Fresh air requirement question

[stevo]

Hello I have a customer who has asked for a high wall mount inverter h/p & for a separate fresh air supply! it is for an office within a factory environment, office size = 40 cubic mts, max bodies = 10.... they don't want anything too sophisticated.. was thinking of a 4" duct fan with fresh air intake grille run from it's own separate electrical supply switch ( they can then operate it at will) The room is approx 4 to 6mts from exterior. I realise the a/c will have to be sized accordingly to cope with added fresh air gain ! would 8 litres per second per person be near ? at present our design source is unavailable.. can anyone give me some ideas/calcs etc.......thanks :)

[penair]

Hi stevo
Check out ASHRAE or similar governing bodies for the required air change for an occupied office airconditioning application. General cooling load W/m2 (air conditioned area) for a general office can be as high as 180 W/m2. Take into account the heat load from lighting, computers, fax machines etc. 8 litres per second per person is a bit extreme for fresh air make up...... are they on fire??? I would'nt have a manual switch for the office staff to operate at will. Put it on a timer to suit occupancy hours with manual overide or cycle it with operation of the aircon system, whatever you do, just cover yourself by not doing it on the cheap just to satisfy the client, if it doesnt work your phone will be ringing....

[Temprite]

Check this out.

global.mitsubishielectric.com/bu/air/office_p/l_a1_b.html

[thebigcheese]

is there not a place on the indoor unit that u can fit a fresh air duct onto?

[stevo]

is there not a place on the indoor unit that u can fit a fresh air duct onto?

No not on the high wall mount systems !....cheers

[nh3simman]

would 8 litres per second per person be near ?

8 l/s/p is a bit high.

If there is no smoking, you can go down to 2.5 l/s/p for a typical office. Fresh air is a big load so you should keep it to a minimum.

You are right to base the quantity on the number of people. I have seen estimates based on a fraction of the supply air but this is when you don't have any better figures.

[stevo]

Thanks for your replies ! anyone else with ideas ? :)

[frank]

CIBSE guide recommends 8 l/s/person as a minimum fresh air requirement (20 l/s/person for airconditioned spaces) or 6 air changes per hour for an office environment.

So, for 10 persons you would require 80 l/s or 0.08m3/s or for 6ACH you would require 0.67m3/s.

The additional cooling load, UK conditions would be 1.51kw or 1.26kw respectively

0.08m3/s would require a duct @ 150mm dia to give a reasonable velocity and pressure drop.

[stevo]

Many thanks Frank, Just the info i can use ! :)

[nh3simman]

CIBSE guide recommends 8 l/s/person as a minimum fresh air requirement (20 l/s/person for airconditioned spaces) or 6 air changes per hour for an office environment.

Good advice, always check your local requirements.

In SA with +35degC ambient and poorly sealed buildings, we try to keep the fresh air to a minimum.

ASHRAE does not actually give a fresh air recommendation but rather a guideline.

[frank]

ASHRAE does not actually give a fresh air recommendation but rather a guideline.

The English CIBSE Guide is also a "Guide" but it is adopted by most design engineers to be incorporated into new build projects, as it sets a baseline for the performance of the services.

[gwilliamson]

Hello I have a customer who has asked for a high wall mount inverter h/p & for a separate fresh air supply! it is for an office within a factory environment, office size = 40 cubic mts, max bodies = 10.... they don't want anything too sophisticated.. was thinking of a 4" duct fan with fresh air intake grille run from it's own separate electrical supply switch ( they can then operate it at will) The room is approx 4 to 6mts from exterior. I realise the a/c will have to be sized accordingly to cope with added fresh air gain ! would 8 litres per second per person be near ? at present our design source is unavailable.. can anyone give me some ideas/calcs etc.......thanks :)

Both Hitachi & Daikin make fresh air wall mounted splits although i believe Hitachi are dis continuing theirs.

[Abby Normal]

8 l/s/p is a bit high.

If there is no smoking, you can go down to 2.5 l/s/p for a typical office. Fresh air is a big load so you should keep it to a minimum.

You are right to base the quantity on the number of people. I have seen estimates based on a fraction of the supply air but this is when you don't have any better figures.

2.5 l/s is bare bones minimum in many places. Think the first number ever used was 4 CFM per in the Houses of Parliament. Heard an urban legend they used to give coal miners 5 cfm per to keep productivity up.

[nh3simman]

2.5 l/s is bare bones minimum in many places. Think the first number ever used was 4 CFM per in the Houses of Parliament. Heard an urban legend they used to give coal miners 5 cfm per to keep productivity up.

I wouldn't say 2.5l/s/p is a bare bones minumum.
We add outside air for 2 reasons

Supply of O2
removal of CO2People need 0.2l/s of O2. This is a fresh air rate of 1 l/s/p.

[stevo]

Good advice, always check your local requirements.

In SA with +35degC ambient and poorly sealed buildings, we try to keep the fresh air to a minimum.

ASHRAE does not actually give a fresh air recommendation but rather a guideline.

Pheww thank god for that ay !!! :cool: