Direct Expansion Cooling Efficiency (COP) 


Applicability 
Packaged DX equipment 

Definition 
The cooling efficiency of a direct expansion (DX) cooling system at ARI rated conditions as a ratio of output over input in Btu/h per W, excluding fan energy. Fan energy shall be modeled separately according to Section 3.7.3 of this document. For all unitary and applied equipment where the fan energy is part of the equipment efficiency rating, the equipment COP shall be calculated as follows: For EER equipment ratings (Equation 3.7.5.21) $$COP_{nfcooling} = 7.84\times 10^{8} \cdot EER \cdot Q + 0.338 \cdot EER$$ For SEER equipment ratings (Equation 3.7.5.22) $$COP_{nfcooling} = 0.0076 \cdot SEER^{2} + 0.3796 \cdot SEER$$ where


Units 
Unitless ratio 

Input Restrictions 
As designed. Specify the COPnf as described above based on SEER for packaged equipment with net cooling capacity less than 65,000 Btu/h from manufacturer’s literature. For equipment with capacity above 65,000 Btu/h, use EER. 

Baseline Rules 
Calculate COP_{nfcooling} using the EER or SEER from the table below:

Direct Expansion Cooling Efficiency Adjustment Curve 


Applicability 
Packaged DX equipment 

Definition 
A curve that varies the cooling efficiency of a direct expansion (DX) coil as a function of evaporator conditions, condenser conditions, and for small packaged equipment, partload ratio. (Equation 3.7.5.23) $$PLR = \frac{Q_{operating}}{Q_{available(t_{wb},t_{odb/wt})}}$$ (Equation 3.7.5.24) $$EIR\_FPLR=a+b\cdot PLR+c\cdot PLR^{2} +d \cdot PLR^{3}$$
For aircooled DX systems (Equation 3.7.5.25) $$EIR\_FT = a + b\cdot t_{wb}+c\cdot t_{wb}^{2}+d\cdot t_{odb}+e\cdot t_{odb}^{2}+f\cdot t_{wb}\cdot t_{odb}$$ For watercooled DX systems (Equation 3.7.5.26) $$EIR\_FT = a + b\cdot t_{wb}+c\cdot t_{wb}^{2}+d\cdot t_{wt}+e\cdot t_{wt}^{2}+f\cdot t_{wb}\cdot t_{wt}$$ (Equation 3.7.5.27) $$P_{operating}=P_{rated}\cdot EIR\_FPLR\cdot EIR\_FT\cdot CAP\_FT$$ Where
Note: if an aircooled unit employs an evaporative condenser, todb is the effective drybulb temperature of the air leaving the evaporative cooling unit. 

Units 
Data structure 

Input Restrictions 
User may input curves or use default curves. If defaults are overridden, the software must indicate that supporting documentation is required on the output forms. 

Baseline Rules 
Use default curves. 
Number of Cooling Stages 


Applicability 
DX systems with multiple stages 
Definition 
This applies to systems with multiple compressors or multiple discrete stages of cooling. This system is a packaged unit with multiple stages of cooling. Systems with unequally sized compressors may have additional cooling stages. 
Units 
None (Integer) 
Input Restrictions 
As Designed 
Baseline Rules 
All baseline DX system single stage. 
Total Cooling Capacity by Stage 


Applicability 
DX systems with multiple stages 
Definition 
This provides the total cooling capacity of each cooling stage, at ARI rated conditions. The capacity is expressed as an array, with each entry a fraction of the total rated cooling capacity for the unit. For example, if the stage cooling capacity is 4 tons (48,000 Btu/h) and the total cooling capacity is 8 tons (96,000 Btu/h), the capacity is expressed as “0.50” for that stage. 
Units 
Array of fractions 
Input Restrictions 
As Designed 
Baseline Rules 
Not Applicable 
Sensible Cooling Capacity by Stage 


Applicability 
DX systems with multiple stages 
Definition 
This provides the sensible cooling capacity of each cooling stage, at ARI rated conditions. The capacity is expressed as an array, with each entry a fraction of the total rated sensible cooling capacity for the unit. For example, if the stage sensible cooling capacity is 3.5 tons (42,000 Btu/h) and the total sensible cooling capacity is 7 tons (72,000 Btu/h), the capacity is expressed as "0.5" for that stage." 
Units 
Array of fractions 
Input Restrictions 
As designed 
Baseline Rules 
Not applicable 
Supply Air Temperature Reset by Stage 


Applicability 
DX systems with multiple stages 
Definition 
This provides the cooling supply air temperature setpoint deviation from the cooling design supply air temperature, specified in the building descriptor Cooling Supply Air Temperature. 
Units 
The temperature reset is expressed as an array, with each entry a deviation from the design supply air temperature. For example, an entry of “5” for a stage would indicate a 5°F reset (for example, 60°F from 55°F). 
Input Restrictions 
As designed. Array of temperature differences, in degrees F 
Baseline Rules 
Not Applicable 
Number of Heating Stages 


Applicability 
DX systems with multiple stages 
Definition 
The number of heating stages provided by the system. Multiple stages could be provided via a heat pump or via a multiplestage gas furnace. 
Units 
Integer 
Input Restrictions 
As designed 
Baseline Rules 
Not Applicable 
Heating Capacity by Stage 


Applicability 
DX systems with multiple stages 
Definition 
This provides the total heating capacity of each heating stage, at ARI rated conditions. The capacity is expressed as an array, with each entry a fraction of the total rated cooling capacity for the unit. For example, if the stage heating capacity is 48,000 Btu/h and the heating capacity is 96,000 Btu/h, the capacity is expressed as “0.50” for that stage. 
Units 
Array of fractions 
Input Restrictions 
As Designed 
Baseline Rules 
Not Applicable. 
Heating Supply Air Temperature by Stage 


Applicability 
DX systems with multiple stages 
Definition 
This provides the heating supply air temperature setpoint deviation from the design heating supply air temperature, specified in the building descriptor Cooling Heating Supply Air Temperature. 
Units 
The temperature reset is expressed as an array, with each entry a deviation from the design supply air temperature. For example, an entry of “10” for a stage would indicate a 10°F reset (for example, 95°F from 105°F). 
Input Restrictions 
As Designed. Array of temperature differences, in degrees F 
Baseline Rules 
Not Applicable 
Supply Fan Low Speed Ratio 


Applicability 
DX systems with multiple stages and twospeed fans 
Definition 
This specifies the low fan speed setting on a Single Zone VAV system or DX system with multiple cooling stages. 
Units 
None (fraction) 
Input Restrictions 
As Designed 
Baseline Rules 
Not Applicable. 
Supply Fan Low Power Ratio 


Applicability 
DX systems with multiple stages and twospeed fans 
Definition 
This specifies the fraction of full load fan power corresponding to low fan speed operation on a Single Zone VAV system or DX system with multiple cooling stages. 
Units 
None (fraction) 
Input Restrictions 
As Designed 
Baseline Rules 
Not Applicable. 
Piping Insulation 


Applicability 
All projects 
Definition 
Thermal insulation on piping systems for service hot water, steam piping, chilled water for cooling and hot water for space heating. 
Units 
List (see above) 
Input Restrictions 
Not modeled 
Baseline Rules 
Not modeled 
Minimum Unloading Ratio 


Applicability 
Packaged systems which use hotgas bypass during low load conditions 
Definition 
The upper end of the hotgas bypass operating range. This is the percentage of peak cooling capacity below which hotgas bypass will operate. 
Units 
Ratio 
Input Restrictions 
As designed. The user must enter this descriptor for each DX cooling system. If hotgas bypass is not employed, a value of 0 may be entered. A maximum of 0.5 is allowed for units with a peak cooling capacity of 240 kBtu/h (20 tons) or less, and a maximum value of 0.25 is allowed for units with a peak cooling capacity greater than 240 kBtu/h. 
Baseline Rules 
No hotgas bypass 
Minimum HGB Ratio 


Applicability 
Packaged systems which use hotgas bypass during low load conditions 
Definition 
The lower end of the hotgas bypass operating range. The percentage of peak cooling capacity below which hotgas bypass will no longer operate (i.e. the compressor will cycle). 
Units 
Fraction between 0 and 1 
Input Restrictions 
As designed. The user must enter this descriptor for each DX cooling system. If hotgas bypass is not employed, a value of 0 may be entered. 
Baseline Rules 
Not applicable 
Condenser Type 


Applicability 
All direct expansion systems including heat pumps 
Definition 
The type of condenser for a direct expansion (DX) cooling system. The choices are:

Units 
List (see above) 
Input Restrictions 
As designed 
Baseline Rules 
Baseline HVAC systems 1 through 6 are all air cooled. 
Condenser Flow Type 


Applicability 
All direct expansion systems including heat pumps 
Definition 
Describes water flow control for a watercooled condenser. The choices are:

Units 
List (see above) 
Input Restrictions 
Default to fixed flow. If the variableflow is selected, the software must indicate that supporting documentation is required on the output forms. 
Baseline Rules 
Not applicable. Baseline DX systems are always air cooled. 