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Lines 15 and 16 (supply- and outdoor-air flow)

Supply-air flow is calculated from Equation 1 using an assumed supply-air temperature. A temperature of around 58°F usually is a good place to start for package rooftop equipment. Smaller equipment applied at loads close to rated capacity might have a slightly higher supply-air temperature. Units with capacities greater than the load might deliver lower supply-air temperature. The spreadsheet helps with the trial-and-error process of balancing supply-air flow and supply-air temperature -- each change in flow changes unit capacity and resulting supply-air temperature, which, in turn, changes the flow required to meet the space sensible load.

Outdoor-air flow is the outdoor-air quantity (usually the minimum) a unit will introduce at peak load conditions. It might be determined from ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality; a table in the building code; or the amount necessary to make up exhausts and pressurize the building.

Lines 18 to 21 (indoor-air conditions)

Enter the design indoor dry-bulb temperature and humidity. Look up the corresponding wet-bulb temperature and specific humidity on a psychrometric chart, or use a calculation tool.

Lines 23 to 25 (return-air plenum)

If a design utilizes a return-air plenum, return air at room temperature passes through the plenum on its way to the rooftop unit. As it passes through the plenum, return air picks up heat from recessed lights, the plenum walls, and the roof (if on a top floor).

Return-air-plenum-temperature rise is a trial-and-error, iterative heat-balance calculation. Heat into a plenum comes from the plenum walls, roof, and recessed lights (about 35 percent of the power input to recessed fluorescent lights). Heat leaving the plenum is heat picked up in the return-air stream plus heat retransmitted through the ceiling. Heat in has to equal heat out. The iteration process is as follows:

• Step 1: Calculate room airflow with no heat transmission from the plenum into the occupied space.

• Step 2: Calculate plenum-temperature rise using the airflow from Step 1, with BTUH_S equal to the plenum heat gain, with no retransmission through the ceiling.

  BTUH_S = 1.1 x cfm x ΔT

• Step 3: Recalculate room airflow, including heat transmission from the plenum (Q = U x A x ΔT), using the ceiling U-value and the plenum-temperature rise from Step 2 as delta-T. (Ceiling U-value consists of a still-air film [heat flow down] on the room side, the ceiling tile, and a still-air film on the plenum side. Air velocity in the plenum is low enough to use the value for still air. Ceiling U-value usually is around 0.33 hr-sq ft-°F per British thermal unit.)

• Step 4: Calculate a new plenum-temperature rise (repeat Step 2) with the new airflow to get a new plenum delta-T.

• Step 5: Recalculate room heat gain and airflow with the delta-T between the two calculated previously. Repeat until the result converges.

Determining return-air-plenum temperature is part of calculating cooling load. Typical values are about 2°F for interior office spaces and 4°F to 5°F for a top floor, which experiences heat gain through the roof.

In the case of ducted return, there is no return-air-plenum-temperature rise. The spreadsheet does not explicitly recognize return-duct-temperature rise. Return-duct-temperature rise usually is rather low, unless the duct passes through unconditioned space. Return-duct-temperature rise could be entered in place of return-air-plenum-temperature rise.