Toll booth ventilation

[EnergyProfessional]

I have to replace furnaces for parking ramp toll booths. Basically little 4x6 booths that get 100% OA from a furnace (they have window AC for cooling). The booths are inside a naturally ventilated parking ramp (about 30' in from the perimeter).

I don't have data on the existing system, and don't want to rely on what someone else designed.

- 2011 ASHRAE Handbook HVAC Applications 15.26 gives some basic information.

but where would I find information on required ventilation rate? I looked up the code, ASHRAE, OSHA etc. and didn't find anything useful. Do I just calculate heating load and supply that much air? Assuming it is 100% OA it always would be positive pressure.

That location also likely doesn't allow a fancy control system to have VAV etc. I have some ideas on how to vary flow with some CO measurements, dP etc. But I may have to settle for just some simple thermostat.
(the booths are used less and less as there are more and more automated payment systems - out of the 2 booths only one is used most of the time).

Currently there are 2 furnaces for 2 booths and I likely keep it that way to have full redundancy and less complex controls.

Does anyone have some advice or can point me to some sources?

 

[EnergyProfessional]

I did some load calculations and came to realize the discharge air (directly dumbed on the employee) really determines how large the unit needs to be and what the flow rate is.
I basically calculated a load for the booth (obviously infiltration is huge, but the rest is not insulated either). If I have higher DAT, the flow rate is less, and I have less OA, hence reducing unit size.

If I use 102°F DAT, I get 320 cfm and require 52,000 btu/h in heating. If I use 82°F DAT, I need 777 cfm and 101,000 Btu.

obviously I need to determine which flowrate is needed (to keep pollutants out), and what DAT is acceptable. The booth is only 4x7 and ~8' high.
the operable window (to the cars paying) is about 1x2'
any advice on how to narrow this down?

I could imagine 320 or 777 cfm both would be sufficient to keep pollutants out. Obvioulsy higher is better for ventilation, but the lowered DAT and higher airflowrate could make it uncomfortable.

 

[Drazen]

taking air from the ramp does not count as ventilation at all. if you do not have ducted supply from outdoor discussion about ventilation is irrelevant, but you should take care about of point of taking the air for furnace, to avoid sucking local pollution, even occasionally. it is best to have it far away from corners.

minimum supply air temperature in case where space diffusion cannot be achieved should be proposed within some norm. what i use in europe requires minimum delta t of 15 degrees celsius.

 

[EnergyProfessional]

thanks or the response. I forgot to mention that the air comes from outside the parking ramp via duct.
I also read that in general to have 20-30°F (16°C) more is comfortable.

 

[ChasBean1]

Ventilating requirements are more than met by the operable windows in a toll booth. Heating is a crap shoot. I'd suggest a small heater at about 300 cfm and 100°F to maintain ~150 feet per minute out the window. Winds will occasionally come in; the toll taker will have an opportunity to warm at the vent if there are wind issues.

 

[EnergyProfessional]

I did some more research and it turns out:
- the existing system uses 2 return paths: one around the MAU, and other from the booth to the MAU. 4 dampers in total with unknown how the dampers are controlled.
- the MAU burner can't turn lower than 40% (direct-fired) and the burner needs some minimum flow all the time.
- the small units I need can't have VFD
- currently employees complain about cold/hot/cold cycles and occasional penetration of exhaust from vehicles. They also occasionally open windows on both sides to deal with arriving and leaving cars. this make me think they need a lot of flow.
- In winter (assume -10°F, -15°F per code) I still want to discharge 100°F, but in fall/spring i obviously need less dT, so I need some variable flow)

I drew up (attached) what I think I would do:
- reuse the re-circulation around the MAU, abandon the recirculation from the booth (it seems complex to have 4 dampers, and I don't see the benefit, and returning from the booth may draw exhaust in)
- control the burner based on space temp (or should I control based on duct discharge air?)
- control the dampers so that the employee can turn from 100% OA to a lower OA percentage.
- i contemplate if I use the MAU supplied Maxitrol controls (our staff and everyone else hates Maxitrol), or a standalone Honeywell T775

does that sound good or should I do something different?