Return Ducting

[hvacers]

Can any body give me ideas how to design return ductings.


For example i have 50 TR AHU supplying 20000 CFM to an area? does my return must also be the same as my supply. for example it is just an office space

 

[imok2]

Keep Main supply velocity about 2000 FPM and return about 1500 FPM
A good design of friction rate per 100 feet of duct should be about 0.08 to 0.1 inwc

 

[hvacers]

what if the the return method is ceiling return?? how to size the main bell mouth return and how many room return diffuser rule should i follow?

 

[willard3]

My advice to you is not to use a ceiling return plenum. They are full of unexpected surprises and can't be balanced properly.

 

[Waramanga]

hey willard,
could you explain your objection to ceiling plenum return. these are the most common return air paths here in australia for large office building, probably due to the ease of doing fit-out, no real problems in balancing that I have come accross. thanks

 

[hvacers]

the building i am handling is a ceiling return setup. i want to know what is the ruling when it comes to making return diffusers. please help me

 

[ChrisConley]

Locate ceiling returns in every space that is seperated. Size the returns generously, based on the expected airflow. Use manufacturer's charts. Eggcrate style is typically acceptable.

We typically provide a return air 'boot' on the grille to minimize crosstalk, and for visuals. Double check your return air path, and provide transfer grilles as required to get the air back to your air handler.

 

[willard3]

Waramanga

I was retained to trouble-shoot a plenum return system that was less than a year old. I took the balance report which showed cfm readings that mimicked the contract documents. The balance contractor and I read all the diffusers and the actual reading variance was way above 50%. The balance report was obviously done without reference to the actually-measured cfm's. I have seen this many times.

We took down the ceilings and added a ducted return and variance was less than 10%. Any place where air delivery is important, as in an hospital, return plenums are a disaster.