HVAC Calculations
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    Calculations for the size of the HVAC system in Macalister hall will be completed through two methods. The first method will be based on cfm and tonnage estimates set forth in ASHRAE. The second method, which is more detailed, involves the use of the Carrier E-20 simulation program to calculate the loads.

ASHRAE Estimation Standards:

ASHRAE sets forth standards for the estimation of the cfm and tonnage in a building.  Using a 20 cfm/person standard and a reheat system, ASHRAE sets forth the numbers as follows:

Estimated Cooling Load (Tons): .25 to .35 tons per 100 square feet of total building area

Estimated Heating Load (MBH): 1.5 to 2.5 MBH per 100 square feet of total building area

Estimated CFM: 75 to 125 cfm per 100 square feet of total building area

Chilled Water GPM: 2.4 gpm per ton of cooling

Hot Water GPM: Heating MBH divided by 10

For our estimations, we will use the midpoints of these values to result in an answer that is neither too liberal nor too conservative.

ASHRAE Estimation Method for Macalister Hall:

The total square footage of conditioned space in Macalister Hall is as follows:

28,400 ft2 in the basement

24,400 ft2 in the first floor

13,500 ft2 on each tower floor

10,500 ft2 in the faculty club

Total Conditioned Area: 117,300 ft2

Based on the square footages calculated above and the ASHRAE standards outlined previously, the building loads are calculated in the following table:

Cooling Load

Heating Load

Total CFM

Chilled Water

Hot Water

350 Tons

2350 MBH

117300 cfm

840 gpm

235 gpm

 

Carrier E-20 Program

   The Carrier E-20 program is much more accurate than the previously mentioned estimation. Through this software program, the building loads are calculated while taking into account construction materials, directional facing, infiltration, schedules of occupancy, equipment loads, people loads, and other setpoints within the HVAC system. The data input to the program is outlined below.

Philadelphia Region Temperatures

Season

Dry Bulb (F)

Wet Bulb (F)

Daily Range (F)

Winter

10

N/A

N/A

Summer

93

75

14

 

Philadelphia Elevation Above Sea Level: 26 ft

Philadelphia Latitude Location: 40

Construction Materials Information:

The following sections show the two main forms of construction for Macalister Hall. The tower is composed of a 6" precast concrete panel on the outside, a large air space, and a interior course of 4" concrete block. The first floor has a 4" brick exterior, a 1" air gap, and 8" concrete block wall.

                                  1st Floor Wall Section                                    Tower Wall Section

 

From the above wall sections, I calculated the overall U value of the walls (Btu/hr/ft2/F) based on the materials used and the standards set forth in ASHRAE. The tabulated values are as follows:

1st Floor Construction:

Construction Material

R-Value (hr x ft2 x F/ Btu)

U-Value (Btu/hr/ft2/F)

Outside Air Resistance

0.33

3.03

4" Face Brick

0.43

2.33

1" Air Gap

0.91

1.10

8" CMU

2.02

0.50

Inside Air Resistance

0.69

1.45

Totals

4.38

8.41

 

Tower Construction:                                      

Construction Material

R-Value (hr x ft2 x F/ Btu)

U-Value (Btu/hr/ft2/F)

Outside Air Resistance

0.33

3.03

6" Precast Concrete Panel

3.22

0.31

6" Air Gap

0.91

1.10

4" CMU

1.11

0.90

Inside Air Resistance

0.69

1.45

Totals

6.26

6.79

 

Typical Window Construction:

Assumed an aluminum double paned window with a thermal break and light shades on the inside. These assumptions lead to the following values:

Overall U-Value: .537 (Btu/hr/ft2/F)
Shading Coefficient: .454

 

Typical Roof Construction:

Assumed a built-up roof on a 22-gage steel deck with R-7 board insulation. These assumptions lead to the following value:

Overall U-Value: .121 (Btu/hr/ft2/F)

 

Typical Lighting Loads: 1.5 Watts/ft2

 

Typical People Loads: 1 person/ 150 ft2 doing office work:

Sensible Load: 245 btuh
Latent Load: 205 btuh

 

Typical Infiltration Losses: 2 air changes/hour

 

Typical Equipment Loads: .5 Watts/ft2

 

Setpoints and Safety Factors:

72 F Cooling Setpoint
85 F Unoccupied Cooling Setpoint
70 F Heating Setpoint
60 F Unoccupied Heating Setpoint
52 F Discharge from the Preheat Coil
20 cfm/person ventilation
2" Fan Static
Building occupied from 7 am - 10 pm
10% factor of safety

 

Equations Used by E-20 to Calculate Loads:

1.    Heating Load: Q = U x A x T

Where: 

Q = Heat transfer rate, Btu/hr
U = Overall heat transfer coefficient, Btu/hr/ft2/F
A = Surface area through which heat flows, ft2
T = Temperature difference across which heat flows, F

The area of wall was calculated using a floor to floor height of 12'-0" in the tower and 15'-0" on the first floor.

2.    Cooling Load: Q = U x A x CLTDc

Where:

Q = Cooling load for roof, glass, or wall, Btu/hr
U = Overall heat transfer coefficient for roof, glass, or wall, Btu/hr/ft2/F
A = Area of roof, glass, or wall, ft2
CLTDc = Corrected cooling load temperature difference, F

The CLTDc is a modified value for the temperature difference that takes into account heat storage and time lag effects.

3.    Solar Radiation through glass: Q = SHGF x A x SC x CLF

Where:

Q = Solar radiation cooling load for glass, Btu/hr
SHGF = Maximum solar heat gain factor, Btu/hr-ft2
A = Area of glass, ft2
SC = Shading coefficient
CLF = Cooling load factor for glass

The SHGF is based on orientation and time of year and the SC is based on the type of drapery placed on the window.

4.    Lighting Load: Q = 3.4 x W x BF x CLF

Where:

3.4 = conversion coefficient between Watts and Btu/hr
Q = Cooling load from lighting, Btu/hr
W = Lighting capacity, Watts
BF = Ballast factor
CLF = Cooling load factor for lighting

The BF accounts for heat losses in the ballasts of fluorescent lights and the CLF accounts for heat storage in the lighting fixtures.

5.    People Loads: Qs = qs x n x CLF, Ql = ql x n

Where:

Qs & Ql = Sensible and latent heat gains, Btu/hr
qs & ql = Sensible and latent heat gains per person, Btu/hr-person
n = Number of people
CLF = Cooling load factor for people

Carrier E-20 Results:

From the above setpoints and equations, the information was entered into the Carrier E-20 program and the following results were given:

Cooling Load

Heating Load

Total CFM

Chilled Water

Hot Water

300 Tons

2100 MBH

90000 cfm

720 gpm

210 gpm