ASHRAE Journal - April 2010 - 36

Smaller Unit Cooling Coil Face Velocity Cooling Coil Pressure Drop Wheel Pressure Drop Wheel Total Effectiveness (Cooling) Wheel Capacity (Cooling) Supply Fan Brake Horsepower Exhaust Fan Brake Horsepower 508 fpm 0.96 in. H2O 1.1 in. H2O 71% 180 MBh 4.8 hp 3.8 hp

Larger Unit 401 fpm
Dehumidification Capacity, lb/h

140 120 100 80 60 40 20 0 45% 114.4 118.5

123.0 115.6 108.2 111.1 103.7 96.2 88.8 107.0 100.8 93.3

0.60 in. H2O 0.79 in. H2O 76% 190 MBh 3.3 hp 2.5 hp

99.6

92.2

84.7

50%

55%

60%

2% 65%

Table 5: Energy-related impacts of equipment sizing.

Space Relative Humidity (Upper Limit)

engineers consider 60% RH an acceptable upper limit, while others choose 50%, 55%, 65%, or some other limit. 2. Determine the design latent load for each zone (qLspace). This is the latent load that occurs within the boundaries of the zone. It typically consists of moisture generated by people or other sources within the zone, as well as infiltration or diffusion of humid air from outdoors or from adjacent zones.9 These loads are often determined with the help of load calculation software. 3. Calculate the required humidity ratio of the conditioned air (Wca) delivered by the dedicated OA unit. If the dedicated OA system is being designed to offset the entire indoor latent load, in addition to the ventilation latent load, then the conditioned outdoor air must be dry enough to offset the latent load in each zone, such that the humidity ratio in every zone is maintained at or below the desired upper limit. Equation 4 is used to determine the required Wca for each zone. Then, the humidity ratio of the conditioned outdoor air delivered by the unit must be the lowest (or worst-case) of all the zones it serves. Wca = Wsp – [qLspace / (0.69 × Voz)] (4)

Figure 4: Impact of Woa and Wsp on required dehumidification capacity (qL), without total-energy recovery (Jacksonville, Fla.)
Dehumidification Capacity, lb/h 80 60 49.9 40 20 0 45% 50% 55% 48.7 46.5 47.7 51.3

49.1

46.8 45.5

44.6 43.2

42.4 41.0

44.3 42.0
AS H DP RA Co E D nd esi iti gn on

39.8

60%

Space Relative Humidity (Upper Limit)

65%

Figure 5: Impact of Woa and Wsp on required dehumidification capacity (qL), with total-energy recovery (Jacksonville, Fla.).

where Wca= required humidity ratio of the conditioned air leaving the unit, grains/lb Wsp = desired upper limit for humidity ratio of the occupied space, grains/lb qLspace = design latent load in the zone, Btu/h Voz = design zone outdoor airflow, cfm To demonstrate this process, consider an example elementary school located in Jacksonville, Fla. A dedicated outdoor air unit is being designed to deliver air directly to four classrooms, removing the entire ventilation latent load and the entire indoor latent load.
36	 ASHRAE	Journal	

1. For this example, the zone dry-bulb temperature setpoint during the cooling season will be 74°F (23°C) and the acceptable upper limit for indoor humidity is 60% RH. This equates to an indoor humidity ratio (Wsp) of 75.2 grains/lb (10.8 g/kg). 2. Table 3 (see Page 35) lists the design latent loads (qLspace) and required outdoor airflows (Voz) for each zone. 3. Equation 4 is used to calculate the required conditioned-air humidity ratio (W ca) for each zone. For example, the required humidity ratio of the conditioned outdoor air delivered to Classroom 102 is 57.6 grains/ lb (8.24 g/kg). Wca= 75.2 grains/lb – [5465 Btu/h / (0.69 × 450 cfm)] = 57.6 grains/lb As the results in Table 3 show, Classroom 102 requires the driest air. Therefore, for this example, the dedicated outdoor air unit must be sized to dehumidify the outdoor air to 57.6 grains/lb (8.24 g/kg), which equates to a 52°F (11°C) dew
ashrae.org	 	 April	 2010

AS H DP RA Co E D nd esi iti gn on

0.4%

1%

0.4% 1% 2%



ASHRAE Journal - April 2010

Table of Contents for the Digital Edition of ASHRAE Journal - April 2010

ASHRAE Journal - April 2010
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
The Science of Evaporation is Key to Defense in Murder Trial
Selecting DOAS Equipment with Reserve Capacity
Technology Award Case Studies: Greening Hospitals
Technology Award Case Studies: Sustainable Remedy for Hospital
Building Sciences
Emerging Technologies
Technical Topics: Selecting Efficient Fans
Technical Topics: Dual-Capacity Heat Pumps
IAQ Applications
International Column
Classified Advertising
Advertisers Index
ASHRAE Journal - April 2010 - Intro
ASHRAE Journal - April 2010 - ASHRAE Journal - April 2010
ASHRAE Journal - April 2010 - Cover2
ASHRAE Journal - April 2010 - 1
ASHRAE Journal - April 2010 - 2
ASHRAE Journal - April 2010 - Contents
ASHRAE Journal - April 2010 - Commentary
ASHRAE Journal - April 2010 - 5
ASHRAE Journal - April 2010 - Industry News
ASHRAE Journal - April 2010 - 7
ASHRAE Journal - April 2010 - 8
ASHRAE Journal - April 2010 - 9
ASHRAE Journal - April 2010 - 10
ASHRAE Journal - April 2010 - 11
ASHRAE Journal - April 2010 - 12
ASHRAE Journal - April 2010 - 13
ASHRAE Journal - April 2010 - 14
ASHRAE Journal - April 2010 - 15
ASHRAE Journal - April 2010 - 16
ASHRAE Journal - April 2010 - 17
ASHRAE Journal - April 2010 - Letters
ASHRAE Journal - April 2010 - 19
ASHRAE Journal - April 2010 - Meetings and Shows
ASHRAE Journal - April 2010 - 21
ASHRAE Journal - April 2010 - The Science of Evaporation is Key to Defense in Murder Trial
ASHRAE Journal - April 2010 - 23
ASHRAE Journal - April 2010 - 24
ASHRAE Journal - April 2010 - 25
ASHRAE Journal - April 2010 - 26
ASHRAE Journal - April 2010 - 27
ASHRAE Journal - April 2010 - 28
ASHRAE Journal - April 2010 - 29
ASHRAE Journal - April 2010 - Selecting DOAS Equipment with Reserve Capacity
ASHRAE Journal - April 2010 - 31
ASHRAE Journal - April 2010 - 32
ASHRAE Journal - April 2010 - BRC1
ASHRAE Journal - April 2010 - BRC2
ASHRAE Journal - April 2010 - 33
ASHRAE Journal - April 2010 - 34
ASHRAE Journal - April 2010 - 35
ASHRAE Journal - April 2010 - 36
ASHRAE Journal - April 2010 - 37
ASHRAE Journal - April 2010 - 38
ASHRAE Journal - April 2010 - 39
ASHRAE Journal - April 2010 - 40
ASHRAE Journal - April 2010 - 41
ASHRAE Journal - April 2010 - Technology Award Case Studies: Greening Hospitals
ASHRAE Journal - April 2010 - 43
ASHRAE Journal - April 2010 - 44
ASHRAE Journal - April 2010 - 45
ASHRAE Journal - April 2010 - 46
ASHRAE Journal - April 2010 - 47
ASHRAE Journal - April 2010 - 48
ASHRAE Journal - April 2010 - 49
ASHRAE Journal - April 2010 - Technology Award Case Studies: Sustainable Remedy for Hospital
ASHRAE Journal - April 2010 - 51
ASHRAE Journal - April 2010 - 52
ASHRAE Journal - April 2010 - 53
ASHRAE Journal - April 2010 - Building Sciences
ASHRAE Journal - April 2010 - 55
ASHRAE Journal - April 2010 - 56
ASHRAE Journal - April 2010 - AP1
ASHRAE Journal - April 2010 - AP2
ASHRAE Journal - April 2010 - AP3
ASHRAE Journal - April 2010 - AP4
ASHRAE Journal - April 2010 - 57
ASHRAE Journal - April 2010 - 58
ASHRAE Journal - April 2010 - 59
ASHRAE Journal - April 2010 - Emerging Technologies
ASHRAE Journal - April 2010 - 61
ASHRAE Journal - April 2010 - 62
ASHRAE Journal - April 2010 - 63
ASHRAE Journal - April 2010 - Technical Topics: Selecting Efficient Fans
ASHRAE Journal - April 2010 - 65
ASHRAE Journal - April 2010 - Technical Topics: Dual-Capacity Heat Pumps
ASHRAE Journal - April 2010 - 67
ASHRAE Journal - April 2010 - 68
ASHRAE Journal - April 2010 - 69
ASHRAE Journal - April 2010 - IAQ Applications
ASHRAE Journal - April 2010 - 71
ASHRAE Journal - April 2010 - 72
ASHRAE Journal - April 2010 - 73
ASHRAE Journal - April 2010 - International Column
ASHRAE Journal - April 2010 - 75
ASHRAE Journal - April 2010 - 76
ASHRAE Journal - April 2010 - 77
ASHRAE Journal - April 2010 - 78
ASHRAE Journal - April 2010 - 79
ASHRAE Journal - April 2010 - 80
ASHRAE Journal - April 2010 - 81
ASHRAE Journal - April 2010 - 82
ASHRAE Journal - April 2010 - 83
ASHRAE Journal - April 2010 - 84
ASHRAE Journal - April 2010 - 85
ASHRAE Journal - April 2010 - Classified Advertising
ASHRAE Journal - April 2010 - 87
ASHRAE Journal - April 2010 - Advertisers Index
ASHRAE Journal - April 2010 - Cover3
ASHRAE Journal - April 2010 - Cover4
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