Economizer damper control in heating season

[raptors076]

Hello All,

ASHRAE 90.1 recommends supply air temmperature to control the economizer dampers and cooling coil valve. The standard provides clear reasoning for doing it.

My question is related to heating season. From your experience ,did anyone had any issues with coil freezing if similar approach is used in heating season. i.e. SAT controls economizer damper and heating valve. Heating valve only comes ON when dampers at minimum OA position. Is there any reason you would recommend any changes in the below sequence?

SAT controls the outside air damper, preheating coil, and heating coil
- Free cooling mode - Economizers controls SAT
- Heating - Outside damper will be at minimum position and preheat coil valve will open first to maintain SAT. Once preheat valve is 90%,heating valve will start opening to maintain SAT.
- Low mixed temperature limit can be separate control loop. i.e when MAT falls below 6 degree C, preheat valve opens to maintain MAT to 10C.

Thanks

 

[willard3]

What is the order of coils in the AHU? Is the HW coil first or the CW coil first in OA?

 

[raptors076]

The sequence is outdoor air damper, pre-heat coil, heating coil, & cooling coil. Thanks

 

[lilliput1]

Coil arrangement should be preheat coil, cooling coil then reheat coil if the AHU serve only one space. If multiple spaces are use each should have variable air volume box & terminal reheat coil. If heating coil is upstream of the cooling coil, the cooling coil will be heated up. The heating coil should be controlled to maintain space heating temperature, not supply air temperature. The preheat coil should be controlled in sequence with the OA damper closing to minimum position to heat the air downstream of the preheat coil to 50F.

 

[Drazen]

Yes, preheat coils can freeze when water flow is too low, it is therefore not customary to modulate flow of preheat coils.

However, I believe your description of heating sequence is not complete. Preheat coils should not open befor heat coils in all scenarios, but only when the outdoor temperature falls bellow trigger point, on higher outdoor temperature heat coil should modulate alone.