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Lines 27 to 35 (return fan)

A return fan, if present in a unit, adds heat to the air passing through it.¹ Enter the static pressure and fan efficiency from the fan selection so the spreadsheet can calculate fan heat gain. The return-fan-brake-horsepower calculation includes allowances of 5 percent for belt and drive losses and 90 percent for motor efficiency (assuming the motor is in the air stream). Users can alter those values (or any of the formulas in the spreadsheet) to suit their application.

Lines 37 and 38 (mixed-air conditions)

The spreadsheet calculates mixed-air conditions. Mixed air is the combination of minimum outside air and return air (after it has passed through a return-air plenum and return fan, if present).

Lines 40 to 45 (blow-through supply fan)

A blow-through supply fan, if present in a unit, adds heat to the air passing through it. Enter the static pressure and fan efficiency from the fan selection so the spreadsheet can calculate fan heat gain. The supply-fan-brake-horsepower calculation includes allowances of 5 percent for belt and drive losses and 90 percent for motor efficiency (assuming the motor is in the air stream).

Blow-through-supply-fan heat raises the temperature of air entering a cooling coil. Higher entering-air temperatures increase cooling-coil capacity (basically because of a larger difference between entering-air temperature and mean refrigerant temperature in the coil). That capacity increase is part of why a blow-through unit delivers more space-cooling capacity than the same unit in a draw-through configuration. On the other hand, a blow-through unit tends to be physically larger than the same unit in a draw-through configuration. The reason is the space required at the fan outlet for fan discharge air to spread out and cover the cooling coil evenly.

Lines 47 to 51 (cooling-coil entering-air conditions)

The spreadsheet calculates the dry-bulb and wet-bulb temperatures of the air entering a cooling coil. These are the entering-air conditions to use with the manufacturer's data or selection program. These temperatures will change with changes in outdoor-air conditions, outdoor-air quantity, supply-air quantity, and design indoor conditions and are affected by whether the unit has a return fan or a blow-through supply fan.

Lines 53 to 60 (cooling-coil performance)

Unit total and sensible capacities come from the manufacturer's performance data or selection program. The spreadsheet calculates the resulting cooling-coil leaving-air conditions.

Lines 62 to 67 (draw-through supply fan)

A draw-through supply fan, if present in a unit, adds heat to the air passing through it, raising the supply-air temperature, sometimes by several degrees. Enter the static pressure and fan efficiency from the fan selection so the spreadsheet can calculate fan heat gain. The supply-fan-brake-horsepower calculation includes allowances of 5 percent for belt and drive losses and 90 percent for motor efficiency (assuming the motor is in the air stream).

The increase in supply-air temperature is a disadvantage of the draw-through supply fan -- fan heat raises the temperature of the air supplied to the diffuser, increasing the airflow required to meet the space sensible-cooling load. For a system, such as a laboratory's, that has high minimum airflows and uses a lot of reheat, draw-through-supply-fan-temperature rise can be an advantage ¡ª the fan energy, which is necessary to move the air, offsets reheat energy that otherwise would be required. Some engineers like draw-through fans because they lower the relative humidity of the air going down a duct, reducing the risk of condensation in the duct. (The air leaving the cooling coil is nearly saturated. Adding a few degrees of fan heat raises the dry-bulb temperature without changing the moisture content, so relative humidity decreases.)

Lines 69 to 74 (duct-rise allowance)

The spreadsheet has an option to allow for duct rise. If a duct passes through unconditioned space, the heat gain into the duct is part of the rooftop-unit cooling load (psychrometrically, it is a space load) and must be recognized.

If an uninsulated supply duct passes through a return-air plenum, supply air will pick up plenum heat. Removing heat from the plenum will reduce heat transmission to an occupied space through the ceiling. For that reason, allowing for duct rise in a return-air plenum without recognizing a corresponding reduction in heat gain from the plenum to an occupied space will result in a small amount of double counting. Some engineers ignore that double counting because it functions as a small safety factor.

Lines 76 and 77 (available net capacity)

The spreadsheet calculates the net cooling capacity available to a space (after deductions for fan heat and other losses). These are the capacities to compare to the calculated space cooling loads.

The difference between unit capacity and available net or space capacity is significant. In the examples on the spreadsheet (which are from real jobs), space sensible-cooling capacity (the BTUH_S in 1.1 x cfm x ÄT) is only about half of the nominal unit tons.

Lines 79 to 82 (capacity-to-load comparison)

These lines show whether a selected unit meets calculated loads. Excess capacity (safety factor) shows up as a positive percentage; shortfalls show up as negative percentages.

Engineers need to use judgment when evaluating these results. A shortfall of a few percent might be OK, especially if remedying it means jumping to a much larger unit size. An oversized unit will short-cycle, reducing effective dehumidification capacity.

If a unit is low on sensible cooling, but has excess latent capacity, increasing the airflow can shift some latent capacity to the sensible side. As airflow increases, entering-air temperature decreases (slightly), while unit sensible-cooling capacity increases.

Lines 91 to 106 (load summary)

These lines summarize the components that make up the load.

Conclusion

The spreadsheet is a tool to help engineers select rooftop units and perform a heat balance on the air stream in a rooftop-unit system. As with any computer program or application, the spreadsheet is subject to data-entry errors and mistakes introduced by moving cell data or modifying formulas. The spreadsheet is not password-protected, and the formulas are not locked. Users are responsible for how they use the spreadsheet.

Reference

1. Williams, G.J. (1989, January). Fan heat: Its source and significance. Heating/Piping/Air Conditioning, pp. 101-103, 108-112.

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Did you find this article useful? Send comments and suggestions to Executive Editor Scott Arnold at scott.arnold@penton.com.

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A longtime member of HPAC Engineering's Editorial Advisory Board, Kenneth M. Elovitz is an engineer and the in-house counsel for Energy Economics Inc. His knowledge of and experience with HVAC, electrical, and life-safety systems allow him to understand system function and performance, including interactions among disciplines.

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