Best Way To Mix Outside Air and Return Air 1

[SRBKDW]

I have a project where we are installing two (2) new 12 ton McQuay Vision AHUs in a stacked configuration. The AHUs will be installed in a new mechanical room. The mechanical room will be located on an outside wall so we will be providing an exhaust air louver to provide full economizer capability. I'm assessing options for the best way to control and mix the outside air and return. Options I'm considering are:

1. Using the mechanical room as a mixing box. Control dampers for outside air at the outside air louver and for return air at the return air grille.
2. Inline mixing boxes installed as part of the AHUs with control dampers built into the mixing boxes. The top unit would have inline outside air with return air coming in the top of the mixing box and the bottom unit would have inline outside air with the return air coming from the side of the mixing box.

Let me know what people think and if there other options I should consider. Thanks in advance for your help.

 

[ChasBean1]

I vote for Option 2 - there are a few reasons for this.

1. When you make the mechanical room the plenum, suddenly it's an "outside" space. You'll have moisture and skin issues to consider on the INSIDE walls. Vapor barriering and insulating the room could prove costly.

2. Even if you duct the OA and make the room a return only plenum, there are concerns about storage, especially of VOC-emitters. Where's the best place for the painters to stash their stuff for the night? How about that mechanical room...

3. There are code concerns with respect to combustibles and flammables located in plenums. Return air plenums typically need pass-through systems to be fire rated. Establishing the necessary fire rating could add cost.

4. Because people need to go in and out through the door, opening and closing the room door could alter outside air proportions and affect building pressurization, which could lead you to more problems.

 

[MintJulep]

I second everything Chas said.

Additionally, using a proper mixing box allows you to design a controlled mixing space, which in-turn allows you to get accurate measurements of the mixed air conditions.

I suggest installing an actual air mixer as well, to avoid stratification through your coils. One possible manufacturer is blender products.

 

[digitrex]

Can someone please advice me what is the ideal mixing ratio of fresh/return air for office environment?

I am an electrical engineer but I have to take care of airconditioning system in my company.

 

[remp]

Another reason not to use the room as a mixing chamber is because you have 2 AHUs in the one room. Using one common mixing chamber for 2 units assumes both AHUs have the same ratio of outsdie air/return air and both AHUs will operate in economy cycle indetically. Both AHUs are unlikely to be serving identical spaces with identical outsdie air quantities.

Digitrex,
The amount of outside air (minimum) depends on the population density of the space and the code requirements for outside air per person. Look up your local code requirements and if living in US - ASHRAE, UK - CIBSE, AUSTRALIA - AS1668.2

If you want economy cycle you will end up with an O/A intake the full size of the AHU supply air volume, but its a good idea to have a fixed minimum O/A damper for min O/A and a modulating O/A damper for the remainder to bring you up to 100%O/A....good luck!

 

[digitrex]

Thank you Billy. Our AHU's O/A intake has only one manual damper and it is fully open at this moment. Our AHU is quite old and the chilled water is controlled by a 3-way valve.

There is a wide variation of room temperature(between 20deg.C and 28deg.C) between a hot day and a cool day.

I think O/A temperature influences a lot in the room air temperature. Therefore, I am thinking that reducing the O/A intake will narrow the temperature variations between hot and cool days.

If you say that our AHU is undersize, well, it will be a difficult task and cost more to replace it. Do you think my logic of reducing the O/A intake to improve the temperature regulation will make any sense?

 

[remp]

The cheapest option would be to set the O/A damper to a minimum and lock it in position. Get your air balance contractor to take a measurement a the damper so as to set the required O/A quantity... a good rule of thumb would be 10l/s/person. I hope you have a return air damper also on the AHU, for the remianer of the air.

Having 100%O/A certainly ups the load demand on the coil.

Regarding O/A infulencing room temperature, if the coil was not sized for 100% O/A origionally, then it definatly would.

Are you going to duct the return to the plantroom and the unit. Is there a return air fan per AHU or just one, or is it an exhasut fan?

 

[digitrex]

Billy,

We have one single fan for AHU and it draws in both O/A and return air. There are air dampers on O/A, return air and supply air ducts.

I think reducing the O/A intake to the AHU is the cheapest option to try?

 

[ChrisConley]

Back to the orginal question, I also recommend parallel blade dampers, rather than opposed blade dampers in the mixbox.

Parallel blade dampers promote mixing by 'aiming' the airstreams at one another. OBD will require an air blending device to avoid stratification withing the unit.