Wednesday, August 09, 2006

Carrier HAP Displacement Ventilation

Not to be outdone, the folks at Carrier HAP software support also have some recommendations for modeling displacement ventilation systems:

1. TRACE has a feature that models the return air plenum as a separate space. This lets you define the size of the plenum and heat going into and out of it. The calculate a load for it as if it was a room and also calculate its temperature. The seem to be recommending you use this feature to model the upper stratified zone of the room as a "plenum space" and assign portions of the room loads like lights, etc.. to the plenum space.

In HAP we don't have an equivalent feature. But you could define a return air plenum and assign part of your lighting, wall and roof loads to the plenum as a way of derating the heat going to the occupied zone in the space. You still need a way to deal with people and plug loads. Perhaps use the schedules to derate these heat gains, or just specify derated sqft/person and W/sqft to begin with. What you'll miss is the portion of these heat gains that goes into the upper zone. And you'll miss the kWh consumption for plug loads if you're doing an energy analysis (could put those back in via miscellaneous energy in the building inputs). Don't know if TRACE lets you put people and office equipment inside a plenum space. Maybe they have a problem with people and plugs too.

2. On the system side it sounds like they recommend modeling a constant volume AHU that uses a bypass so you have part of the air going thru the cooling coil and part going through a bypass and the two streams mixing downstream. Something equivalent in HAP might be this:

a. Choose a 3-Deck Multizone system. In cooling mode it mixes neutral (untreated) with cold deck air in the proper proportions to get the proper SAT to meet the load.

b. Specify the central cooling with 55 F SAT and constant supply temperature control.

c. You'll want to use user-defined sizing on the sizing tab so you can directly specify the supply airflow. This airflow should be sufficient to meet the peak zone sensible load with a 60 to 65 F supply temperature at the supply diffuser. To figure out what this airflow for 60 to 65 F SAT is to begin with, you may need to run the system once with SAT specfied as 60 to 65 F. Then go back and edit the air system, change the SAT to 55 F and switch to user defined sizing to put in the airflow. Then run calculations a second time. The result is the supply temperature at the diffuser will be 60 F or above, and it will be a mixture of 55 F cold deck air plus untreated neutral deck air.

We recommend looking carefully at the results like the component loads on the System Design Load Summary and the system operating details on System Psychrometrics to get a feel for whether everything is working the way you intend it to, and is representing what you think displacement ventilation behavior should be.

Best Regards,
Carrier HAP Support

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