmic Mean Temperature of evaporator







Josip
14-08-2006, 02:25 PM
Hi, need help :)

I need some information how to control temp from the subject. By PLC, PC, with temp sensor at cooler outlet for 3way-valve for glycol, it must be changeable and controlled

This is an example of cross flow (air stream is perpendicular to glycol pipes in cooler

Yes I know we have to controll room temperature/humidity but this is a new customer request and now....

Some links about...;)
Any input is welcome:)

Thanks,

Best regards, Josip :)

US Iceman
14-08-2006, 04:12 PM
Hi Josip,

Is the evaporator a normal air cooling coil? Cooling coil, draw through fans, drain pan; Is this what you have?

Is the glycol flowing through the tubes, instead of refrigerant?

What is the glycol and air temperature you need to control?

And, where is the three-way control valve located? Is it controlling the inlet flow to the coil, or is it controlling the outlet flow into the coil?

I'm beginning to work on a project where a similar requirement exists, so we might learn something together.;)

Your question was "how to control the LMTD of an evaporator". Are you trying to re-set the glycol outlet temperature to maintain a constant RH in the supply air?

Obviously, the LMTD is comprised of four temperatures; inlet air, outlet air, inlet glycol, and outlet glycol.

If you can provide some additional information on the actual process and what your customer wants, we can find a good solution.

Sounds interesting...

Best Regards,
US Iceman

Josip
14-08-2006, 08:15 PM
Hi, US Iceman :)

Lets define (problem) task :)

Onion
-Cool down from +45C to +25C within seven (7) days. Using outside air only if colder than the product temperature, this air could be used for cooling down the product. Mechanical cooling is only used when outside air is not suited.
- Kept @ +25C in 14 days @ 70-75% RH
- Cool down from +25C to +15C within seven (7) days @ maximum RH of 70%
- Cool down from +15C to +0C within 14 days @ maximum RH of 70%

Potato
- Cool down from +35C to +15C within seven (7) days.
Using outside air only if colder than the product temperature, this air could be used for cooling down the product. Mechanical cooling is only used when outside air is not suited.
-Kept @ +15C in 14 days @ 85% RH
-Cool down from +15C to +4C within seven (7) days @ 85% RH



Is the evaporator a normal air cooling coil? Cooling coil, draw through fans, drain pan; Is this what you have?

Yes, but not exactly, it is some special execution with cooling coil and with separate post heating coil with hot glycol defrost system. During cooling process in cooling coil will be a cold glycol and during defrosting there will be a hot glycol.


Is the glycol flowing through the tubes, instead of refrigerant?

Yes


What is the glycol and air temperature you need to control?

Glycol in/out -10C/-5.5C final air temp in the room +4C for potato and 0C for onion.



And, where is the three-way control valve located? Is it controlling the inlet flow to the coil, or is it controlling the outlet flow into the coil?

Motorized 3-way valve will be on inlet pipe with additional circulation pump for each piece or pair of coolers. Is there some catch to install it in some other position?


I'm beginning to work on a project where a similar requirement exists, so we might learn something together. ;)

Agree ;) , but now I have to give an answer and later on to see if that can work or not :( what I do not like.


Your question was "how to control the LMTD of an evaporator". Are you trying to re-set the glycol outlet temperature to maintain a constant RH in the supply air?

Definitely :) We have to try to keep humidity constant within requested period of time for cooling

Hope those additional informations will be of some help ;)

Now I have to wait for good answers :) Thanks

Best regards, Josip :)

US Iceman
14-08-2006, 08:56 PM
Hi Josip,

I will try to get some information for you later today. I just looked at the site quickly to see what was happening.

Peter_1
14-08-2006, 10:12 PM
Josip, when we have such a project, I give the requirements like you describe to a PLC programmer and he translates this to a working PLC program.
We have for such a cases 3 capable persons.

I also think that JUMO (Germany) has a capable controller, altough I think this is a typical task for a PLC programmer.
It seems on the first sight not that difficult.

Do you need often to defrost it? I ask you this because your temperature are well above 0C.

You can use also a stand alone electrical heater/cooler like we did in the attached picture: 3 way valve and a separate small pump, pumping the warm glycol. Works very well.

But why do you need to control the LMTD? This is not completely clear for me.

http://www.vdhproducts.nl/ is a Electronic Control manufacturer specialist for the vegetables and fruit sector. Perhaps they have a controller right of the shelves.

We also have in our village an electonical designer who can make almost everything with microcontrollers.

US Iceman
15-08-2006, 02:17 AM
Hi Josip,

If I understand what you are doing, you have this:

A fan/coil unit which has cold glycol circulated through the primary cooling coil. On the air exit side of this coil, you have another glycol coil used for reheat (humidity control and temperature increase as needed).

The primary cooling coil has a need to be defrosted by circulating warm glycol through the coil.

From my understanding of these systems the primary need is to select the control valve to match the change in heat transfer performance of the coils. An equal percentage control valve tends to provide a linear response to the change in heating/cooling capacity. This helps to ensure you have control of the process at the coil(s).

In addition, the control of the liquid system (the cold or hot glycol loops) is a function of the glycol piping system. This can be addressed by using a constant volume pumping system, or a variable volume pumping system. With the variable volume pumping system you have several different methods you can use.

The control algorithm used simply adjusts the respective cooling or heating requirement to match the needs.

By changing the volume of glycol flow through the coil, you are controlling the exiting air temperature, right? The inlet glycol temperature does not change. Only the outlet glycol temperature.

This would control the amount of dehumidification of the leaving air from the cooling coil. The air reheat coil then controls the final temperature and humidity. I think you also have to allow for the fan motor heat as part of the air temperature rise.

I was trying to remember a conversation I had with someone about this subject. I believe he said it was easier to control the temperature and final humidity if you over-cooled the air, then reheated the air to the temperature you need, then add the water to control the RH.

At one of my former employers we used to do this for engine test cells. Unfortunately, I did not get involved in this often enough to really learn what I would like to.




Motorized 3-way valve will be on inlet pipe with additional circulation pump for each piece or pair of coolers. Is there some catch to install it in some other position?


The information I have always heard is to place the control valves on the outlet of the coil(s). When the pressure drop occurs across the control valves any air that is released can be vented, without air binding the coils.



(***information on control valves***)
http://www.maintenanceresources.com/ReferenceLibrary/ControlValves/CashcoFlowChar.htm

http://www.spiraxsarco.com/learn/modules/6_5_01.asp

http://www.belimo.us/library/applications/DocumentLibraryManager/upload/Doc_Vlve%20Apps%20Guide_1999_3.pdf -- this is a good reference book to review.


(*** information on variable volume pumping***)
http://www.bellgossett.com/Press/VVFund.htm

http://www.bellgossett.com/Press/techtk699a.htm -- these are good sources of information on pumping systems.

(***information on flow balancing valves***)
http://fhaspapp.ittind.com/literature/files/583.pdf -- something you will need to balance the flow during commissioning.

I will continue to look for information Josip. I think it is possible, we just have to find all of the answers for you.

Lc_shi
15-08-2006, 04:02 AM
Just read it. a very good topic.
Josip ,it's better to understand if you add a sketch for your system:)

I'm not sure what you really mean to control the LMTD of evaporator. Does it mean to control the evap cooling capacity?

regards
LC

Peter_1
15-08-2006, 09:47 AM
Josip, forgot to add the picture

Peter_1
15-08-2006, 09:51 AM
By changing the volume of glycol flow through the coil, you are controlling the exiting air temperature, right? The inlet glycol temperature does not change. Only the outlet glycol temperature.

This is exactly the same a VRV/VRF is performing, only, the throttling device is then an EEV.

Josip
15-08-2006, 07:04 PM
Hi, guys :)


thanks a lot to all of you, now I have to check all links you sent and read all files and then I will make an answer.

All is under construction now;)

Best regards, Josip :)

Peter_1
15-08-2006, 09:21 PM
Just a small add Josip: personally I think that as long as you have a proportional valve, the PID algorithm will take care of the movement of the valve.

You will of course have to experiment a little bit with the PID values but it doesn't matter what type of valve you have but I should go for a linear flow.

If more then one coil is used, a 2 way valve can be used and the speed of the pump (VFD) is then pressure controlled. Some pumps even have this technique build into it.
We used the valves of Staeffa (which is now integrated in the Siemens group)

We did it once with 2way valves on all the coils,except on the largest coil where we had installed a 3 way valve, ensuring that the system was never completely blocked.

Even a small Logo of Siemens or the similar device of Tlmcanique or the similar device of Crouzet are capable performing this task I think.
There are even wireless GSM modules available for the Logo.

The nicest application we once had was an airflow of 35.000 m/h (20.600ft/min) which had to be maintained at a temeprature of 17C +/-0.5K (62.6F) and as long as free heat or free cooling was available, we had to use it.
The free cooling came from outside, the free heating from inside the factory.
There was a cooling coil, connected to a Carrier chiller we installed and a steam coil, connected to the excisting steam net in the factory.
A subcontractor installed the steam lines.

Also the pressure had to stay within very narrow margins but I forgot the exact figures. This pressure was controlled by inlet vanes on the turbines.

There could be a variation in pressure drop when the registers on the inlet turned to take outisde or inside air and if they took a position between the two for mixing the air.

This whole setup was for ensuring a very steady cooling flow of falling melted polyprop (carpet yarns) for making carpets.

Everything was controlled by a Siemens S7/300 PLC and an OP. But afterwards, we found out that Staeffa had a device right of the shelves for this which costed only a fraction of the total price we paid for it.

It was installed +/- 10 years ago and it's still running.

Andy
18-08-2006, 02:10 PM
Josip, when we have such a project, I give the requirements like you describe to a PLC programmer and he translates this to a working PLC program.
We have for such a cases 3 capable persons.

I also think that JUMO (Germany) has a capable controller, altough I think this is a typical task for a PLC programmer.
It seems on the first sight not that difficult.

Do you need often to defrost it? I ask you this because your temperature are well above 0C.

You can use also a stand alone electrical heater/cooler like we did in the attached picture: 3 way valve and a separate small pump, pumping the warm glycol. Works very well.

But why do you need to control the LMTD? This is not completely clear for me.

http://www.vdhproducts.nl/ is a Electronic Control manufacturer specialist for the vegetables and fruit sector. Perhaps they have a controller right of the shelves.

We also have in our village an electonical designer who can make almost everything with microcontrollers.

Ask about the Proba. The company I work for assisted VDH in developing the Proba for glycolbased cooling on Banana Rooms.
This application is very close to this and well within what the Proba will do.

I recommend it:)
It would be possible to use PLC, but better gain the working knowledge on a Proba first.

Iceman we are currently installing 12 banana rooms in Sacramento California. All have Proba on them.

Kind Regards Andy:)

Kind Regards Andy

US Iceman
18-08-2006, 09:26 PM
Andy,

How did you get a job in California from Ireland?

That's a long drive to work each morning.:D

Renato RR
19-08-2006, 10:19 AM
Problably,
they get yob and give it to someone in Sacramento to doit instead of them (Subconstractor).

Renato

Andy
19-08-2006, 12:15 PM
Andy,

How did you get a job in California from Ireland?

That's a long drive to work each morning.:D

Job is in partnership with an american company. We supply european equipment that is not normally available in the states, design the systems, supervise the installation and commission the controllers and the control valves (the other company does the chilled water and provides the site labour).

Me I won't be going out unless Iam needed, two of my team, an installation supervisor and a commissioning engineer are going out end of this month.

We do this type of work all over the world, sending teams out for up to 5 months at a time.

Sweden, Columbia, England, Scotland, china and the US, we have installations all over the world:)

Still it's a bit of a travel to work every day:D

Kind Regards Andy:)