Psychometric exercise

[Abby Normal]

Dan

The room point is the condition you want to maintain in the space.

The supply point is how cool and dry the air must be to compensate for the heat and humidity gains that are just created in the space. (or come in through the ceiling, exterior walls, the windows)

The return point shows that the room air is slightly warmed. Maybe it uses a plenum ceiling and the lights are heating up the air a bit.

The 'return' air and the outside air are mixed, the typical system is providing some fresh air. The lengthof the line segment between "return" and "Mix" is proportional to the outside air percentage. The length of line segment between "mixed" and "outdoor" is proportional to hown much return air is mixed with the outside air.

This process most likely is a draw through fan, and the fan and motor will heat the air. There could also be supply ducts in unconditioned space, so even if the duct work is insulated it will still pick up heat. Therefore the coil must over cool the mixed air down to a bit cooler than the required supply condition to compensate for the reheat.

The line segment between "leaving coil" and "supply" is reheat after the air is cooled. Could be a deliberate reheat or something you have limited controil over such as fan heat or ducts in unconditioned space.

To determine mixed air properties, plot the condition of the two air streams on a chart and connect with a straight line. The mixture will be on that line.

A quick way of determining the mixture, with reasonable accuracy is to do a weighted average of the dry bulb temperatures and the volumetric flows. Puts you in the ballpark.

Tmix= (T1 x Airflow1 + T2 x Airflow2)/(Airflow1 + AIrflow2)

The error for doing this based on volume and not mass is minimal usually. Once you calculated Tmix, locate where that dry bulb temperature cuts the mixing line you drew, and read off the wet bulb, the RH, the enthalpy etc.