Fan Coil Unit Design

[hvacnewbie]

Hi all,

Im glad i found this place, i hope many great engineers here can help me. As a graduate and starting a new job as a HVAC design engineer. I still dont understand a few things.

why is it better to use temperature differential of 8 degrees instead of 10 for the chilled water.

 

[lilliput1]

No it is not. Ten Degrees F is the usual but optimum design is about 14 or 15°F. The higher the delta T, the less gpm and the less the size of the pipes. It can't be too high though because it would require deeper coils, mor fins - both harder to clead and impose more air pressure drop.

 

[ChrisConley]

Delta T is also dependant on the inital temperature of your chilled water and the capabilities of your chiller.

Are you speaking Celcius or Fahrenheit? Most of us are bilingual, but it does make a big difference.

 

[hvacnewbie]

Sorry, i didnt make it clear enough.

Im from the UK, its Celcius that i was refering to.

Thanks for the replies.

I still dont understand, i mean i know what you are saying but can you give an explaination as to why higher dela T means less pressure and lower pipe sizes?.

 

[ballsonwalls]

Pressure drop in a pipe is determined by the D'arcy Weisbach equation. This states that pressure drop is a function of the velocity of the fluid of the pipe (among other things). The required GPM for heat transfer is GPM = BTUH/(500 * delta T) where delta T is leaving water temperature minus entering water temperature. As delta T increases, GPM decreases. As GPM decreases, velocity decreases. As velocity decreases, pressure drop decreases. As GPM decreases, pipe size decreases.

 

[ballsonwalls]

the pressure drop is proportional to the velocity squared

 

[hvacnewbie]

Wow, that was a quick reply. Thanks

I think i need to wikipedia D'arcy Weisbach equations and study them to get my head around this.

I have another questions, When you have calculated your solar heat gains and internal gains. I get Sensible and Latent total. Why is it that for selecting fan coil units you only have to specify the sensible loads and not the total loads which inlcudes the latent?

 

[ballsonwalls]

It is actually Darcy-Weisbach. My mistake.

 

[TADiep]

"I have another questions, When you have calculated your solar heat gains and internal gains. I get Sensible and Latent total. Why is it that for selecting fan coil units you only have to specify the sensible loads and not the total loads which inlcudes the latent?"

The sensible heat directly warms up the room and refers to the dry bulb temperature. Latent heat affects the humidity of the room and refers to the wet bulb temperature.

Depending on the climate region your project is located, accounting for latent heat gains can be very important or neglible.

I am in California, US and the climate here is moderately dry except near the coast. Even near the coastline, the humidity levels are not that extreme where latent loads need to be taken into account when selecting units. Sensible loads are the primary loads in this climate. Latent loads are often times omitted unless its a special design application.

On the other hand, if we were in a humid climate, say Hawaii or Mississippi US, than latent and sensible loads MUST be taken into consideration when selecting units. There is alot of moisture in the air and the unit must be able to cool and dehumidify the space to comfortable levels.

You may want to read ASHRAE standard 55 for more info on human thermal comfort.



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A green thought..."We don't inherit the earth from our ancestors, we borrow it from our children." (unknown)