Supply Fans

Applicability

All fan systems

Definition

Software commonly models fans in three ways.

• Power-per-unit-flow. The simple method is for the user to enter the electric power per unit of flow (W/cfm). This method is commonly used for unitary equipment and other small fan systems.
• Static pressure. A more detailed method is to model the fan as a system whereby the static pressure, fan efficiency, and motor efficiency are specified at design conditions.
• Break horsepower. A third method is to specify brake horsepower at design conditions instead of fan efficiency and static pressure. This is a variation of the second method whereby brake horsepower is specified in lieu of static pressure and fan efficiency.

The latter two methods are commonly used for VAV and other larger fan systems.

Units

List: power-per-unit-flow, static pressure or brake horsepower

Input Restrictions

As designed. The power-per-unit-flow method shall be used when no fan performance data are available for the proposed design cooling system, e.g. only EER or SEER are available.

Baseline Rules

The baseline building fan system for Systems 1, 2, 9, and 10 is specified using the power-per-unit-flow method. For systems 3-8 and 11-13, the break horsepower method shall be used for the baseline building.

Applicability

All fan systems

Definition

The design air flow rate of the supply fan(s) at design conditions. This building descriptor sets the 100% point for the fan part-load curve.

Units

cfm

Input Restrictions

As designed. This input should be at least as great as the sum of the design air flow specified for each of the thermal blocks that are served by the fan system. For multiple deck systems, a separate entry should be made for each deck.

Baseline Rules

The program shall automatically size the air flow at each thermal block to meet the loads. The design air flow rate calculation shall be based on a 20 degree temperature differential between supply air and the return air for zones. The design supply air flow rate is the larger of the flow rate required to meet space conditioning requirements and the required ventilation flow rate. (The ventilation requirements may dictate supply air flow rate requirements for laboratory spaces and other spaces with special process requirements.) The supply fan design air flow rate shall be the sum of the calculated design air flow for the thermal blocks served by the fan system.

For laboratory spaces, the design air flow rate calculation shall be based on a 17 degree temperature differential rather than 20. 

For baseline systems 9 and 10, the design air flow rate calculation shall be based on the temperature difference between a supply air temperature setpoint of 105 °F and the design space heating temperature setpoint, the minimum outdoor air flow rate or the airflow rate required to comply with applicable codes, whichever is greater.

If the proposed design includes a fan(s) to provide non-mechanical cooling (ventilation only), the baseline building design shall include a separate fan to provide non-mechanical cooling, sized and controlled the same as the proposed building design. 

Applicability

All fan systems

Definition

A description of how the supply (and return/relief) fan(s) are controlled. The options include:

• Constant volume
• Variable-flow, inlet or discharge dampers
• Variable-flow, inlet guide vanes
• Variable-flow, variable speed drive (VSD)
• Variable-flow, variable pitch blades
• Variable-flow, other
• Two-speed
• Constant volume, cycling (fan cycles with heating and cooling)

Units

List (see above)

Input Restrictions

As designed

Baseline Rules

Prescribed based on the baseline building HVAC system type. See the table below

Applicability

All fan systems, except those specified using the power-per-unit-flow method

Definition

The design shaft brake horsepower of the supply fan(s).

Fans for parallel fan-powered boxes are not included in fan power calculations.

Units

Horsepower (hp)

Input Restrictions

As designed. If this building descriptor is specified for the proposed design, then the Static Pressure and Fan Efficiency are not required.

Baseline Rules 

See Table 3.7.3-2

Applicability

All fan systems, except those specified using the power-per-unit-flow method

Definition

The design static pressure for the supply fan. This is important for both fan electric energy usage and duct heat gain calculations.

Units

Inches of water column (in. H₂0)

Input Restrictions

As designed. The design static pressure for the supply fan does not need to be specified if the supply fan brake horsepower (bhp) is specified.

Baseline Rules

Not applicable. When Static Pressure and Fan Efficiency are entered for the proposed design, the baseline building shall use the brake horsepower modeling method.

Applicability

All fan systems, except those specified using the power-per-unit-flow method

Definition

The efficiency of the fan at design conditions

Units

Unitless

Input Restrictions

As designed. The supply fan efficiency does not need to be specified if the supply fan brake horsepower (bhp) is specified.

Baseline Rules

Not applicable. When Static Pressure and Fan Efficiency are entered for the proposed design, the baseline building shall use the brake horsepower modeling method.

Applicability

All supply fans, except those specified using the power-per-unit-flow method

Definition

The full-load efficiency of the motor serving the supply fan

Units

Unitless

Input Restrictions

As designed. Not applicable when the power-per-unit-flow method is used.

Baseline Rules

Table 3.7.3-4.  

Horsepower

Efficiency

Horsepower

Efficiency

Horsepower

Efficiency

1

85.5

15

92.4

75

95.4

1.5

86.5

20

93.0

100

95.4

2

86.5

25

93.6

125

95.4

3

89.5

30

93.6

150

95.8

5

89.5

40

94.1

>= 200

95.8

7.5

91.7

50

94.5

10

91.7

60

95.0

Applicability

All supply fans

Definition

The position of the supply fan relative to the cooling coil. The configuration is either draw through (fan is downstream of the coil) or blow through (fan is upstream of the coil).

Units

List (see above)

Input Restrictions

As designed.

Baseline Rules

Draw through

Applicability

All supply fans

Definition

The position of the supply fan motor relative to the cooling air stream. The choices are: in the air stream or out of the air stream.

Units

List (see above)

Input Restrictions

As designed.

Baseline Rules

In the air stream

Applicability

All variable flow fan systems

Definition

A part-load power curve which represents the percentage full-load power draw of the supply fan as a function of the percentage full-load air flow. The curve is typically represented as a quadratic equation with an absolute minimum power draw specified.

Units

Unitless ratio

Input Restrictions

As designed. If data is unavailable, a default fan curve shall be selected from COMNET Appendix H for the fan type used in the proposed design.

(Equation 3.7.3-1)

$$Greater of$$

$$PLR = a+b\cdot FanRatio+c\cdot FanRatio^{2}+d\cdot FanRatio^{3}$$

$$PLR=PowerMin$$

where

Baseline Rules

Only applicable baseline building systems 5-8 and 11. The part load for the baseline building fan shall be determined from either Table 3.7.3-6 for Equation 3.7.3-1 with a = .0013, b = 0.1470, c = 0.9506 and d = 0.0998.

Table 3.7.3-5 Baseline Building VSD Performance

Applicability

Fan systems that use the power-per-unit-flow method

Definition

The supply fan power per unit of flow.

Units

kW/cfm

Input Restrictions

As designed or specified in the manufacturers’ literature. For units with rated total cooling capacities less than 120,000 Btu/h, the user may default to a value calculated as follows:

 (Equation 3.7.3-2)

$$Fan_{kW}=0.365\times  \frac{Q_{rated}}{30,000}$$

where

Baseline Rules

Applicable to baseline systems 1, 2, 9, and 10. See Table 3.7.3.2