Make-up air for commercial kitchens is a challenging issue to handle. Especially for larger hoods within small rooms, the air flow could cause drafts making it uncomfortable for the occupants.  Even worse it may effect the performance of the hoods and allow smoke and odours to spill out.

There are many ways to mitigate it with expensive diffusers but one cost effective, tried and true solution is to install an egg crate return air grille with plexiglass hanging below it.

Why the egg crate?

It’s all about the free area, a 24”x24” egg create grille has a much more area than a 24”x24” square diffuser. 

As the linear equation goes for air flow:

Q = V*A 

Q = the flow rate in cubic feet per minute (cfm)

V = air velocity in feet per minute (fpm) 

A = free area of the diffuser in square feet (sft) 

Rearranging the equation to measure velocity:

V = Q/A

We see from this equation that with the same amount of air flow (Q) the only way to decrease drafts (V) is by increasing the free area (A)

Lets look at two cases:

A) Standard 24”x24” square office diffuser  

B) Evolve Mechanical Solutions 24”x24” egg crate & plexiglass

A) Standard Diffuser:

Q = 500 cfm

A = not provided by manufacturer however they state at 5’-0” away from the diffuser they measure 150 fpm

V = 150 fpm @ 5’

B) Egg crate & plexiglass:

Q = 500 cfm

A = 3.61 square feet (24”x24” free area listed by manufacturer)

V = 139 fpm @ 0’

Although the velocities are relatively the same, it’s the DISTANCE at which that velocity are measured at that’s important.

When we compare apples to apples our egg crate velocity would be much lower when measured 5’ away thus minimizing drafts and maximizing hood performance.

Why the plexiglass?

Even with a much lower velocity there may be an incident of drafts due to dumping.

Dumping is a term for when cold air is supplied to a warm room.  You may have heard the term warm air rises correct?  This is because the warm air is less dense and rises up, likewise cold air is denser and it drops down creating a draft.

With the plexiglass installed it forces the cold air out horizontally such that it doesn’t discharge directly down onto the occupant.  Further it allows the cold air to mix with the warmer room air creating a more neutral buoyancy and minimizing the dumping/ drafting effect.

As always Evolve Mechanical Solutions looks forward to comments, suggestions or ideas.  Please let us know what you think!

James Hicks P.Eng

Evolve Mechanical Solutions