Cooling Capacity vs Supply Air Flow Rate 1

[SGSA]

Hi everybody!

So I did a head load calculation using HAP, using terminal units for a VRF system.
Lets take a look at one of the rooms in the office block. I am using SI units, but it doesn't really matter because the question I am asking is theory/practical based.
Room 1 is 17.2m2. And the total cooling capacity calculated is 3.3kw. This comes out as 192w/m2, which is totally expected for the hot and humid climate I am in.
The design supply air flow rate is calculated to be 233 l/s. The sensible coil load is 2.5kw, which means a SHR of 0.76 (which I don't see as a problem).

Now, selecting the equipment. From the range of ceiling cassettes I am choosing from:
Cassette 1 is rated 3.6kw (12000 BTU) in cooling with a maximum supply air flow rate of 167 l/s.
Cassette 2 is rated 5.6kw (18000 BTU) in cooling with a maximum supply air flow rate of 222 l/s.

Cassette 1 is the best match according to cooling capacity. However it doesn't provide the air flow rate I need according to the calculation. I don't want to oversize the unit by going for cassette 2, because that could lead to additional problems on it's own. Any advise? Does the supply air flow rate actually matter when considering cassettes? Because I see it being more useful in applications with diffusers where you actually have control of the supply air flow rate.

Thanks for any assistance!

 

[lukaiENG]

Cassette 1 is going to require a higher temperature difference between supply and return temperatures to deliver the capacity. either is it providing notably colder air than your calculation is assuming, or perhaps it is assuming a higher return air temperature than you are assuming. I would look at the temperatures used in the data sheets for cassette 1 and see how they compare to your assumptions.

if you are supplying colder air this is probably fine, although in your humid climate you may need to consider condensation.

 

[MP Texas Engineering]

Hi SGSA,

Great question as I recently have run into this problem as well. The way I see it, is you need to meet your capacity with an equally sized cassette, and you need to meet your air flow rate because of comfort issues. I base my design airflow off of cfm/sqft or air changes/hour, both work well and I see you are close to 2.75 cfm/sqft with your design air flow rate. Cassette 1 provides a 1.85 cfm/sqft. Personally I think you are oversized on your design air flow rate and something between 1 and 2 cfm/sqft will work. So Casstte 1 seems to work fine in my opinion. I use the ASHRAE handbook to decide my air flow requirements usually, lower on interior locations and higher on the building exterior.

 

[BronYrAur]

How is ventilation air brought into the space? If through a supplemental system, that will be additional air flow that you can add to the cassette.... Assuming it's conditioned air that is.

 

[SGSA]

Great responses guys!
@LukaiENG - Absolutely right! I did lower the room set-point to 22 deg (default was 23.9 deg), which may be overkill. By raising that a little, the supply air flow rate does decrease.
@MP Texas Engineering - The calculated air flow rate does seem a bit oversized.
@BronYrAur - Direct ventilation at ambient conditions, which unfortunately adds considerably to the heat gain.

 

[Drazen]

what does it mean SHR of 0,76 is not seen as a problem? Does it match with your selected unit SHR or not? And, have you accounted outdoor air load or not?

Normally, indoor VRF units are completely unable to deal with outdoor air latent gain, this is biggest issue with VRF in general, as units with much higher latent capacity work on much different freon temperature regime.

HAP can offer you some help, but you need to configure it to match your actual condition, otherwise you will get useless results. In what you describe, it can happen that your indoor units will create waterfall on ceilings if the design in question is in some very humid area.

 

[willard3]

SHR in Tejas is unlikely to be 0.76 on a design day.

VRF units are designed with maximum 2-row coils that will not change the wet bulb temperature much.

I agree with Drazen, you have not accounted for OA or people load in your calculations.

 

[trashcanman]

Stop trying to make catalog data fit your calculations. It usually does not fit. Finalize the cooling/heating load and select a unit that will meet the load. The cfm will be what the unit provides. The catalog data comes from extensive testing. Have the vendor/sales rep do the selection and provide written document for the selection. That is a CYA thing.