heating coil sizing

[mechanicaldude]

Can somebody give me a good explanation why typically when you size heating coils in VAV systems (either reheat coils for VAV boxes or in the VAV AHU) is it customary to size the coil for about 50% of the maximum (cooling) airflow rate. I usually see in the sequence of operation that when heating is called for in the space the boxes close down to 50% and then the heating coil is activiated. why is it done this way ??

Thanks for your help.

 

[ChrisConley]

I don't usually see this, but the reason would be load. My guess is that you are in a cooling dominated climate, so the requirement for heating is ~50% the requirement of cooling.

This will allow a reduction in fan energy (why move 100% of the air when you don't need it all).

The sequences I write tend to be more based around free cooling: boxes open to minimum for heating and cooling, boxes open to 100% for free cooling.

 

[imok2]

A cooling coil may have a water temperature difference of 10/12*F delta T whereas a heating coil may have a delta T of 20*F so the amount of air will be reduced so as not to develope extreme temperature fluctuations(hunting)

 

[DRWeig]

In cooling, you're limited to a delta-T (air side) of 15 to 20 degrees F. In heating mode, if you can go up into the 40 to 60 degrees F delta T range, you reduce the supply air velocity and, hence, the draftiness of the room when heating. When cooling, folks tend to appreciate the air movement...


Them's my two cents worth...

Good on y'all,

Goober Dave

 

[blutfort]

Ditto to what DRWirg said. Plus wouldn't the reduced air flow into the room help minimize fan energy. Isn't that a big plus of a VAV system.

 

[ChasBean1]

You have to go back to the purpose of a VAV system. Their mission in life is to heat by reducing cooling, saving energy by cutting down on airflow. It costs to cool, then add heat, so why not just reduce the cooling? Reheat is an add-on concept because you still need enough ventilating air and need to dehumidify. It's pretty common that people get too cold when you just throttle back the cooling.

So the heating process is designed in two main steps: 1) reduce cooling airflow, then wait and see if that warms the space enough. If the space is still too cool, 2) begin to bring on reheat (while still at the reduced flow) to control room temperature.

 

[Jabba007]

Reheat does not dehumidify.

That is my biggest pet-peeve of misconception of HVAC people.

Reheat does not dehumidify. It removes NO moisture. It does reduce the RH of the supply air, but the ONLY thing that removes moisture is a cold coil. The reheat allows the cooling coil to run and supply dehumidified air, without overcooling the space. Essentially, it nothing but a false load for the zone.

Jabba

 

[SAK9]

cd122395:

Please read the attached pages and your doubts will be cleared.

 

[SAK9]

Sorry did not upload the file in the first post

 

[marcoh]

In a VAV box with heating, the VAV box modulates the air flow down to the minimum air flow (eg 50% of maximum air flow) before the heating comes on.