Do we always need a heating coil in an AHU?

[noviceengineer]

Good day:

On a current job we are designing we have 8140 CFM supply air. We also have 900 CFM outdoor air coming in. Upon doing our calculation for the mixed air temperature coming into the air handling unit, we arrived at 64ºF (heating side). This unit serves a field of variable air volume devices later on down the system, which in turn heat up the air from mixed air temp to around 100ºF.

My calcs were based upon -10ºF outside air in the winter time (out of the Ashrae book) and 72ºF return air temp.

Is it a safety thing why we always put in heating coils?
Can we maybe get by without them in some cases?

Thanks!

 

[ChasBean1]

Noviceengineer - I figured it with 67°F return temperature and 15% RH (might be more likely what you'll see on a -10°F day). I still get about 58.5°F mixed air temperature. You have reheat at the terminal boxes. If you have a chilled water coil, install freeze protection just upstream to ensure the mixed air mixes at the right proportions and doesn't freeze the coil. Without a preheat coil, you should also ensure there is appropriate mixing by considering the use of an air blender. Then yes, you are correct in your assumption that you would not require a heating coil installed in the AHU with this system. Heating coils in mixed air systems such as this are usually for design conditions, but I think they are often for protection of the designer, perhaps as a backup to a malfunctioning economizer damper set. Keep in mind heating coils add to first cost and operating cost (more fan power to overcome friction). Keep on your current track! -CB