Direct Evaporative Cooling VS Humidification

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Dears Srs.,

 

I am simulation a service building with cooling loads, about 4000W/m2 with a schedule 100% activates in all hours. My system is an air handling unit with return and free cooling. The air handling unit has a cooling coil, a heating coil and a direct evaporation cooling coil 100% activated. The system has humidification and the minimum room relative humidity is 30%.  The temperatures of insufflation are 14-20 ºC for cooling and 20-31 for heating. The room set point is 18-27 ºC.

When I run the simulation, in winter the cooling consumption is 0 because the operation of free cooling. On the other hand, in Winter I have consumption of humidification to control the humidity to 30 %. However the direct evaporative cooling coil does not work as I expected because the direct evaporative cooling coil should help in humidification of the rooms. My question is: Why the direct evaporative cooling coil does not work in winter, when I have height consumption of humidification. In practice the direct evaporative coil work all the year.

 

 

The Best Regards,

 

 

Ricardo

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Ricardo-

The reason that the evap cooling does not operate during winter is because it does not operate in heating mode. I cannot think of a way to get it to operate in heating mode.

Ultimately, the problem is that you are paying for humidification in winter. Maybe it is insignificant? If it is significant, what you could do is set up a second piece of heating equipment in your humidification plant, make it free (by setting the energy rate to something like .01 Mbh or KW) and then schedule that piece of equipment to be on during the heating months only. Subsequently you might have to schedule the other piece of equipment off during those times (depending on which is the first piece of equipment).

Of course, then you run into the issue of a little bit of free heat, but that would have to be a judgement call.

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Bob,

Thanks for your replay, but I expect that in the winter the direct evaporation cooling coil cool the air and in the same time humidificated it. The purpose is not heating the air but, instead, cooling and humidification the air. In practice the direct evaporation cooling coil is 100% activated. At the moment I can´t do it in the Trace.    

Thanks for your help

Ricardo

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Yup, I definitely understand the intent. It just cannot be modeled directly in TRACE 700 (since evap cooling won't operate in heating mode). 

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Dears Srs.,

 

I think that I have solved the problem exposed before. The simulation program Trace 700 doesn’t work at the same time with free-cooling and evaporative cooling. To solve the problem I put a scheduled in the operation of free cooling. This scheduled turn off the free cooling when humidity below the humidity set-point. When the free cooling turn off, the evaporation cooling turn on, cooling the spaces and at the same time controlling the humidity.

I think this is the best solution for this problem.

Thanks for your help.

 

The Best Regards

 

Ricardo    

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Dears Srs,

 

At the moment I’m doing a thermal simulation of a Data Center with Trace 700. The Data Center has miscellaneous load of 4000W/m2, part of this is electric equipment and other part is cooling load. The equipment is 100 % activated and the cooling load is only sensible. My system is composed with an air handling unit with variable volume and displacement ventilation. The air handling unit has a heating coil, cooling coil and a direct evaporative cooling coil. All the coils are 100% activated. The supply air temperatures are 20-35ºC and the set point of the room is 25ºC (+2ºC-7ºC) (18-27ºC). The minimum relative humidity is 30%. The AHU is allowing to work with free-cooling.

In this case, the evaporative cooling coil not only cooling the air but also humidifies the air. The evaporative cooling coil turn on when the humidity below 30 %. The evaporative cooling coil doesn’t work in same time with free-cooling, when the evaporative coil turn on the free-cooling turn off. To simulate this situation I put a schedule in the free-cooling that turn off the free-cooling when the humidity below 40%. I put 40% instead of 30%, because I want to define a safety margin.

When I analyze the results, the relative humidity isn’t controlled. At the same time, the supply humidity ratio differs with humidity ratio in the room. In fact, the supply humidity ratio must be equal with the humidity ratio in the room because the cooling load is only sensible.

 

My questions are: Why supply humidity ratio differs with humidity ratio in the room? Why I don’t control the minimum humidity with this way? What is the best way to revolve this problem?

 

Best Regards

 

Ricardo

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Ricardo-

WARNING- The TRACE 700 viewer has been known to cause premature aging, and vision loss!

The first question for me is, "why is the humidity ration increasing within the SYSTEM?"

To troubleshoot:

  • Do any rooms contain infiltration?
  • Do any rooms contain people?
  • Do any rooms contain custom misc loads (that might have latent component"?
If the answer to all of these questions is "No", then it is still possible that your HR would change simply based on the transfer functions within TRACE (if there is flux in the supply air's HR).
You could test this theory by setting all of the rooms within the said system to have a "Moisture Capacitance" of none (done in the thermostat template or Room tab of create rooms).
It is really important to solve this problem first before you can more on to your other questions!
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Bob many thanks for your amiability.

 

My answer to your questions is no. There are no infiltrations, no people and equipment with latent loads. My simulation is definite with “Moisture Capacitive  - none”. My problem is that the humidity ratio in supply air is different that in the room. Because of this, relative humidity in room diverges.

I think the problem is the definition of the evaporative cooling coil because when I turn off the evaporative cooling coil the humidity ratio in supply air is equal with the room.

Bob Can I send you the simulation file? If you permit it and if you have a little available time please send me an email to my email rlicinio@gmail.com.

 

 

Many Thanks

 

Ricardo Marques   

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Hey RL-

Could you please post directions on how you get access to the "TRACE 700 viewer"??

Thanks

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Hey

 

To get the access to TRACE 700 viewer in full year you have to substitute the original files  "T700Special", "t700view" and  "Editors700" to others.

If you want this files give me your email and I send it.

 

Best Regards

 

Ricardo

 

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The simulation files of the direct evaporative cooling is in attachment.

 

Thanks

Ricardo Marques

 

File Attachment: 
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The simulation file of direct evaporative cooling is in attachment.

 

Thanks

 

Ricardo Marques

File Attachment: 
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Dears Srs.,

 

The problem of my simulation is the pychrometric state point thar don’t close. I saw this is in the pychrometric state point report. To resolve this issue I try play with this supply temperatures but I can’t resolve this problem. I think the problem is the heat gain is only sensible consequence the sensible heat gain is 1.00. The Trace 700 is confused with that situation.

 

My question is: How I should simulated a building with sensible heat gain equal to 1.00? How I can resolve this problem of pychrometric state points?

 

Best Regards

 

Ricardo

 

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Ricardo-

When you have problems with the psychrometric loop, the first thing to make sure you do is specify the supply air temperature, so that airflows do not get calculated incorrectly.

Then, if the sensible heat gain is in fact 1.00, the humidity in the room will simply have to fluctuate (it will of course be dry).

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