Dear Matthew,

..." Is there a way to force HAP to supply a constant air flow through a "system"
(this includes specifying return air and outside air requirements) and
calculate the cost of providing the cooling and heating water to the "system"
and "zone" reheats? Any explanations would be helpful. "...

The air quantity through the cooling coil that is indicated by the HAP is the minimum air quantity that warrants that you get the desired space conditions. It is called the Dehumidified Air Quantity, (DA). The air that you deliver through the ducting to the space is called Supply Air, (SA). Normally DA is equal to SA. You have to maintain this flow of the mixture of return/outside air over the coil or you will not obtain the desired temperature and RH in the space.

Now, if by some reason you want to increase the flow of air, make a bypass of Return Air only into the plenum of the fan. The return air is thermaly neutral to the space and will increase the air quantity without afecting your psychrometric process.

This is done by example in night clubs or other loads with a very large latent component. The supply air quantity in these applications is some times too small due to the very low Apparatus Dewpoint, and you will have to increase the air quantity to ensure that you have a proper distribution of air in the space. Aim at least to maintain 6 air changes per hour in the space.

Some high quality air handling units of modular design, have an external or internal bypass duct that runs over the coil.

You can make the bypass of air with a mix of outside air and return air directly from the mixing box passing over the cooling coil, but the use of bypass of return air only is better to achieve a good control over the RH in the space.

Kindly feel free to demand more information on this subject.

Best Regards
Eduardo Barrigana