Off-coil temperature and airflow effect on total cooling capacity

[Wsyzxxn]

Hi everyone,

I have a project which requires 100% outdoor air, so make-up air units are used. Our rooms require strict humidity conditions for operation so a lot of dehumidification will be needed. The existing make-up air units have a design off-coil temperature of 18deg but this will not be sufficient to reach the absolute humidity for our room conditions.

Our actual outdoor airflow rate is less than the design airflow rate that the make-up air units were designed for. My train of thought was that having lower airflow will allow the make-up air unit's off-coil temperature to be lower, for the same cooling capacity. I would think more contact time for the air to pass through the coils.

However, my supplier told me that the total cooling capacity would be reduced for a lower airflow. I'm not sure why? As long as the supply and return chilled water temperature and water flowrate stay constant, I don't see an issue with maintaining the same total cooling capacity. To me, having a lower off-coil temperature and reduced airflow will increase the latent capcity while reducing the sensible capacity.

Is my understanding wrong? Is the supplier correct? Would like to hear more opinions. Thank you!

 

[EnergyProfessional]

I tall depends on the climate, and the conditions you need inside. Also infiltration and other sources of water.

if cooling alone doesn't give you the result, you can look into the wraparound heatpipe and ERV solutions that run air through different stages to get you a lower humidity ratio than the final leaving temperature would suggest.

Yes, lower airflow will reduce cooling capacity. among other things, we use airflow volume control to control cooling capacity. I don't know what your supplier said exactly, but they could clarify that for you.

 

[willard3]

CFM X 1.085 X delta T = BTU/hr
If you lower CFM, BTU/hr will decrease

 

[EnergyProfessional]

For cooling, it is a bit more complex due to dehumidification.
And with a given coil, a lower air-flow will result in somewhat higher dT. Not enough added dT to keep the BTU-number the same, of course.

a cooling coil capacity depends on the 2 fluid flows, temperatures, and air psychrometry. The manufacturer basically has a software that accounts for psychometrics, heat transfer etc. It is all dynamic and changes once you change flowrate, or temps. it is not linear.

 

[MintJulep]

You logic holds for each cubic unit of air.

But you are decreasing the cubic units of air per unit of time.

 

[willard3]

As CFM decreases, sensible heat ratio may also change, ie, wet bulb performance may change

 

[GT-EGR]

You need to ask your supplier a different question. First determine what supply air temperature you need to get the RH you want, and then have them run their software to find out how much airflow the unit is able to cool down to that temperature.

If you leave everything fixed, as you lower the airflow quantity across the same coil, you will see your cooling capacity decrease, but you’ll also see the leaving air temperatures start to lower and maybe approach the space RH/dewpoint that you want. Leaving air temperature at the actual makeup airflow you need should be your goal, don’t worry about total capacity, that is just secondary calculation.

 

[BronYrAur]

You made the assumption that your chilled water supply and return temps will remain the same as they were for your original airflow condition. Why do you think they will remain the same?

 

[ProcessHVAC]

The manufacturer is likely correct. It will likely not work exactly how you are planning. Heat transfer to the coil is directly related to turbulence, by reducing the air speed, you are likely reducing the heat transfer on the coil. As others have mentioned, this would also have an impact on chilled water temperatures. By not fully loading the coil, the return chilled water temperature will be cooler than design. A different coil could work for getting temperature lower at a reduced CFM.

 

[moideen]

Is the system built with heat recovery wheel and heat pipe? Or evaporator coil selected based ambient air temperature as ON coil temperature?
What is the humidity design conditions?
The total unit capacity calculated by 4.5*CFM*delta enthalpy=total cooling load
The total latent load capacity calculated by .68*CFM*delta W=total Latent load
When airflow decreases which decreases the total latent capacity along with sensible capacity
Since the air is leaving from the cooling coil as saturated condition (if not using reheat electrical coil or heat pipe), it will increase the RH though the unit decreasing the humidity ratio,
Also, look at the chilled water circuit, there gradually decreasing the capacity because delta T across the unit is decreasing. When decreasing design air flow then chilled water delta T also decreasing if chilled water flow rate is constant