C-i

APPENDIX C. DESIGN OPTION ANALYSIS - SUPPORTING DOCUMENTATION

TABLE OF CONTENTS

C.1 MODELING TOOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

C.1.1 DOE/ORNL Heat Pump Design Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

C.1.2 Heat Exchanger Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-5

C.1.3 Compressor Map Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-5

C.1.4 Fan Curve Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-6

C.2 BASELINE MODELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-6

C.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-6

C.2.2 7.5-ton Standard-Efficiency Rooftop Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-7

C.2.3 7.5-ton High-Efficiency Rooftop Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-10

C.2.4 15-ton Standard-Efficiency Rooftop Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-11

C.2.5 15-ton High-Efficiency Rooftop Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-13

C.3 DESIGN MODIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-15

C.3.1 Design Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-15

C.3.2 7.5-ton Standard-Efficiency Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-17

C.3.3 7.5-ton High-Efficiency Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-18

C.3.4 15-ton Standard-Efficiency Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-19

C.3.5 15-ton High-Efficiency Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-21

C.4 R-410A ANALYSIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-23

C.4.1 7.5-ton R-410A Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-23

C.4.2 15-ton R-410A Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-25

LIST OF TABLES

Table C.1.1 Compressor Map Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-6

Table C.3.1 Design Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-16

LIST OF FIGURES

Figure C.1.1 Cycle Balance Logic Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-3

Figure C.1.2 Refrigerant Balance Logic Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-4

Figure C.2.1 Evaporator Data (ORNL input page) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-8

Figure C.2.2 Condenser Data (ORNL input page) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-9

Figure C.2.3 Model Calibration with Test Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-10

Figure C.2.4 Evaporator Data (ORNL input page) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-12

Figure C.2.5 Condenser Data (ORNL input page) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-12

Figure C.2.6 Evaporator Data (ORNL Input page) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-14

Figure C.2.7 Condenser Data (ORNL input page) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-14

Figure C.3.1 7.5-ton High-Efficiency Baseline and Design Options . . . . . . . . . . . . . . . . . . C-19
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Figure C.3.2 15-ton Standard-Efficiency Baseline and Design Options . . . . . . . . . . . . . . . C-21

Figure C.3.3 15-ton High-Efficiency Baseline and Design Options . . . . . . . . . . . . . . . . . . C-23

Figure C.4.1 7.5-ton R-410a Baseline and Design Options . . . . . . . . . . . . . . . . . . . . . . . . . C-25

Figure C.4.2 15-ton R-410a Baseline and Design Options . . . . . . . . . . . . . . . . . . . . . . . . . . C-26
C-1

APPENDIX C. DESIGN OPTION ANALYSIS - SUPPORTING DOCUMENTATION

C.1 MODELING TOOLS

The task of modeling the performance of rooftop units and evaluating the impact of

multiple design options involved the use of several different engineering models, each serving a

specific role in the overall process. The primary model was the Oak Ridge National Laboratory

(ORNL) Heat Pump model, which predicted the overall performance of the rooftop unit. The

Department used secondary models, which accounted for certain components of the rooftop unit,

to obtain necessary data to input into the ORNL model. Compressor performance data (in the

form of a nine-term polynomial function) provided data on the compressor operation—namely,

the refrigerant mass flow and power consumption. The Department found the air-side pressure

drop across the condenser and evaporator coils using a custom heat exchanger performance

model called UAIRE. The Department then used the pressure drop in conjunction with the

corresponding fan or blower curve to determine the fan or blower power. 

C.1.1 DOE/ORNL Heat Pump Design Model

The ORNL developed the Hypertext Markup Language (HTML)-based model, with

Department of Energy support, for use as a reference tool with the ability to predict performance

trends for air-to-air heat pumps and air conditioners. Available on the ORNL website at

http://www.ornl.gov/~wlj/hpdm/doehpdm.html, the model accepts user input, performs selected

calculations, and presents results in a comprehensive manner.

Input

The required inputs for the ORNL model are numerous, and are summarized but not

explicitly listed here. The user first chooses to model either a heat pump or air-conditioning

system, and selects one of 15 possible refrigerants. The model provides the following three

options for calculating system refrigerant charge balance: one in which the high side of the

system (in terms of either condenser exit subcooling or flow control requirements) and low-side

(compressor inlet superheat) conditions are specified; and two options that allow the user to

specify a refrigerant charge, and then either the high- or low-side specifications. In each case, the

third, unspecified variable is calculated when the model is run. If the low side of the system is to

be specified, the model allows for the use of four options for detailing flow control: condenser

subcooling, capillary tube, short-tube orifice, or thermal expansion valve. 

For compressor selection, the model offers ten different compressors, using four different

refrigerants and with capacities ranging from 26,000 Btu/h to 36,500 Btu/h, that are

preconfigured and can be used with or without modification. It is also possible to input data from

any other compressor map for use in the model, using coefficients determined according to the

Air-Conditioning and Refrigeration Institute (ARI) Standard 540-99, “Positive Displacement

Refrigerant Compressors and Compressor Units.” In general, the variables for both the
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evaporator and condenser coils are extensive, and allow for both air-side and refrigerant-side

specifications. The final details that the model requires as input variables are the fan locations

within the unit and dimensional data for the refrigerant lines. 

ORNL designed and verified the simulation model for basic air conditioners and heat

pumps, but the model has optional flexibility that allows users to simulate a wider range of

air-conditioner designs. The model’s flexibility is, in part, attributed to scaling factors:

customizable multipliers for compressor capacity, compressor power, compressor mass flow,

coil heat transfer (refrigerant-side and air-side), and coil pressure drop (refrigerant- and air-side).

These scaling factors enable users to customize and fine-tune the model to suit their specific

needs. The Department, for example, used the scaling factors to calibrate the model for each

baseline product to published test data. Once the Department established the scaling factors for

each baseline, it held them constant during all of the design option simulations.

Calculations

Depending on the user-selected input, the model calculates the performance of the airconditioning system following one of the three logic diagrams shown in Figures C.1.1 and C.1.2.

Quantities calculated in this system balance are capacity (in Btu/h), sensible heat ratio (SHR),

condenser heat transfer coefficient multiplied by area (UA) (Btu/h-/F), evaporator UA

(Btu/h-/F), and refrigerant charge (lbm). 
C-3

Figure C.1.1 Cycle Balance Logic Diagram

OflNL in-.74 7C1<roo 

Basic Cycle Balance 

Low- and High-Side-Specified 

GUESS: 

High- snd L ow-Side 

Pressures 

S PECIFY: 

Inlet Air Temps 

Condenser 1-"---1 Subcooling (6 Tsc) o r Flow Control Device 

Evaporator 

Superheat (t!.T SH) 

mcm Compressor 

C:c~o~n~d~~n~.~~,) t!.T sc, calc 

Flow Control 

, _____ -'N;:;'-< m or t!.T sc 

match 

? 

Fixed 

Subcooling t!.T sc, .pec 

Adjus t 

Inlel Air 

Temp T air,ev 

t!.TSH No 

No 

match 

? 

Ve. 

Print 

Result s 

S lOP 
C-4

Figure C.1.2 Refrigerant Balance Logic Diagram

Presentation of Results

The results page of the ORNL model contains information on the system as a whole as

well as information on each of the components. The system-wide information is based on the

capacity, using that value to calculate the energy-efficiency ratio (EER) and the SHR. The EER

is calculated using the system capacity and the total power, which is the sum of user-supplied

power values for the condenser and evaporator fans and the calculated compressor power. The

sensible capacity is calculated from the appropriate indoor air conditions, then compared to the

overall system capacity. The SHR is reported as a decimal value. All of these values are

presented immediately at the top of the results page, with the remainder of page one devoted to

general system operating conditions and page two presenting detailed evaluations of each

individual component.

The majority of the first page of results presents the refrigerant conditions at any of seven

points within the system. Each point that is represented displays the refrigerant pressure,

temperature, and saturation temperature, as well as properties that are specific to that point
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within the system. For example, at the condenser inlet, the refrigerant enthalpy and superheat are

reported, and at the condenser exit, the refrigerant subcooling is shown. Also shown on the first

results page is the calculated compressor mass flow and power, as well as the relevant air

conditions for both the condenser and evaporator.

The second page of results provides a detailed analysis of each component of the system.

Data given for the compressor include, but are not limited to, EER, capacity speed (rpm),

pressure ratio, and any scaling factors or multipliers that were used. Data reported for both the

condenser and the evaporator include face area, face air velocity, total air-side area, and coil

overall heat transfer coefficient multiplied by the face area (UA). Details are also given for the

flow-control device. The refrigerant-charge calculation results complete the page, displaying not

only the total charge of the system, but the dispersion of the refrigerant throughout the

system—shown as the percentage of refrigerant present in different locations.

C.1.2 Heat Exchanger Model

The custom heat exchanger model, called UAIRE, accepts input regarding the physical

specifications of a heat exchanger coil (e.g., rows, area, fin pitch) and the airflow over the coil

(cfm), and then calculates the associated air properties concerning airflow through the coil. The

central interest in this program for the rooftop unit modeling is the calculation of air pressure

drop across the coil, and the Department used the UAIRE program to find this value for both

condenser and evaporator coils. The Department then used the air-side pressure drop (DPAIR) in

baseline and design-option modeling.

C.1.3 Compressor Map Analysis

For both of the units, DOE obtained compressor maps from the compressor manufacturer

for the exact compressors that it used. From this map, DOE created a curve fit to determine the

coefficients of the following equation, from ARI 540-99:

F(TS,TD) = C1 + C2TS + C3TD + C4TS

2

 + C5TDTS + C6TD

2

 + C7TS

3

 + C8TDTS

2

 + C9TSTD

2

 + C10TD

3

Where: 

Ts is the compressor suction saturation temperature, 

TD is the compressor discharge saturation temperature, and 

C1-10 are the performance coefficients. 

The Department carried out this type of analysis for the compressor mass flow and power tables

from the map (see Table C.1.1), and used the analysis accordingly in the ORNL model.
C-6

Table C.1.1 Compressor Map Values

7.5-Ton Standard

Efficiency

7.5-Ton High

Efficiency

15-Ton (Standard and

High Efficiency)

Coefficients Mass

flow

Power Mass

flow

Power Mass flow Power

C1 -1569.8 1762.18 -789.24 274.874 466.636 6855.719

C2 -25.981 95.684 -12.129 32.255 42.869 33.268

C3 50.346 -41.812 27.712 19.526 10.088 -66.524

C4 0.0037 1.4892 0.0188 0.9154 0.0740 0.4502

C5 0.5312 -2.4394 0.2550 -0.8211 -0.3209 -0.7235

C6 -0.4497 0.9138 -0.2265 -0.0207 -0.0345 1.0297

C7 0.0006 0.0027 0.0003 0.0068 0.0004 0.0012

C8 0.00003 -0.0171 -0.0002 -0.0114 0.0011 -0.0061

C9 -0.0020 0.0179 -0.0007 0.0051 0.0012 0.0053

C10 0.0012 -0.0047 0.0005 0.0006 -0.0001 -0.0009

C.1.4 Fan Curve Analysis

Given specifications for either the evaporator or condenser coil (e.g, rows, depth, area)

and airflow, DOE established a baseline air-side pressure drop for the coil using the UAIRE

program. It then used the pressure drop with fan performance data provided in public product

literature to determine the baseline fan power, which is typically given in terms of shaft

horsepower. The Department multiplied the horsepower value by the number of fan motors in

the rooftop unit (typically two condenser fans and one evaporator blower), and converted it to

watts. It finally corrected this value for motor efficiency (estimated at 70 percent for condenser

fans and 85 percent for an evaporator blower, unless stated otherwise), resulting in the input

power to the fans. 

C.2 BASELINE MODELS

C.2.1 Introduction

The Department established a baseline model for each of the four units that it used in the

design option process. This baseline served as a starting point for all of the design options, and

was equivalent to the published ARI rating data for each of the units. For this process, DOE

gathered all input data from unit catalogs taken from each manufacturer, and from the physical
C-7

teardowns of the units. Once it had entered all relevant data into the ORNL model, DOE

matched the unit capacity and then the EER, determined by the total unit power. Once these

three values corresponded with the published data, DOE checked other performance variables

against available test data and engineering judgement to ensure that the model was reasonably

predicting unit performance. Among these variables were refrigerant pressures throughout the

system, superheat and subcooling values, refrigerant mass flow, and air conditions around both

coils.

C.2.2 7.5-ton Standard-Efficiency Rooftop Unit

The subject of the first model was a standard-efficiency rooftop unit with a nominal

capacity of 7.5 tons.

ITS Testing Results

Intertek Testing Services (ITS) performed a series of tests on the unit that was to be torn

down to determine the capacity and EER at ARI (80/67 return air temperature, 95/F ambient)

and integrated part-load value (IPLV) (80/67 return air temperature, 80/F ambient) rating

conditions. The outcome of this testing indicated capacity and EER numbers better than the

published levels, so the modeling process moved forward with the ITS results as the target

values. The tested capacity was 92,741 Btu/h, with a 10.453 EER value (total unit power of 8872

W). ITS performed several trials at each set of test conditions and reported an extensive set of

data for each trial. The data included refrigerant temperatures and pressures and air conditions

recorded at various points within the system. With the new numbers set as the targets, DOE

completed the modeling process as described in the following section.

Model Inputs

The first major piece of information that was used for this baseline model was a set of

data from the appropriate compressor map. This unit has two compressors set up in a parallel

arrangement. The particular compressor used in this product has a rated capacity of 41,700 Btu/h

and a rated EER of 11.0. The Department input these values as needed into the ORNL model,

along with coefficients for mass flow and power. The Department modeled the two compressor

circuits as a single circuit with capacity, power, and mass-flow scaling factors set to 2.0.

The fixed-orifice flow control technique was selected in the model. The modeled unit

used 12 short-tube orifices to regulate refrigerant flow, so the measured length and diameter of

the orifices was entered along with the total number of parallel orifices in the system.

The next major information needed for the ORNL model was a complete physical

description of the evaporator. The input data for the evaporator can be seen in Figure C.2.1

below, which is an image directly from the ORNL model. The user supplies all of the data that

are seen in the various text boxes on the page, many of which are determined previously to being

input into the model. The air conditions (temperature, humidity, and external dP, seen in Figure
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Figure C.2.1 Evaporator Data (ORNL input page)

C.2.1) are specified by the ARI standard, and all coil information is taken from the manufacturer

catalog or from the teardown data. The evaporator blower power was taken from the ITS data,

and verified with the appropriate catalog data. The last significant input is the refrigerant-side

heat-transfer coefficient of 1.55. This value was supplied by Oak Ridge engineers in their

version of the baseline model (discussed in a subsequent section below) and verified by the

Department.

As with the evaporator, DOE completed a similar input page for the condenser coil; this

is shown in Figure C.2.2. Again, the user supplies all information in text boxes, with the same

sources for air conditions—with the exception of external dP, which is not specified for the

condenser, and coil dimensions. The condenser fan power is found from the fan manufacturer’s

fan performance curve, and is a function of the air-side pressure drop across the coil, as

determined by the UAIRE model. The refrigerant-side coil dP correction factor was set at 0.70 to

correspond with the refrigerant pressure characteristics seen in the ITS data (DPCOIL ~ 10 psi).
C-9

Figure C.2.2 Condenser Data (ORNL input page)

The last inputs involve unit configuration and the refrigerant-line data. For this unit, the

indoor blower, condenser fan, and the compressor shell are located after the respective coil. This

option was chosen in the ORNL model, which adds the appropriate waste heat for each device to

the airstream. Due to the method of modeling two circuits with one compressor, the refrigerant

line flow area was doubled and the overall length cut in half. This was an attempt to correctly

predict pressure drop within the refrigerant line.

Model Results

With the data input as described in the preceding section, the model yields results that are

quite close to the values reported in the ITS data—a calculated capacity of 91,037 Btu/h (1.8

percent difference from ITS) and an EER of 10.334 (1.1 percent difference). These results were

within the range of error allowed in the ARI test standard. Figure C.2.3 shows a comparison of

the data.
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Capacity (Btu/hr) 91,037 92,7412

EER 10.334 10.452

Compressor Power (W) 7,325 7,430 +/- 2.0%

Evaporator Blower Power (W) 910 880 +/- 2.0%

Condenser Fan Power (W) 575 562 +/- 2.0%

Evaporator Pressure (PSIA) 96.8 98.7 +/- 2.0%

48.8 Average: 502 

(48.7 - 51.2)3

Predicted by

Model

Measured by 

ITS Testing1

Evaporator Temperature (oF)

-1.8%

-1.1%

-1.4%

+3.4%

+2.3%

-1.9%

1.2 oF

Difference

Condenser Pressure (PSIA) 279.2 282.2 +/- 2.0%

121.3 Average: 122.12

(120.6 - 123.7) Condenser Temperature ( 3 oF)

-1.1%

0.8 oF

1 Using Test “A” methods

2 Calculated from measured data 

3 Range calculated from pressure error band 

Figure C.2.3 Model Calibration with Test Data

Oak Ridge Participation

Oak Ridge National Laboratory engineers simultaneously completed their own version of

this baseline model. Because these engineers are continuously involved with all details of their

HTML-based model, the Department considered their input valuable as an addition to its own

work. The model the ORNL engineers created utilized a different, but equivalent, approach to

modeling the dual compressors (doubling the compressor map before finding coefficients, then

using scaling factors set to one), but modeled the rest of the unit in essentially the same manner.

The results were slightly lower than the final values in the model and closer to the published

ratings with a capacity of 89,500 Btu/h (0.36 percent lower than the model), and an EER of

10.11 (0.72 percent lower than the model’s value).

C.2.3 7.5-ton High-Efficiency Rooftop Unit

Modeling the 7.5-ton high-efficiency unit was much the same as the standard efficiency,

due to the vast similarity of the two units. Most of the equipment in the unit (e.g., condenser and

evaporator coils, condenser fans) was the same in both of the units; the only major difference

was a change from a reciprocating compressor to a scroll compressor. The scroll technology

offers greater efficiency than the reciprocal compressor and this is reflected in the compressor

coefficients used in the model. The Department used the same approach to model the dualcompressor configuration, where it set scaling factors for capacity, mass flow, and power equal

to 2.0. The compressor’s rated capacity is 45,500 Btu/h with an EER of 11.4.
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The flow control device and coil characteristics (both evaporator and condenser) for this

unit were the same as that of the standard-efficiency unit described in Section B.2.2.

The high-efficiency unit has a more powerful evaporator fan blower, rated at 2.9

horsepower, compared to the standard-efficiency unit’s 2.4-horsepower motor. This difference

has little impact on the modeling, however, because the ARI rating is measured at standardized

air conditions (dP External = 0.25 inches of water). This pressure setting, along with the use of

the same evaporator coil, ensures that the evaporator blower power will remain relatively

constant despite the higher peak motor horsepower rating.

The results of the model were very close to the published values: model capacity is

90,148 Btu/h and the EER 11.029 (total power of the unit is 8173.4 W).

C.2.4 15-ton Standard-Efficiency Rooftop Unit

Modeling the compressors in the 15-ton unit presented a challenge that had not been

previously encountered, since the unit contains one 10-ton compressor and one 5-ton

compressor. The Department could not model these as it had the 7.5-ton units; instead, with an

adjusted scaling factor, it combined the performance maps for both compressors to model a

single compressor. The Department obtained coefficients for the capacity, mass flow, and power

of each compressor, and created a standard performance map for each compressor. It then added

corresponding values from each of the two maps, simulating the two compressors operating

simultaneously under the same conditions. The Department performed a regression analysis (see

section B.1.3) on the resultant maps for mass flow and power, and entered these coefficients into

the ORNL model. The Department also determined a standard capacity from the combined map,

and entered it into the model. 

Considering the approximations made with the compressor modeling, it was necessary to

use the scaling factors in the model to achieve the desired unit performance. Of particular

importance were the compressor mass flow and power-scaling factors. Through an iterative

process, DOE adjusted the mass flow so the capacity would approach 174,000 Btu/h. The

Department also adjusted the compressor power to make a change in the overall unit power and,

in effect, the unit EER, because condenser fan power and evaporator blower power were fixed.

The Department set the mass flow factor at 1.102, and the power factor at 1.125. Both remained

constant for all subsequent design option modeling.

As with the 7.5-ton units, this system uses a short-tube orifice to regulate the refrigerant

flow through the evaporator coil. The Department entered the measured orifice information into

the model.

The input data for the evaporator and condenser coils appear in Figures C.2.4 and C.2.5,

respectively; they were taken from the manufacturer’s catalog and the teardown data. The
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Figure C.2.4 Evaporator Data (ORNL input page)

Figure C.2.5 Condenser Data (ORNL input page)

evaporator blower power of 1560 W comes directly from the manufacturer’s catalog for airflow

and pressures specified by the ARI test standard.
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The Department changed the refrigerant-side pressure drop scaling factor for the

condenser to five, in order to maintain a 10-psi refrigerant drop through the coil. The condenser

fan power calculation considers the two 0.5-hp condenser fans operating within the unit,

resulting in a power input of 1070 W.

The final capacity of the unit, as determined through this process, was 174,091 Btu/h

(+0.05 percent, compared to published rating), with an EER value of 9.703 (+0.03 percent, from

the published rating).

C.2.5 15-ton High-Efficiency Rooftop Unit

The high-efficiency unit uses the same compressors as the standard unit, but has larger

and deeper heat exchanger coils. As a result, DOE inserted the compressor coefficients that it

used for the standard unit directly into the high-efficiency model. 

A short-tube orifice is used in this unit, as with the standard-efficiency unit, to regulate

the flow of refrigerant into the evaporator. Again, DOE entered the measured orifice parameters

into the model.

As stated above, both the evaporator and condenser coils were quite different from those

used in the standard-efficiency unit. The data pertaining to the coils are shown below in Figures

C.2.6 and C.2.7.
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Figure C.2.6 Evaporator Data (ORNL Input page)

Figure C.2.7 Condenser Data (ORNL input page)
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The final capacity of the unit as determined through this process was 180,984 Btu/h

(+0.55 percent, compared to the published rating), with an EER value of 11.507 (+0.06 percent,

from the published rating).

C.3 DESIGN MODIFICATIONS

C.3.1 Design Paths

The object of the design option simulation process was to increase the efficiency of each

rooftop unit to at least a 12.0 EER value, while staying within performance limits established by

the Department in discussions with manufacturers. The performance limits that were maintained

are presented below in Table C.3.1 and, in almost every case, the modeled rooftop units were

enhanced up to these limits. These performance limits prevented some of the design option

units—essentially those models that were built on a standard-efficiency baseline unit—from

reaching the target efficiency level. This is why the Department did not consider the standardefficiency results for the presentation in Section 5.8. These results are included here for

documentation purposes, and in order to add a further level of clarity to the design-modification

approach.

The process of modeling the design modifications is described in detail in this section for

each unit. For all units, DOE set the target efficiency level at 12.0 and modified the units until

this level was reached or until the unit reached the stated operating limits. The Department

completed modifications independently on each of the four baseline units; the process for each

will be discussed in the following sections.
C-16

Table C.3.1 Design Constraints

Compressors a. With multiple compressors of the same size, each acts with the same

performance characteristics. This enables the use of scaling factors.

b. Compressors are located after the condenser coil, so waste heat is

added to the exiting condenser air stream.

c. Compressor capacity is scaled to maintain relatively constant

system capacity. DOE performed compressor scaling in discrete

increments to use capacities of commercially available compressors

at all times.

Evaporator d. Evaporator blower motor efficiency is assumed to be 85%.

e. Refrigerant side heat transfer “correction multiplier” is 1.55 (taken

from ORNL participation).

f. Evaporator blower is located after the coil, so the motor waste heat

is added to the indoor air stream.

g. Maximum evaporator temperature is 52/F.

h. Maximum sensible heat ratio is 0.750.

Condenser i. Condenser fan motor efficiency is assumed to be 70%.

j. Refrigerant pressure drop across the coil is maintained at

approximately 10 psi. This is accomplished by changing the number

of parallel circuits in the condenser coil when the pressure drop

increases beyond this level.

k. Condenser fans are located after the coil, so the motor waste heat is

added to the leaving condenser air stream.

l. Minimum condenser temperature is 117/F for R-22, and 116/F for

R-410A.

m. Minimum refrigerant exit temperature is 102/F, for each refrigerant.

Refrigerant

Lines

n. If there are parallel refrigerant circuits, the total area (sum of

individual areas) is maintained by assuming one line, then setting

the diameter to the necessary size for this larger tube. The tube

length is kept at the original value.

Short Tube

Orifice

o. This method of flow control is used in every model, although actual

systems may used a TXV (R-410A units especially). The orifice can

approximate a TXV that is in a fixed position, as it would be at full

load during ARI testing.

p. Diameter is variable to four significant figures.

q. Length is variable (only used as a coarse adjustment), although

typically held constant.
C-17

C.3.2 7.5-ton Standard-Efficiency Unit

The standard-efficiency unit did not offer a high probability of making the jump from the

10.1 ARI rated EER to a 12.0, so DOE used a slightly different approach for the design option

simulation process. Rather than attempting to attain a 12.0 EER, DOE modeled four different

changes to determine the effect each would have on the unit performance. This information

proved useful later when it was considered while choosing the specific paths that DOE selected

to enhance the high-efficiency units.

The Department completed four separate design modifications for this baseline unit, all

of which focused on the heat exchanger coils. For each coil, DOE increased the depth by one

row (models 1 and 3), and then increased the coil face area along with the depth increase

(models 2 and 4). The first two models allowed the unit to remain in the same box size, because

the dimensions are more dependent on coil area than on coil depth, but the last two models

included a box size “jump” (increase in box size to the next available volume). Each model is

described below and a summary of the results is in Figure C.3.1.

Condenser Depth Increase

The addition of one row to the condenser coil of this unit resulted in a three-row coil and

increased the PAIR ∆ by 0.0478 inches of water, which in turn decreased the airflow across the

coil, 6200 cfm, and slightly increases the condenser fan power to 610 W. The larger condenser

coil also initially resulted in an increase in overall unit capacity, but this was maintained by

scaling back the compressor capacity by 2.5 percent to 40.6 kBtu/h. The result was a unit with a

10.62 EER, almost 0.3 EER higher than the baseline.

Condenser Area Increase

The condenser coil was increased by 25 percent, resulting in a 25-ft2

 face area and

necessitating a box-size jump. The Department also added one row to the coil to maximize the

EER gain. The resulting three-row coil had a PAIR ∆ that was slightly higher than the baseline,

since the effects of the additional area and the added depth offset each other. The effect of this

marginal increase in PAIR ∆ was a decrease in condenser airflow to 6400 cfm from the 6500 cfm

baseline, with a negligible effect on fan power. The condensing temperature was 117.5/F, and

the exit temperature was 102.7/F. In order to maintain capacity of this unit, DOE scaled back the

compressor capacity by 2 percent to 41.1 kBtu/h. The result was a unit with a 10.77 EER, more

than 0.4 EER higher than the baseline.

Evaporator Depth Increase

The addition of one row to the evaporator caused the PAIR ∆ to increase by 0.0428 inches

of water, which translated to a 55 W jump in evaporator blower power to 965 W. The

Department scaled back the compressors by 5 percent to account for the additional unit capacity

that was caused by the enhanced evaporator, to a total value of 39.4 kBtu/h. The constraints
C-18

pertaining to this modification were the maximum allowable evaporator temperature, and the

maximum SHR. This unit pressed the allowable levels, but did not exceed them, as the

evaporating temperature was 51.6/F and the SHR was 0.745. The result was a unit with a 10.59

EER, almost 0.3 EER higher than the baseline.

Evaporator Area Increase

The Department increased the evaporator area by 25 percent for this design option,

resulting in a face area of 11.125 ft2 and a PAIR ∆ that was 0.0437 inches of water less than the

baseline. The lower PAIR ∆ resulted in a blower power that was 50 W lower than the baseline

value of 910 W. Scaling the compressors back by 4 percent to a capacity of 40.0 kBtu/h

controlled the capacity. Since the evaporator conditions were already approaching the design

constraints, DOE made no further modifications to the evaporator. The evaporating temperature

was 50.7/F, and the SHR 0.736. The result was a unit with a 10.63 EER, 0.3 EER higher than the

baseline.

C.3.3 7.5-ton High-Efficiency Unit

The 7.5-ton high-efficiency unit had a rated capacity of 90,000 kBtu/h and an EER of

11.0, and served as the baseline for the design option process that yielded the target 12.0 EER

unit. In order to achieve this gain in efficiency, DOE modified the condenser coil, the evaporator

coil, and the compressors. The final result was a unit with a capacity of 90,350 Btu/h and an EER

of 12.05.

Condenser Area Increase

Increasing the condenser coil face area by 50 percent and adding a row resulted in a drop

in PAIR ∆ of 0.0473 inches of water, which effected a 300 cfm increase in condenser airflow

(6500 cfm, baseline) and a 60 W drop in fan power (620 W, baseline). The Department scaled

down the compressors for this option by 2 percent to 42.0 kBtu/h to correct for the initial

increase in unit capacity. The condensing temperature, 117.8/F, and exit temperature, 102.1/F,

were within the design constraints. The result was a unit with an 11.71 EER, almost 0.6 EER

higher than the baseline.

Condenser Depth Increase

The Department added one row to the condenser coil, bringing the total to three rows,

which caused a rise in PAIR of 0.0478 inches of water. The change in PAIR ∆ ∆ led to a slightly

lower condenser airflow value of 6200 cfm (6500 cfm, baseline) and an increase in fan power of

35 W (620W, baseline). Along with the coil change, DOE scaled down the compressors by 2

percent to 42.0 kBtu/h, in order to maintain the rated capacity. The condensing temperature for

this unit was 118.5/F and the exit temperature was 102.3/F, both above the minimum values. The

result was a unit with an 11.51 EER, almost 0.4 EER higher than the baseline.
C-19

Product

Specs

Approach Baseline Physical Characteristics Performance Characteristics

Tc = 121.0 F

Te = 48.9 F

Subcool = 10.8 F

Superheat = 20 F

Compressor: 6883 W

Fan: 620 W

Blower: 700W

SHR = 0.728

11.14-EER

Tc = 117.8 F

Te = 48.8 F

Subcool = 11.7 F

Superheat = 20 F

Compressor: 6533 W

Fan: 560 W/ +300 CFM

Blower: 700W

SHR = 0.729

+0.57 -EER

box:

 +70% volume

condenser:

+50% area

compressor:

2 x 42.0 kBtu/hr

Tc = 118.5 F

Te = 48.9 F

Subcool = 11.9 F

Superheat = 20 F

Compressor: 6545 W

Fan: 655 W/ -300 CFM

Blower: 700W

SHR = 0.730

+0.37 -EER

condenser:

+1 row

Compressor:

2 x 42.0 kBtu/hr

Tc = 117.4 F

Te = 50.7 F

Subcool = 11.2 F

Superheat = 20 F

Compressor: 6289 W

Fan: 560 W/ +300 CFM

Blower:650W

SHR = 0.739

+0.91 -EER

box:

+70% volume

condenser:

 +50% area 

evaporator:

+25% area 

compressor:

2 x 40.6 kBtu/hr

Condenser Area +50% Area Condenser Depth +1 Row Area Combination

+50% Cond., +25% Evap.

Figure C.3.1 7.5-ton High-Efficiency Baseline and Design Options

Design Combination

Since the condenser area increase yielded the largest gain in unit EER, DOE chose that

option to build on, in order to design a 12.0 EER unit. The condenser area increase made a jump

in box size necessary, so the next step to increase unit performance would be an increase in the

face area of the evaporator. Thus, additional cost beyond that of the coil itself would not be

incurred.

The addition of the evaporator modification to this unit caused an increase in system

capacity as well as EER, which was handled by scaling the compressors back again. The

compressors chosen for this unit were 6 percent smaller in capacity than the baseline, rated at

40.6 kBtu/h, and each had the same EER. The unit had an evaporating temperature of 50.7/F and

a SHR of 0.739, which met design constraints. The result was a unit with a 12.05 EER, almost

0.9 EER higher than the baseline. The results are summarized in Figure C.3.1. below.

C.3.4 15-ton Standard-Efficiency Unit

As with the 7.5-ton standard-efficiency unit, the 15-ton standard-efficiency unit did not

offer a high probability of making the jump from the 9.7 ARI rated EER to 12.0 EER. With this

in mind, DOE modeled the same four changes as with the 7.5-ton unit to determine the effect

each would have on the unit performance. These changes were the same for both the condenser

and evaporator coils, and included a depth increase of one row (models 1 and 3), and then a coil

face area increase which may or may not also have included a depth increase (models 2 and 4).

Each model is described below, and a summary of the results is seen in Figure C.3.2.
C-20

Condenser Depth Increase

The additional row on the condenser coil of this unit made the coil depth four rows, and

the corresponding PAIR ∆ increase by 0.0484 inches of water. In addition to the coil

modification, DOE changed the condenser fans to produce a higher airflow over the coil. The

Department increased the fan blade diameter to 28" (26", baseline), and applied the new P ∆ AIR

to the larger fan curve to find the appropriate airflow and fan power values. The Department

determined the new airflow to be 11800 cfm (10200 cfm, baseline), and the new fan power to be

1215 W (1070 W, baseline). These changes resulted in an increase in overall unit capacity, but

this was maintained by scaling back the total compressor capacity by 3.4 percent to 168.0

kBtu/h. As with the earlier models, the design constraints that limited this option were the

minimum condensing temperature and the minimum condenser exit temperature. Both of these

limits were maintained, as the condensing temperature was 121.3/F, and the exit temperature

102.4/F. The result was a unit with a 10.47 EER, almost 0.8 EER higher than the baseline.

Condenser Area Increase

The Department increased the condenser coil face area by 35 percent to 36.6 ft2

, and

added one row to make a four-row coil. The resulting PAIR ∆ was slightly lower than the

baseline, and was again used to find airflow and fan power required by the 28" diameter

condenser fans that were used in place of the 26" fans in the baseline unit. The effect of these

changes was an airflow value of 12300 cfm (10200 cfm, baseline), and a fan power of 1400 W

(1070 W, baseline). The Department changed the compressors in this unit to maintain overall

capacity, and scaled them back by 3.8 percent to 167.4 kBtu/h. The condensing temperature was

118.2/F, and the exit temperature was 102.2/F. The result was a unit with a 10.71 EER, over 1.0

EER higher than the baseline.

Evaporator Depth Increase

The Department increased the evaporator by one row in depth, making the coil three rows

deep. In addition to the depth increase, DOE changed the evaporator blower to an 18" blower,

which nearly cut the blower power in half. The PAIR ∆ that corresponded to the coil change

increased by 0.0268 inches of water when the airflow was set at 5100 cfm (5300 cfm, baseline),

and the evaporator blower power was 830 W. The Department scaled back the compressors by 9

percent to account for the additional unit capacity that was caused by the enhanced evaporator, to

a total value of 158.3 kBtu/h. The constraints that pertained to this modification were the

maximum allowable evaporator temperature, and the maximum SHR. This unit was easily within

the allowable levels, since the evaporating temperature was 50.0/F and the SHR was 0.717. The

result was a unit with a 11.11 EER, over 1.4 EER higher than the baseline.

Evaporator Area Increase

The Department employed a 50 percent increase in evaporator face area for this design

option, resulting in a 26.25 ft2

 coil with a PAIR ∆ that was 0.0256 inches of water lower than the
C-21

Product

Specs

Approach Baseline Physical Characteristics Performance Characteristics

Tc = 126.7 F

Te = 45.5 F

Subcool = 15.1 F

Superheat = 10 F

Compressor: 15,312 W

Fan: 1070 W/10,200 CFM

Blower: 1560 W /

 5300 CFM

SHR = 0.694

9.7-EER

Condenser Area

+35% Area, +1 Row

Tc =118.2 F

Te =45.3 F

Subcool =13.2 F

Superheat =10 F

Compressor:13,296 W

Fan:1400 W/12,300 CFM

Blower: 1560 W /

 5300 CFM

SHR =0.694

+1.01 -EER

box:

+46% volume

condenser:

 +35% area, +1 row 

condenser fans:

+2” diameter

compressor:

2 x 84.0 kBtu/hr

Condenser Depth

+1 Row (4 total)

Evaporator Area +50% AreaEvaporator Depth

+1 Row (3 total)

Tc =121.3 F

Te =45.3 F

Subcool =16.3 F

Superheat =10 F

Compressor:13,857 W

Fan: 1215 W/11,800 CFM

Blower: 1560 W /

 5300 CFM

SHR =0.694

+0.77 -EER

condenser:

+1 row 

condenser fans:

+2” diameter

compressor:

2 x 84.0 kBtu/hr

Tc =125.9 F

Te =49.4 F

Subcool =14.4 F

Superheat =10 F

Compressor:13,964 W

Fan: 1070 W/10,200 CFM

Blower: 790 W/5300CFM

SHR =0.711

+1.35 -EER

box:

 +46% volume

evaporator:

+50% area

compressor:

2 x 87.5 kBtu/hr

Tc =125.7 F

Te =50.0 F

Subcool =14.3 F

Superheat =10 F

Compressor:13,795 W

Fan: 1070 W/10,200 CFM

Blower: 830 W/5100CFM

SHR =0.717

+1.41 -EER

evaporator:

+1 row

compressor:

2 x 87.5 kBtu/hr

Figure C.3.2 15-ton Standard-Efficiency Baseline and Design Options

baseline. With the lower PAIR ∆ , DOE increased the evaporator blower to an 18" blower (15",

baseline) to maximize the efficiency gain. This change resulted in a blower power of 790 W,

slightly more than half the baseline value. The Department scaled back the compressors by 8

percent, to a capacity of 160.0 kBtu/h, to control the capacity. The evaporator conditions were

well within the limits, with an evaporating temperature of 49.4/F, and the SHR 0.711. The result

was a unit with an 11.05 EER, more than 1.3 EER higher than the baseline. Figure C.3.2 shows

the results.

C.3.5 15-ton High-Efficiency Unit

The high-efficiency 15-ton unit offered a couple of different options in order to get to a

12.0 EER, since it was already close to that efficiency level at the baseline rating of 11.5. The

Department explored three different possibilities, one falling just short of the desired efficiency

and the other two exceeding the target. The first design option was an increase in evaporator

depth, the second an increase in condenser depth, and the last an increase in condenser area.

Evaporator Depth Increase

Since the baseline model for this unit already had an evaporator temperature of 51.5/F,

there was very little room for evaporator modification while keeping within the design

constraints. For this reason, DOE modeled a one-row evaporator depth increase to determine

whether it was a feasible design choice to modify the evaporator at all. The one-row increase

brought the coil to four rows in all, and caused an increase of 0.0117 inches of water in PAIR ∆ .

Along with this change, DOE reduced the evaporator airflow to 5000 cfm (5300, baseline), and

scaled down the compressors by 2 percent to 176.4 kBtu/h. These changes did cause an increase

in evaporator temperature over the baseline, to 52.7/F, which is higher than the applicable limit.

However, the SHR was much lower than the maximum level with a value of 0.717. Since the
C-22

SHR was well within the limit, DOE accepted the high evaporator temperature. This choice of

allowing an elevated evaporator temperature was relatively inconsequential, however, since the

modified unit did not reach the target EER level. The result was a unit with an 11.81 EER, 0.3

EER higher than the baseline.

Condenser Depth Increase

This design option increased the condenser depth from three rows to four, and included

an increase in condenser fan diameter, as well as a slight change in compressors. The depth

increase caused a jump in PAIR ∆ , which DOE calculated at the higher airflow that accompanied

the 28" fans (26", baseline). The Department calculated the PAIR ∆ to be 0.0315 inches of water

higher than the baseline, at an airflow of 12600 cfm (11000 cfm, baseline). Along with the

condenser changes, DOE scaled down the compressor capacity by 2 percent to 176.4 Btu/h in

order to maintain the overall unit capacity. After all of these changes were modeled, DOE

verified the condenser temperature limits: the condensing temperature was 119.8/F, and the exit

temperature was 102.7/F. The result was a unit with a 12.15 EER, over 0.6 EER higher than the

baseline.

Condenser Area Increase

This design approach involved finding the minimum condenser area increase that would

enable the unit to meet the 12.0 EER target. A 10 percent increase in coil face area, accompanied

by a 2" increase in condenser fan diameter and a 2 percent decrease in compressor capacity,

accomplished this goal. The face area increase resulted in a PAIR ∆ that was slightly higher than

the baseline, when considered with the increase in fan size, and made the total condenser face

area 39.05 ft2

. The Department used a total compressor capacity of 176.4 kBtu/h in the model,

with a condenser airflow value of 13800 cfm (11000 cfm, baseline). The Department checked the

condenser temperatures to verify unit performance, and met the design limits: the condensing

temperature was 120.3/F, and the exit temperature 103.1/F. The result was a unit with a 12.03

EER, over 0.5 EER higher than the baseline. Figure C.3.3. summarizes the results.
C-23

Product

Specs

Approach Baseline Physical Characteristics Performance Characteristics

Tc = 125.37 F

Te = 51.5 F

Subcool = 16.4 F

Superheat = 10 F

Compressor: 13,418 W

Fan: 1070 W

Blower: 1240 W

SHR = 0.722

11.5-EER

Tc =120.3 F

Te =51.5 F

Subcool =14.0 F

Superheat =10 F

Compressor:12,344 W

Fan:1450 W/+2,800 CFM

Blower:1240 W

SHR =0.723

+0.5 -EER

condenser:

 +10% area 

condenser fans:

+2” diameter

compressor:

1 x 58.5 kBtu/hr

1 x 115.0 kBtu/hr

Tc =119.8 F

Te =51.4 F

Subcool =14.3 F

Superheat =10 F

Compressor:12,283 W

Fan: 1365 W/+1,600 CFM

Blower: 1240 W

SHR =0.723

+0.7 -EER

condenser:

 +1 row 

condenser fans:

+2” diameter

compressor:

1 x 58.5 kBtu/hr

1 x 115.0 kBtu/hr

Tc =125.0 F

Te =52.7 F

Subcool =16.2 F

Superheat =10 F

Compressor:13,115 W

Fan: 1070 W

Blower:1105 W/-300CFM

SHR =0.719

+0.3 -EER

evaporator:

+1 row

compressor:

1 x 58.5 kBtu/hr

1 x 117.0 kBtu/hr

Condenser Area +10% AreaCondenser Depth +1 RowEvaporator Depth +1 Row

Figure C.3.3 15-ton High-Efficiency Baseline and Design Options

C.4 R-410A ANALYSIS

C.4.1 7.5-ton R-410A Unit

The Department modeled the 7.5-ton R-410A unit to match the baseline performance of

the 7.5-ton standard-efficiency unit (see A.2.2), since there are no commercially-available units

that could be used as they were in the R-22 analysis. The Department constructed a baseline

model for the 410A unit, using physical characteristics that were common in R-22 units of the

same size. These characteristics included details about the condenser and evaporator coils, fans

and blowers, refrigerant lines, and unit configurations. The only components that DOE used for

the baseline models that were not based on the R-22 units were the compressors. The

Department selected these from Copeland’s line of R-410A compressors, which are currently

commercially available. Once it had established valid baseline models, DOE completed the

design option modeling following the same methodology as it used in the R-22 analysis.

7.5-ton R-410A Baseline Model

The evaporator coil was 8.9 ft2

 and 3 rows deep, with a 15" evaporator blower. The

condenser coil was 16.5 ft2

 and 2 rows deep, with two 22" condenser fans powered by 0.25 hp

motors. Refrigerant line sizes were identical to those of the R22 unit, and the flow was controlled

with a short tube orifice. The short tube orifice is not typically used with R-410A units, based on

available residential units that range in size up to five tons, but instead a thermal expansion valve

is employed. For the modeling process, the short tube orifice can simulate a thermostatic

expansion valve (TXV) that is held at a constant position during full loading. The last component

to be determined for the baseline model was the compressors. The Department chose Copeland
C-24

Scroll compressors that had a capacity of 41.2 kBtu/h and an EER of 10.3. These compressors

were very similar in capacity to the compressors used in the 7.5-ton R-22 unit, and they fulfilled

the 90,000 Btu/h capacity requirement for the unit. The results of the baseline nearly replicated

those of the R-22 unit, as the capacity was 90,350 Btu/h (91,040 Btu/h, R-22) and the EER 10.32

(10.33, R-22). 

Design Option Procedure

Reaching the 12.0 EER target efficiency for this unit was not as simple as with the R-22

unit, primarily because the baseline EER was 10.1 rather than 11.0. In order to make up the large

gap between baseline and target, DOE implemented a three-step process. The first of these steps

was enhancing the condenser coil, taking advantage of the lower allowable condensing

temperature in the process. From this unit, DOE added an increase in evaporator depth. The final

step was an option that was used only for the 410A design options, which involved the use of

more efficient fan and blower motors.

The enhancement of the condenser coil involved increasing the coil face area to 20 ft2

,

and adding one row in depth. The resulting PAIR ∆ was nearly equal to that of the baseline

model, since the effects of the area increase and added depth nearly canceled each other out, and

the condenser airflow was 6300 cfm (6315 cfm, baseline) and the fan power 610 W (620 W,

baseline). The condensing temperature for this modified unit was 116.1/F, at the threshold of the

allowable limit, and the condenser exit temperature was 108.2/F. The result was a unit with an

11.21 EER, almost 0.9 EER higher than the baseline.

The next model built off the results of the previous step, and involved adding one row to

the evaporator coil to make a four-row coil. The effect of this change was a 50 W increase in

evaporator blower power, due to the higher PAIR ∆ , and increased capacity. The Department

subsequently scaled down the compressors by 7 percent from the baseline, to 38.3 kBtu/h, and

maintained the capacity. The evaporating temperature increased to 50.8/F, and the SHR to 0.743

due to the changes. The result was a unit with an 11.76 EER, more than 1.4 EER higher than the

baseline.

The final step in this design option process was to introduce electronically commutated

motors (ECM) for both the condenser fans and evaporator blower. These motors are more

efficient than the ones that are typically used, and provided a gain of 12 percent in condenser fan

motor efficiency (82 percent efficiency versus 70 percent) and 5 percent in evaporator blower

motor efficiency (90 percent versus 85 percent). This change decreased the power required for

these motors, lessened the waste heat that they added to their respective airstreams, and

increased unit EER. As this alone was not quite enough to reach the 12.0 EER target, DOE made

a small increase to the face area of the evaporator coil. The Department increased the coil to 10.0

ft2 (8.9 ft2, baseline), which decreased the blower power to 870 W (910 W, baseline). The

condenser fan power was 520 W (620 W, baseline). These changes in turn required another

reduction in compressor capacity to maintain system capacity, and DOE scaled down the

compressors from the baseline size by 9 percent, to 37.5 kBtu/h. The result was a unit with a
C-25

Product

Specs

Approach Physical Characteristics Performance Characteristics

Tc = 116.1 F

Te = 48.2 F

Subcool = 5.8 F

Superheat = 20 F

Compressor: 6580 W

Fan: 610 W/ -15 CFM

Blower: 910W

SHR = 0.727

+0.89 -EER

condenser:

+21%area/+1 row

compressor:

2 x 41.2 kBtu/hr

Tc = 116.6 F

Te = 50.8 F

Subcool = 10.6 F

Superheat = 20 F

Compressor: 6149 W

Fan: 610 W / -15 CFM

Blower:960W

SHR = 0.743

+1.44 -EER

condenser:

+21% area/+1 row

evaporator:

+1 row

compressor:

2 x 41.2 kBtu/hr

Condenser Only 

20 ft2 Area, +1 Row

Cond. & Evap.

Prev. step, +1 Row evap

Tc = 116.3 F

Te = 51.6 F

Subcool = 10.2 F

Superheat = 20 F

Compressor: 5989 W

Fan: 520 W / -15 CFM

Blower:870W

SHR = 0.748

+1.91 -EER

condenser:

+21% area/+1 row

evaporator:

+1 row

condenser fans:

ECM motors

evaporator blower:

ECM motor

compressor:

2 x 41.2 kBtu/hr

Cond-Evap-ECM

Prev. step+ECM +12%Ev.

Tc = 122.8 F

Te = 48.3 F

Subcool = 12.9 F

Superheat = 20 F

Compressor: 7222 W

Fan: 620 W

Blower: 910W

SHR = 0.728

10.10-EER

Baseline

Figure C.4.1 7.5-ton R-410a Baseline and Design Options

12.24 EER, nearly 2.0 EER higher than the baseline. Figure C.4.1. summarizes the results

described above.

C.4.2 15-ton R-410A Unit

As with the 7.5-ton R-410A unit, DOE based this model on typical unit details for R-22

units of the same capacity. The Department chose the compressors from Copeland’s line of

R-410A compressors and, in almost every case, used two equal-size compressors in the unit to

attain the 180,000 Btu/h capacity. The Department modeled the baseline unit to match the

performance of the 15-ton standard-efficiency unit, and employed design options until the unit

neared the 12.0 EER level.

15-ton R-410A Baseline Model

The evaporator coil was 17.5 ft2

 and 4 rows deep, with an 18" evaporator blower. The

condenser coil was 28 ft2

 and 3 rows deep, with two 26" condenser fans powered by 0.50 hp

motors. Refrigerant-line sizes were identical to those of the R-22 unit, and the flow was

controlled with a short tube orifice, which simulated a TXV held at a constant position during

full loading. The compressors chosen have a capacity of 57.5 kBtu/h and an EER of 10.6, and

DOE used three of these to fulfill the 180,000 Btu/h capacity requirement for the unit.

The results of the baseline nearly replicated those of the R-22 unit, although the capacity

more closely mirrored that of the high efficiency R-22 unit. The final baseline values were

182,640 Btu/h (174,090 Btu/h, R-22) and the EER 9.72 (9.70, R-22).
C-26

Product

Specs

Approach Physical Characteristics Performance Characteristics

Tc = 118.2 F

Te = 51.1 F

Subcool = 14.1 F

Superheat = 10 F

Compressor: 12660 W

Fan: 1600 W/ +2,500CFM

Blower:1240W

SHR = 0.724

+1.91 -EER

box:

+47% volume

condenser:

+27% area/+1 row

compressor:

1 x 103.0 kBtu/hr

1 x 55.0 kBtu/hr

Tc = 116.1 F

Te = 51.1 F

Subcool = 11.7 F

Superheat = 10 F

Compressor: 12098 W

Fan: 1545 W/ +2,900CFM

Blower:1240W

SHR = 0.724

+2.41 -EER

box:

+47% volume

condenser:

+57% area, +1 row

compressor:

3 x 50.5 kBtu/hr

Condenser Only 

27% Area, +1 Row

Condenser Max

57% Area, 4 row

Tc = 116.1 F

Te = 51.1 F

Subcool = 11.7 F

Superheat = 10 F

Compressor: 12098 W

Fan: 1320 W/+2,900 CFM

Blower:1170W

SHR = 0.724

+2.67 -EER

box:

+47% volume

condenser:

+57% area, +1 row

condenser fans:

ECM motors

evaporator blower:

ECM motor

compressor:

3 x 50.5 kBtu/hr

Max + ECM

57% Area, 4 row

Tc = 126.3 F

Te = 51.0 F

Subcool = 18.2 F

Superheat = 10 F

Compressor: 15890 W

Fan: 1660 W

Blower:1240W

SHR = 0.721

9.71-EER

Baseline

Figure C.4.2 15-ton R-410a Baseline and Design Options

Design Option Procedure

Similar to the 7.5-ton unit, the design options for this unit had to cover a large jump in

EER, since DOE modeled the baseline to coincide with a standard efficiency level. However,

rather than using a three-step process as it had for the 7.5-ton unit, DOE achieved the 12.0 EER

with a major change to only the condenser coil. This was possible since the baseline model had a

very high condensing temperature, 126.3/F, which allowed for significant condenser change

before the minimum condensing temperature of 116.0/F was reached.

The Department increased the condenser face area to 44 ft2

 (28ft2, baseline), and added

one row to make it four rows deep. These changes resulted in a slightly lower PAIR ∆ , which

yielded a condenser airflow of 13600 cfm (12800 cfm, baseline) and a condenser fan power of

1545 W (1660 W, baseline). The enhanced condenser allowed for a reduction in compressor

capacity of 11 percent, to 153.0 kBtu/h, and came very close to the temperature limits:

condensing temperature was 116.1/F, and condenser exit temperature was 102.2/F. The result

was a unit with a 12.12 EER, 2.4 EER higher than the baseline. Figure C.4.2. shows the results

described above.