# 3.7.5.2 Direct Expansion

90.1-2016 BM

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 3.7.5.2-1)

$$COP_{nfcooling} = 7.84\times 10^{-8} \cdot EER \cdot Q + 0.338 \cdot EER$$

For SEER equipment ratings

(Equation 3.7.5.2-2)

$$COP_{nfcooling} = -0.0076 \cdot SEER^{2} + 0.3796 \cdot SEER$$

where

 COPnfcooling Packaged equipment cooling efficiency without the fan Q the AHRI-rated cooling capacity in Btu/h EER, SEER The energy efficiency ratio or seasonal energy efficiency ratio determined at AHRI test conditions, including the fan.

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 COPnfcooling using the EER or SEER from the table below:

 Equipment Capacity Rated Efficiency Test Procedure <65,000 Btu/h 9.7 SEER ARI 210/240 ≥65,000 Btu/h and <135,000 Btu/h 9.9 EER ARI 340/360 ≥135,000 Btu/h and <240,000 Btu/h 9.1 EER >240,000 Btu/h 8.8 EER

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, part-load ratio.

(Equation 3.7.5.2-3)

$$PLR = \frac{Q_{operating}}{Q_{available(t_{wb},t_{odb/wt})}}$$

(Equation 3.7.5.2-4)

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

For air-cooled DX systems

(Equation 3.7.5.2-5)

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

For water-cooled DX systems

(Equation 3.7.5.2-6)

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

(Equation 3.7.5.2-7)

$$P_{operating}=P_{rated}\cdot EIR\_FPLR\cdot EIR\_FT\cdot CAP\_FT$$

Where

 PLR Part load ratio based on available capacity (not rated capacity) EIR-FPLR A multiplier on the rated power draw (Prated) to account for the part load ratio. See the equation above and choose coefficients from Appendix H based on the type of equipment. EIR-FT A multiplier on the rated power draw (Prated) to account for the wet-bulb temperature entering the coil and the outdoor dry-bulb temperature. See the equation above and choose coefficients from Appendix H. Qoperating Present load on air conditioner (Btu/h) Qavailable Air conditioner available capacity at present evaporator and condenser conditions (in Btu/h). twb The entering coil wet-bulb temperature (°F) twt The water supply temperature (°F) todb The outside-air dry-bulb temperature (°F) Prated Rated power draw at ARI conditions (kW) Poperating Power draw at specified operating conditions (kW)

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.

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 multiple-stage 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 two-speed 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 two-speed 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

Applicability

Packaged systems which use hot-gas bypass during low load conditions

Definition

The upper end of the hot-gas bypass operating range. This is the percentage of peak cooling capacity below which hot-gas bypass will operate.

Units

Ratio

Input Restrictions

As designed. The user must enter this descriptor for each DX cooling system. If hot-gas 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 hot-gas bypass

Minimum HGB Ratio

Applicability

Packaged systems which use hot-gas bypass during low load conditions

Definition

The lower end of the hot-gas bypass operating range. The percentage of peak cooling capacity below which hot-gas 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 hot-gas 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:

• Air-cooled
• Water-cooled
• Air-cooled with evaporative pre-cooler

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 water-cooled condenser. The choices are:

• Fixed Flow
• Two-position
• Variable Flow

Units

List (see above)

Input Restrictions

Default to fixed flow. If the variable-flow 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.