HVAC System Design

 

Choosing an HVAC System:

This is the first step when designing an HVAC system for a building.  One has to look at the building size, location, and type to determine which system will be the most efficient and have either the lower installation or operating costs.  The decision on which HVAC system to use, is one of the biggest assumptions one can make.  Once the system is chosen, it can be optimized for a particular application, but if the wrong system is chosen from the beginning, the work you do to optimize that system is useless.

 

Available Systems:

Using the Energy-10 software package, we are given a total of six different types of HVAC systems that have the ability to both heat and cool a space.  Of these six systems, there are only two unique types of systems.  These include packaged direct-expansion stand-alone units and packaged terminal air conditioner through-the-wall unit.  Due to our office application, we decided that a central packaged unit would be better suited than the through the wall units that would interrupt both the exterior façade and may create noise in the workspace.  This is our first large assumption and will largely influence our loads calculations and annual costs.  Once we chose the type of system, we had to decide between the electric resistance furnace for heating and the gas furnace.  Since the cooling cycle for both of the systems is the same, the decision between the two is due to the heating system.  Although the energy usage is less with the electrical resistance heating, the annual cost of the system is much larger than the cost of the gas furnace due to the excessive cost of electricity. 

System Definitions

 

Exterior Design Conditions:

 

Madison, Wisconsin

 

WINTER

SUMMER

Lat. Deg.

 

Degree Days

 

99%

97.5%

2.5% Coinc.

Daily DB

2.5%

DB

DB

DB

WB

Range

WB

43

7640

-8

-4

87

73

21

74

 

 

HVAC Analysis:

 

Glazing:

The analysis of the HVAC system must first begin with the architecture of the building.  Since the HVAC system is usually designed after the layout of the floor and the façade had been determined, some of the initial assumptions for the systems are no longer assumptions, but rather they are given.  One instance of this is the window sizes and locations.  While designing the HVAC system it may be practical for us to orient the windows differently to save on loads and utility costs, the window placement is typical decided upon by the architect in his architectural program.   In our case, the large percentage of windows in relationship to the wall led us to try and decrease the heat loss through the windows in the winter and decrease the heat gain from the windows in the summer.  The best method of doing this is by changing the glazing of the windows to increase the R-values of the windows.  This increase in the R-value decreases the conductance of the windows since conductance is the reciprocal of thermal resistance.  By decreasing the conductance of the glass from U=0.49 to U=0.12, we were able to reduce the annual energy use by 904,389 kBtu and the annual cost by $5,421.

 

GLAZING

 

Double; U=0.49

Quad Low e 88; U=0.12

SAVINGS

4,060,932 kBtu

3,156,543 kBtu

904,389 kBtu

$44,740

$39,319

$5,421

 

Control Setback:

The next method of decrease the annual energy use and utility costs was to alter the setback and setup conditions.  These conditions allow the interior temperature of the building to increase or decrease to an acceptable range while the building is unoccupied.  One disadvantage to such a control system is that if someone comes into the building while the temperature is fluctuating in the given range, he or she may be uncomfortable.  The second disadvantage is that the system requires a certain lag time to be able to recondition the space to its desired interior design temperature.  This lag time must be closely monitored since it may vary with varying exterior conditions.  By allowing the interior temperature of the building to fluctuate plus or minus 8 degrees Fahrenheit, we were able to reduce the annual energy use by 193,949 kBtu and the annual cost by $1,234.

 

CONTROL SETBACK

 

None

72-64 (heat); 76-84 (cool)

SAVINGS

3,156,543 kBtu

2,962,594 kBtu

193,949 kBtu

$39,319

$38,085

$1,234

 

Duct Leakage:

When changing the control setback conditions, we noticed that Energy-10 was allowing for a 5% duct leakage to the exterior.  We are unsure of the reasoning behind this value, but since all of our ducts are running on the interior of the building, we decided this value must be zero since any leakage from the ducts would only be on the interior of our structure.  This led to a minimal savings of 113,196 kBtu in annual energy use and reduced the annual energy cost by $540.

 

DUCT LEAKAGE (Outdoor)

 

5%

0%

SAVINGS

2,962,594 kBtu

2,849,398 kBtu

113,196 kBtu

$38,085

$37,545

$540

 

 

High Efficiency HVAC:

The next method we chose to reduce the overall energy use was to increase the efficiency of the HVAC equipment.  Since most of our energy is used in the heating season, we decided to change the heating efficiency.  Changing the efficiency of the equipment from 80% to 100% allowed us to decrease the annual energy usage by 903,389 kBtu and decrease the annual cost by $1,662.

 

HIGH EFFICIENCY HVAC (Heating)

 

80% Efficient

100% Efficient

SAVINGS

2,849,398 kBtu

2,528,283 kBtu

321,115 kBtu

$37,545

$35,883

$1,662

 

 

High Efficiency Lighting:

The next step in our analysis was to change the lighting in the building to high efficiency lighting.  This reduces the Watts per square foot for the lighting loads and then our total annual cost.  By increasing the efficiency of the lights from 1.78 W/ft2 to 1.33 W/ft2, we decreased the annual energy use 114,476 kBtu and decreased the annual energy cost $2,741.

 

HIGH EFFICIENCY LIGHTING

 

IL 1.78/EL 0.33 W/ft2

IL 1.33/EL 0.25 W/ft2

SAVINGS

2,528,283 kBtu

2,413,807 kBtu

114,476 kBtu

$35,883

$33,142

$2,741

 

High Efficiency HVAC (Cooling):

The final step in our analysis was to increase the efficiency of our cooling cycle.  Although the cooling cycle is smaller than the heating cycle in our case, we felt that the savings would show in the reduction in annual cost since the cost of electricity is quite excessive.  By increasing the EER from 8.9 to 13.0, we decreased the annual energy use 115,837 kBtu and reduced the annual cost $2,818.

 

HIGH EFFICIENCY HVAC (Cooling)

 

EER = 8.9

EER = 13.0

SAVINGS

2,413,807 kBtu

2,297,967 kBtu

115,837 kBtu

$33,142

$30,324

$2,818

 

Results:

 

            Building Properties:

These building properties were determined using Energy-10.  They were decided upon based on optimizing the HVAC system.

 

Floor Area

30,000 ft2

Floor to Floor Height

15 ft

Wall Construction Type

Steelstud 6 Poly

Wall Thermal Resistance

23.0

Roof Construction

Flat; R=19.0

Floor Construction

Slab on Grade

Window Construction Type

4060 double, low e, U=0.19

Glazing Conductance

quad low-e 88, U=0.12

Interior Design Temp (Cool)

76.0 °F

Setup

84.0 °F

Interior Design Temp. (Heat)

72.0 °F

Setback

64.0 °F

 

 

 

            HVAC System:

These are the final values derived from Energy-10 by optimizing the HVAC system.

 

 

Total

Unit Values

Cooling Load (kBtu)

753,000 kBtu

25.1 kBtu/ft2

Cooling Load (ton)

62.75 ton

478 ft2/ton

Heating Load (kBtu)

183,000 kBtu

6.1 kBtu/ft2

Heating Load (ton)

15.25 ton

1,967 ft2/ton

Airflow (cfm)

29,145 cfm

1.03 ft2/cfm

Annual Energy Use

2,297,967 kBtuh

76.60 kBtuh/ft2

Annual Cost

$ 30,324

$ 1.01 /ft2

Peak Electric (kW)

152.4 kW

5.08 W/ft2

Total Electric (kWh)

436,383 kWh

14.55 kWh/ft2

 

            Duct Layout:

The duct layout of our building consists of dual central vertical main shafts to each floor with branch ducts running horizontally.  The ducts are then connected to diffusers in the space using flexible duct connections.  The return air is handled with return grills in the acoustical ceiling tiles and the ceiling plenum is then used as the horizontal return shaft to the central chase.  The air is then enters the central chase with grills and is then ducted vertically in uninsulated hard duct.  Exhaust air will be handled by venting through the roof of the cafeteria to contain food smells, and through the rooftop.  Ventilation in the bathrooms will be handled by individual fan and duct systems that will exhaust one hundred percent of that air.

 

 

 

Duct Design:

 

DUCT SIZE

Velocity

Airflow

Airflow per Zone

Area of Duct

Duct Diameter

1

1.5

2

2.5

3

3.5

4

fpm

cfm

cfm

ft2

ft

ft

ft

ft

ft

ft

ft

ft

800

29145

14572.5

18.22

4.82

18.22

12.14

9.11

7.29

6.07

5.20

4.55

900

29145

14572.5

16.19

4.54

16.19

10.79

8.10

6.48

5.40

4.63

4.05

1000

29145

14572.5

14.57

4.31

14.57

9.72

7.29

5.83

4.86

4.16

3.64

1100

29145

14572.5

13.25

4.11

13.25

8.83

6.62

5.30

4.42

3.79

3.31

1200

29145

14572.5

12.14

3.93

12.14

8.10

6.07

4.86

4.05

3.47

3.04

1300

29145

14572.5

11.21

3.78

11.21

7.47

5.60

4.48

3.74

3.20

2.80

1400

29145

14572.5

10.41

3.64

10.41

6.94

5.20

4.16

3.47

2.97

2.60

1500

29145

14572.5

9.72

3.52

9.72

6.48

4.86

3.89

3.24

2.78

2.43

1600

29145

14572.5

9.11

3.41

9.11

6.07

4.55

3.64

3.04

2.60

2.28

1700

29145

14572.5

8.57

3.30

8.57

5.71

4.29

3.43

2.86

2.45

2.14

1800

29145

14572.5

8.10

3.21

8.10

5.40

4.05

3.24

2.70

2.31

2.02

1900

29145

14572.5

7.67

3.12

7.67

5.11

3.83

3.07

2.56

2.19

1.92

2000

29145

14572.5

7.29

3.05

7.29

4.86

3.64

2.91

2.43

2.08

1.82

 

Duct sizing can be done on an estimate level by using the simple relationship of Q=VA where Q is airflow in cubic feet per minute, V is velocity in feet per minute, and A is area in square feet.  Since we know the airflow of the building from our load calculations, we can determine the velocity range and size the duct accordingly.  From Air Conditioning Principles and Systems by Edward G. Pita, the maximum allowed duct velocity for Office Buildings 1100-1600 fpm.  Since we would like to minimize the size of our duct, we decided to work in the upper range of this maximum allowable velocity.  By using a simple Excel spreadsheet, we determined that at 1400 feet per minute, we could use a 3’ x 3.47’ duct.  For simplicity, we decided to use a 3’ x 4’ duct.  This leads to an area of 12 ft2 and with an airflow of 14572.5 cfm a velocity of 1215 fpm.

 

Sizing of the Roof-Top Units:

Commercial Self-Contained Unit, 20 through 80 Tons

Commercial Self Contained Unit Courtesy of The Trane Company


The actual sizing of the packaged rooftop unit is done based on the peak-cooling load derived from Energy-10.  The total cooling load for our entire building is 62.75 tons.  Since we are separating the building into two equal zones each supplied by their own unit, we can assume each unit would need to be at least 31.375 tons supplying 14572.5 cfm.  Choosing a unit from The Trane Company’s online catalog, I chose a packaged unit that supplies 15,200 cfm and is rated at 38 tons.  This is the closest model to the requirements I need and meets both the airflow and tonnage.

 

Comparison of Initial and Final Choices:

 

 

Program

Design

Difference

Total Btu Load (kBtu)

  219,540

  753,000

533460

Total Ton Load (ton)

      18.30

      62.75

             44.46  

Btu/ft2

        7.32

      25.10

             17.78

ton/ton

      1,640

         478

1162

Total CFM

    20,100

    29,145

9045

CFM/ft2

      0.670

      0.972

             0.302

 

It is apparent after our building design that our initial assumptions differed greatly from our calculated values. This is due to our limited exposure to actual values for these systems.  If we were more knowledgeable  in these areas, we may be able to estimate these values more accurately.  We feel that the spreadsheet we used to derive our building program could be enhanced to more accurately estimate the HVAC values.  We would also have to determine our building characteristics earlier to make that spreadsheet work to its full ability.

 

If more time was allocated for this project, there are more changes that could be made.  We could have designed a make-up heating or cooling system for this project.  We would probably suggest for a building this size a finned tube hydronic baseboard convector system around the perimeter windows of the buildings with a  cooling tower and gas boiler.  This would more effectively and efficiently deal with the solar loads and higher conductance levels of the windows.

 

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