3.7.5.1 General

90.1-2016 BM

This group of building descriptors applies to all cooling systems.

Cooling Source

Applicability

All systems

Definition

The source of cooling for the system. The choices are:

  • Chilled water
  • Direct expansion (DX)
  • Other

Units

List (see above)

Input Restrictions

As designed

Baseline Rules

The baseline building cooling source is direct expansion for systems 1-6 and chilled water for systems 7-8 and 11-13. Systems 9 and 10 have no cooling capability. See Table 3.7-2.

 

Total Cooling Capacity

Applicability

All cooling systems

Definition

The total cooling capacity (both sensible and latent) of a cooling coil or packaged DX system at AHRI test conditions. The building descriptors defined in this chapter assume that the fan is modeled separately, including any heat it adds to the air stream. The cooling capacity specified by this building descriptor should not consider the heat of the fan.

Units

kBtu/h

Input Restrictions

As designed. Fan energy shall be modeled separately from cooling, unless the fan cycles, so for packaged equipment that has the fan motor in the air stream the Total Cooling Capacity shall be adjusted for the heat generated by the fan using the following procedure .

 (Equation 3.7.5.1-1)

$$Q_{Net} = Q_{Rated} - Q_{Fan}$$

(Equation 3.7.5.1-2)

$$Q_{Fan}=\frac{Q_{Rated}}{COP_{cooling}}-\frac{Q_{Rated}}{COP_{nfcooling}}$$

 

where

QNet

The net cooling capacity of the packaged equipment without consideration of the fan (Btu/h)

QRated

The total cooling capacity of the packaged equipment including the impact of the fan, as rated by AHRI (Btu/h)

Qfan

The heat generated by the fan and fan motor (if fan motor is in airstream), at AHRI rated conditions (Btu/h)

COPcooling

The coefficient of performance (COP) of the packaged equipment with consideration of the fan, where COPRated = EERRated / 3.412

COPnfcooling

The coefficient of performance (COP) of the packaged equipment without consideration of the fan, as determined from equations 3.7.5.2-1 and 3.7.5.2-2

Baseline Rules

The total cooling capacity of the baseline building system is oversized by 15%. Sizing calculations shall be based on 1% dry-bulb and 1% wet-bulb design conditions.

 

Sensible Cooling Capacity

Applicability

All cooling systems

Definition

The sensible heat cooling capacity of the coil or packaged equipment at ARI conditions. The building descriptors defined in this chapter assume that the fan is modeled separately, including any heat it adds to the air stream. The cooling capacity specified by this building descriptor should not consider the heat of the fan.

Units

kBtu/h

Input Restrictions

As designed. For packaged equipment that has the fan motor located in the air stream such that it adds heat to the cooled air, the software shall adjust the sensible cooling capacity as follows:

(Equation 3.7.5.1-3)

$$Q_{s.adj} = Q_{s,rated} + BHP_{supply} \times 2.545$$

 ==Where

Qs,adj

The adjusted sensible cooling capacity of a packaged unit (kBtu/h)

Qs,rated

The ARI rated sensible cooling capacity of a packaged unit (kBtu/h)

BHPsupply

The supply fan brake horsepower (bhp).

Baseline Rules

The sensible cooling capacity of the baseline building is oversized by 15%. Sizing calculations shall be based on 1% dry-bulb and 1% wet-bulb design conditions.

 

Cooling Capacity Adjustment Curves

Applicability

All cooling systems

Definition

A curve that represents the available total cooling capacity as a function of cooling coil and/or condenser conditions. The common form of these curves is given as follows:

(Equation 3.7.5.1-4)

$$Q_{t,available}=CAP\_FT \times Q_{t,adj}$$

For air cooled direct expansion

(Equation 3.7.5.1-5)

$$CAP\_FT=a + b \cdot t_{wb}+ c \cdot t_{wb}^{2}+ d \cdot t_{odb}+ d \cdot t_{odb}^{2}+f\cdot t_{wb}\cdot t_{odb}$$

For water cooled direct expansion

(Equation 3.7.5.1-6)

$$CAP\_FT=a + b \cdot t_{wb}+ c \cdot t_{wb}^{2}+ d \cdot t_{wt}+ d \cdot t_{wt}^{2}+f\cdot t_{wb}\cdot t_{wt}$$

For chilled water coils

(Equation 3.7.5.1-7)

$$CAP\_FT=a + b \cdot t_{wb}+ c \cdot t_{wb}^{2}+ d \cdot t_{db}+ d \cdot t_{db}^{2}+f\cdot t_{wb}\cdot t_{db}$$

Where

Qt,available

Available cooling capacity at specified evaporator and/or condenser conditions (MBH)

Qt,adj

Adjusted capacity at ARI conditions (Btu/h), from equation 3.7.5.1-3

CAP_FT

A multiplier to adjust Qt,adj

Twb

The entering coil wet-bulb temperature (°F)

Tdb

The entering coil dry-bulb temperature (°F)

Twt

The water supply temperature (°F)

Todb

The outside-air dry-bulb temperature (°F)

a, b, c, d, e, f

Cooling capacity curve coefficients taken from Appendix H for each cooling system type.

Note: if an air-cooled unit employs an evaporative condenser, todb is the effective dry-bulb temperature of the air leaving the evaporative cooling unit.
Software may represent the relationship between cooling capacity and temperature in ways other than the equations given above.

Units

Data structure

Input Restrictions

As designed. The equations and coefficients given above are the default.

Baseline Rules

The default curves shall be used for the baseline HVAC system.

 

Coil Bypass Factor

Applicability

All cooling systems

Definition

The ratio of air that bypasses the cooling coil at design conditions to the total system airflow.

Units

Ratio

Input Restrictions

The rated building shall use the factors given below.

Table 3.7.5.1-1: Default Coil Bypass Factors

System Type

Default Bypass Factor

Packaged Terminal Air-conditioners and Heat Pumps

0.241

Other Packaged Equipment

0.190

Multi-Zone Systems

0.078

All Other

0.037

Baseline Rules

Same as the rated building

 

Coil Bypass Factor Adjustment Curves

Applicability

All DX cooling systems (optional input)

Definition

Adjustments for the amount of coil bypass due to the following factors:

  • Coil airflow rate as a percentage of rated system airflow
  • Entering air wet-bulb temperature
  • Entering air dry-bulb temperature
  • Part load ratio

Units

Data structure

Input Restrictions

Where applicable, the following default values shall be used for the coil bypass factor adjustment curve:

(Equation 3.7.5.1-8)

$$CBF_{adj}=CBF_{Rated} \times COIL\_BF\_FFLOW \times COIL\_BF\_FT \times COIL\_BF\_FPLR$$

(Equation 3.7.5.1-9)

$$COIL\_BF\_FFLOW = a + b\cdot CFMR + c \cdot CFMR^{2} + d \cdot CFMR^{3}$$

(Equation 3.7.5.1-10)

$$COIL\_BF\_FT = a + b\cdot T_{wb} + c \cdot T_{wb}^{2} + b\cdot T_{db} + c \cdot T_{db}^{2} +f\cdot T_{wb} \cdot T_{db}$$

Equation 3.7.5.1-11)

$$COIL\_BF\_FPLR = a + b\cdot PLR$$

Where

CBFrated

The coil bypass factor at ARI rating conditions

CBFadj

The coil bypass factor adjusted for airflow and coil conditions

CFMR

The ratio of airflow to design airflow

COIL-BF-FFLOW

A multiplier on the rated coil bypass factor to account for variation in air flow across the coil (take coefficients from Appendix H)

COIL-BF-FT

A multiplier on the rated coil bypass factor to account for a variation in coil entering conditions (take coefficients from Appendix H)

COIL-BF-FPLR

A multiplier on the rated coil bypass factor to account for the part load ratio (take coefficients from Appendix H)

Twb

The entering coil wet-bulb temperature (°F)

Tdba

The entering coil dry-bulb temperature (°F)

PLR

Part load ratio

a,b,c,d,e,f

Separate coefficients from Appendix H for COIL_BF_FT and COIL_BF_FFLOW

Baseline Rules

The above default values shall be used when applicable.