Undersized AHU 1

[gebre]

I inherited the following problem ….

A designer sizes an AHU with the assumption of 50 less people than the actual number, 275. Moreover, he used much less Heat generation factor by the occupancy than the min. ASHRE recommendation, and more… The end result is, we have a 27% undersized AHU (13 ton while it should be atleast 17.6 tons).

My question to you all is, to give me a list of options that I can do, if any, to salvage the current unit (By-the-Way, did I mention that the unit is already installed). I know you need a lot of information, like the air speed, fan power, etc. but at this time I am also waiting for those information. So At this point I just want to know each and every possibilities (system adjustment) assuming the system is not max out. Thanks.

 

[62hog]

Bend over, take it on the rear and then make it right. You will suffer far less in the long run (by correcting it up front and not patching a new system), and make better friends with your clients (who then know that you will take care of them and their pocketbooks).

Another option is to contact the equipment supplier for low cost suggestions on what can be done. They often have good ideas on how to extend the range of their equipment.

At least that is one man's contention.

And remember "A journey of a thousand miles must begin with a single step." Lao-tzu

 

[ChasBean1]

How often are there 275 people and for how long? 27% undersize doesn't strike me as catastrophic. What are the specific problems they encounter - heat or humidity?

Maximize the motor sheave to get max fan speed within the limitations of coil velocity. Recommend keeping less than 500 fpm across the cooling coil. Clean the coils. Change the filters. Look at the economizer (if applicable) for proper operation. Charge the refrigerant if needed. If there are VAV terminal devices, increase airflow at each VAV.

 

[gebre]

Thank you 62hog & ChasBean1 for you advice!

ChasBean1, to answer you question, I think this theatre will be fully occupied most of the time, if not all. One bit of information also, they use half the OA requirement due to intermittent occupancy of the space (ASHRE 62-Sec. 6134). I have some doubt about that too. As the operation of the theatre is only in the morning by almost continuously occupied: first show 10:00-10:45, and the second show will be 11:15 – 12:00 with 30 min. break in between.

The theatre is not up and running yet; but my assumption is, we will have probably both heat as well as humidity problem(100% OA is been used).

I like your idea of maximize the motor sheave to get max fan speed, and I think we are way less than the max. 500 fpm coil velocity. However, I am afraid it might increase the noise problem? What do you think?

 

[DouginMB]

Is there a chance that you are over-ventilating the space? In many applications where occupancies are intermittent(church assembly for example), you can use motorized dampers to shut off the outside when the building is not in use. A CO2 sensor can then be employed to open the damper as needed to maintain a set CO2 level. By minimizing the outside air (and also latent) load, the space might not require the full 17.5 tons.

Also you can pre-cool the space with a programmable t-stat.

 

[ChasBean1]

Noise, yes. You didn't mention that this was a theatre in your first post. Some additional things to consider:

• Are we expecting sold-out performances in every venue?

• I like DouginMB's post above regarding CO2. Keep in mind CO2 is not a toxin in this application, but an indicator of ventilation turnover. 15-20 cfm OA per person was developed to maintain a CO2 of less than about 850 ppm, which corresponds to a study of office occupancy ventilation versus perception of odor and stuffiness. In a theatre, where alternate sources of contaminant generation are lower (no white-out, no copiers, no elevator hydraulic odors, etc.,) as a designer, I would consider cooling to take a general precedence over ventilation. People's respiratory rates are also lower as they are seated.

You might not have action required. I would recommend that you place an air quality data logger (recording CO2, temperature, and humidity) in the theatre for a couple of weeks of use and make a determination based on the data.