Selection of Air Handling Unit (Chilled Water Type)

[PS]

Hello All,

I have a situation where the result of heat load, which was done by a consultant, is 9000 CFM at 20 TR. The heat load summary is given below:

Sensible Heat Gain: 63 kW
Total Heat Gain: 70 kW (20 TR)
Dehumidified Air Quantity: 4250 L/s (9000 CFM)
Inside Design Conditions: 23+/-1 degC at 50%+/-5%RH

Clearly it is 450 CFM/TR. The space is exposed to sunlight and so the sensible heat gain much more than the latent heat gain.

When I checked with few AHU suppliers, they informed me that they manufacture AHU at 400cfm/TR as a standard. They also adviced med to go for a 22.5 TR AHU which would give 9000 cfm. I'm a bit confused. Do I need to match the sensible capacity in AHUs or the total heat capacity? In other words, do I need to select an AHU where the the CFM matches which is rated for 9000 cfm at 22.5 TR or do I need to select an AHU for 20 TR at 8000 cfm?

Please do advice on this regard.

 

[GT-EGR]

For your AHU selection, don’t get too hung up in these CFM/ton numbers - they are just general sanity checks and reference points. What matters is your load calculation and making sure the air you deliver satisfies those loads.

I’m assuming those calculations you did were just your interior loads, which is only a part of your AHU sizing. There are a lot of different ways people approach this to get the same answer. You need to select a discharge air temperature and moisture content that satisfies both your sensible and latent head loads. In a typical building that isn’t used for sweaty or moisture producing activity the sensible load will likely dictate your CFM.

I don’t know the metric conversions, but in English version your sensible heat load = CFM x 1.08 x temp difference between air delivered and the temp of the space served.

The 1.08 is a conversion factor of a variety of inputs including unit conversions, air density, air heat content, etc. it should be easy to look this up.

So if your 9,000 CFM x conversion factor x (space temp - air temp) = your sensible load, then your air handler is selected correctly.

Then you just back check the latent load using a similar calculation, and make sure your air is dry enough. In the English version your CFM x 0.68 x air humidity ratio difference between desired room humidity vs supply air humidity. This number will likely exceed you space latent heat gains - and if so you could try a higher supply air temp and more CFM, or just leave it where you have it.

To find the exact supply air temperature you need to equally satisfy sensible and latent heat gain, you have to plot your sensible heat ratio from your room temp and it will land on the correct supply air temperature you want.

Just don’t forget that your air handler also takes the load of cooling the outside ventilation air that you add in for your space.

 

[PS]

Thank you GT-EGR.

"So if your 9,000 CFM x conversion factor x (space temp - air temp) = your sensible load, then your air handler is selected correctly." Am I suppose to take only the sensible load and proceed with the AHU selection? I get struck everytime in this place. The AHU catalog does not list out either the ADP/bypass factor or the discharge (off-coil) temperature. How do I proceed in this situation? Kindly do advice.

Would anyone suggest few articles where I can gain knowledge about AHU selection? I already posses books on Air-Conditioning where they only talk about ADP, Bypass factor, and discharge temperature. But, in practice, I'm NOT using these parameters to do AHU selection. Kindly do suggest.

Thanks, Again!

 

[moideen]

What is the percentage of fresh air for your system? It influences the coil load.
Moisture content influences the coil load, you can use this formula to find total coil load
4.5*cfm*air enthalpy difference
In your example: 4.5*9000*(28.8-24.4)=15 ton
As GT-EGR said, outside fresh air is not considered, when add the fresh air , it increases the coil load.
Just Google and download Mcquay psychometric analyzer

 

[GT-EGR]

PS, you have the first part complete, you know you have a quantity of air being at the right temperature that will cool the sensible load in the space. You should get in the habit of checking the latent load as well, although in a normal office building if you supply at a standard 55ish deg F temp you should be fine.

Now you have to determine how much return air you will have, and at what dry bulb and wet bulb temp, and combine that with the quantity of outside air you will have, and at what dry bulb and wet bulb temp. That should combine to equal your 9,000 CFM total, at a mixed air dry bulb and wet bulb temperature condition. This determines your total load on the air handler. The coil has to cool your 9,000 CFM, from the mixed air condition, back down to your supply air condition.

If you are using an AHU catalog, they usually have a large menu of CFMs, and a few varying entering air conditions. Once you follow the chart from those points, they usually provide the AHU performance as far as what the discharge air temperature is, and the total load associated with cooling the air to that temperature.

As far as articles, a good AHU catalogue will likely walk through the selection process, which is the best place to start as it will simultaneously teach you to make the selection too. There's a lot more to it than just sizing the coil and figuring out the load.

 

[PS]

thank you @GT-EGR and @moideen.