Negative pressure ventilation system - need guidance 1

[abdelkader90]

Hello,

I have been learning about the flow calculation of negative pressure ventillation systems and i have a couple of questions, i would really appreciate it if someone could help.

The example i will be using to clarify my questions: U shape corridor that has 8 offices (positively pressurized +5 Pa) and 2 labs (negatively pressurized -15)
i will state the steps i followed while trying to calculate the necessary flow rates to achieve the required conditions and the negative/positive pressurization then i will ask two questions and i am sorry if anything sounded stupid or if the post is too long

1- Calculated the internal and external thermal loads for all spaces using HAP 4.9
2- determined the cooling and heating capacity of the fan coil units then determined the flow rate required to achieve the required conditions
3- recalculated the flowrate of each space according to the ACH required (1 ACH for offices and 8 ACH for Labs)
4- compared the flowrate obtained from HAP with the flowrate necessary for the required ACH and chosen the higher
5- for the offices i calculated the leakage flowrate and added it to the supply flowrate
6- for labs i calculated leakage flowrate (leakage area to be the space under the door) and added it to the exhaust flowrate
noz i have the flowrates for each space except of the corridor!
my questions :

1- i know that the corridor is going to be more positively pressurized than offices but how to determine the flowrates or the extra supply flowrate of the corridor that i shall add on the flowrate necessary for cooling?

2- Do i have to carry out a type of flow balance on the corridor by calculating the flow that will leak from the corridor to offices and labs and the total leakage is to be added as extra supply flow to the corridor?

Thank you in advance.

 

[trashcanman]

The airflow should be what the A/C unit requires for the cooling + makeup air (for exhaust and pressurization). Do not try to select an AC unit based on air flow calculated from HAP or "ACH required", it is useless.
It is ok for the corridor to be pressurized.

If the labs are exhausting all the time, consider using a 100% outside air unit for the labs only to simplify controls. DO NOT ALLOW lab exhaust air back into the space.

Now, if the exhaust flow from labs is > or = flow rate for cooling for offices + labs, -----> 100% outside air unit for building. Set HAP cfm for 100% outside air, calculate the cooling load and have vendor provide selection. I have done this many times.

What "leakage" from offices? To where, the labs?

 

[abdelkader90]

first of all thank you for your reply.
allow me to clarify some points ...
for the labs it will be surely 100% fresh air AHU and for the labs exhaust i will use exhaust fans on rooftop and for offices&corridor it is a normal AHU with a moxing chamber.
let me explain how am i calculating the flow rate for each space and please correct me if i am wrong

for each office
supply flow rate = required flow for cooling + the flow rate of the space under the door from office to corridor calculated for a delta (P) of +5 Pa

return air = required flow for cooling

for each lab

supply flow (by ahu)= flow required for ACH of 8 ( i have two values for the supply flow , 1- cooling flow , 2- flow required to have 8 ACH , and i chose the second value because i can't just maintain cooling and not ACH) not sure though

exhaust flow (by exhaust fans) = flow required for ACH of 8 + the air flow rate of spaces under doors calculated with a delta P of -15 Pa


for corridor

i simply do not know , i have the flow rate required for cooling and thats it , although i know that there has to be an added value to make up for the flow going into labs (those are negativly pressurized)

 

[GT-EGR]

It’s hard to tell from your scenario how your fan coils work into this compared to your ventilation air and your exhaust air.

If the fan coils just recirculate in the space, then they don’t actually impact the pressure in the space at all. It’s only if they are ducted to and from multiple spaces that they might cause relative pressure issues - or if you’ve connected your makeup air or exhaust air system directly to its ductwork, if there even is any

I think you have it right as for as the spaces are concerned but haven’t quite put together the whole picture.

Let’s start with the lab spaces - you calculate how much air you need and take the max of either your air change number or cooling number. Your exhaust number should equal that plus whatever transfer air quantity you determined hits your negative pressure target - which is pretty much what you said.

Keep track of this transfer air quantity and specifically what spaces communicate directly with each negative space. So for example if your lab needed 100 CFM more exhaust than supply, than that 100 CFM is going to come from the adjacent space, likely the corridor. So that air is gone from you the building - and needs to come back in from the outside - so the simplest way is to use the 100% outside air unit you have to put that quantity of air into the corridor.

You just need to make an air balance diagram showing all the air in and all the air out and how it gets out whether it’s supplied, exhausted, or returned.

You are on the right track with the corridor, if it gets 800 CFM from offices transferring in, and 600 CFM from labs transferring out, then in theory you have to exhaust the corridor by 200 CFM more than you supply it. Just be sure to allow some fluctuation either way, so that you can increase Or decrease the corridor supply or exhaust once the space is actually balanced to make up for any construction tightness or loseness that doesn’t line up exactly with your calculations.

Additionally - be sure that your recirculating office unit is controlled properly - how will you be controlling the actual return - is it based on space pressure or measured CFM offset, or fixed day 1 CFM offset. This unit can easily have an impact on your bulidnh pressure if you don’t account for this additional outside air component. On the other hand - if you are letting your office supply air transfer to the labs, then that airflow is gone and can’t be returned, so you have to be sure to include more outside air in your unit to makeup for that.