An engineer is calculating the cooling load for each room to be served by a new rooftop unit. Solar and transmission heat gains through the windows, walls, and roof and internal heat gains from lights, people (sensible), and equipment add up to the room sensible heat gain (BTUH_S). Latent heat gains from people and any process moisture gain (e.g., from a fountain or, in a laboratory, a water bath) add up to the room latent heat gain (BTUH_l). With that information, how does the engineer determine the supply-air quantity (in cubic feet per minute) for each room and select a rooftop unit to meet the loads?

Converting BTUH_S to supply-air flow seems simple:

BTUH_S = 1.1 × cfm × (T_room - T_{supply air}) (1)

where:

1.1 = 60 min per hour × 0.24 Btu per pound mass per degree Fahrenheit (specific heat of air) × 0.0764 lb per cubic foot (density of moist air at typical supply-air conditions)

But is it? What should the supply-air temperature be? If the design is based on 56°F supply air, but the selected unit delivers air at 59°F, a room will be about 15-percent short of cooling capacity.

If this were an applied chilled-water unit, the engineer could select a cooling coil to match the load. But the unit is direct expansion, so the engineer has to work with the cooling coils and capacities manufacturers have pre-selected. What size rooftop unit will meet the load and deliver the desired supply-air temperature? Besides meeting the space cooling load (sensible plus latent), the unit must cool outside air and overcome fan heat.

Most rooftop-unit manufacturers provide tables or computer programs to help engineers select units. The user needs to know the ambient temperature (dry bulb), desired supply-air flow, and cooling-coil entering-air conditions (dry bulb/wet bulb). How does an engineer determine cooling-coil entering-air conditions and supply-air flow? Changing the flow of supply air changes the temperature of supply air a unit can deliver, while changing the temperature of supply air changes the flow of supply air required to meet a space's sensible-cooling load (Equation 1).

A manufacturer's selection program reports gross cooling capacity for stated conditions. How does gross cooling capacity relate to space cooling capacity?

This article introduces a spreadsheet tool (Table 1; an electronic version is available for free at http://hpac.com/RTU-ANAL.xls) intended to answer those questions and simplify calculations. The remainder of this article is a section-by-section discussion of the spreadsheet entries. Cells with a white background are data-entry cells; cells with a yellow background are calculations the spreadsheet performs.

The spreadsheet evaluates three units — 75-ton draw-through (Column C), 90-ton draw-through (Column D), and 90-ton blow-through (Column E) — for one project. Columns C, D, and E show how capacity and configuration affect ability to meet the calculated load. Column E also shows how relaxing the design indoor humidity (55 percent instead of 50 percent) affects performance.

Columns F and G of the spreadsheet compare a 75-ton blow-through unit and a 60-ton draw-through unit for another project.

Figures 1 and 2 show the psychrometric process for a 75-ton draw-through unit (“DT-75” in Column C of the spreadsheet) and a 75-ton blow-through unit (“BT-75” in Column F of the spreadsheet), respectively. Column B of the spreadsheet lists the state points in figures 1 and 2.

Lines 11 to 13 (outdoor-air conditions)

The spreadsheet calculates specific humidity (pounds of water per pound of dry air) for design dry-bulb temperature and mean coincident wet-bulb temperature. These values are an important part of documenting design criteria for a project. They are used in subsequent calculations.

In selecting a rooftop unit, some engineers check loads at both peak cooling (dry-bulb) temperature and peak dehumidification (wet-bulb) temperature. Peak dehumidification load usually occurs at a lower outdoor temperature than peak cooling load. The temperature of the air entering the condenser will be lower, so the rooftop-unit cooling capacity (determined from manufacturer data) might increase enough to meet the higher dehumidification load. The spreadsheet makes comparing performance at peak cooling temperature with performance at peak dehumidification temperature easy by calculating the two cases in adjacent columns.

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