ASHRAE Journal - August 2011 - 78

iaq applications
required dew-point temperature (DPT) necessary to remove the space latent load, approximately 48°F (9°C) or lower for many occupancy densities, then to reheat the air continually to a neutral temperature eliminating the need for terminal reheat. But this is not necessary or useful. By avoiding central SA reheat, the system avoids energy waste. Another benefit is that the terminal equipment can be smaller and less expensive to purchase and operate. Space in this column is insufficient to discuss this important topic in detail. However, in general, when the DOAS is supplying spaces with highly variable occupancy, such as a classroom, the KISS principle would suggest supplying the air at a DBT equal to the required DPT, and modulate the SA flow based
Region ID A Enthalpy Wheel Control Regions, Kansas City, Kan., Data, 8,760 Hours
150 140 130 120 110 100 90 Regions A B C D E 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 Dry-Bulb temperature (°F) 70 80 90 0 100 W (grains/lbmDa)

Figure 2: Psychrometric regions related to enthalpy wheel control.
Enthalpy Wheel Control Action Hot and humid OA region, the EW should run full speed.

B

Humid OA region between the RA enthalpy and the required SA humidity ratio: the EW should be off. Operating the EW in this region elevates the enthalpy of the OA entering the cooling coil (CC) and hence the energy use. Warm (hotter than the space) but dry OA region (mechanical dehumidification is not necessary to meet the space latent load using this air directly): the EW should be off. Operating the EW in this region elevates the humidity ratio of the OA entering the CC requiring latent cooling in addition to sensible cooling, increasing the energy use above that needed to sensibly cool the air when the EW is off. Cool (cooler than the space) but dry OA region (mechanical dehumidification is not necessary to meet the space latent load using this air directly): the EW should be off. Operating the EW in this region elevates the humidity ratio of the OA entering the CC, requiring latent cooling in addition to sensible cooling, increasing the energy use above that needed to sensibly cool the air when the EW is off. Cold (colder than the required SA DPT) and dry OA region (mechanical dehumidification is not necessary to meet the space latent load using this air directly): the EW should modulate as necessary to avoid overcooling or meet the design SA DBT (48°F in this illustration). Modulating the EW in this region to just avoid overcooling enables the system to do most, if not all, of the cooling as an economizer allowing the cooling plant to be off. Mechanical cooling is required when the EW is operating full speed.

C

D

E

table 1: Appropriate enthalpy wheel control action.
Region ID A B C D E Hours in Region 2,666 1,255 55 1,261 3,523 Difference Between Operating the EW Full Time vs. Proper Control Hot humid OA, EW on either method, no difference. EW should be off. If EW on, cooling use increases by 10,500 ton hours (TH). EW should be off. If on, cooling use increases 115 TH. EW should be off. If EW on, cooling use increases 18,690 TH. EW speed to modulate holding 48°F SAT. If EW full on, cooling use increases by 45,755 TH.

table 2: Energy penalty when the enthalpy wheel is on all the time vs. proper control.
78 ASHRAE Journal ashrae.org August 2011



ASHRAE Journal - August 2011

Table of Contents for the Digital Edition of ASHRAE Journal - August 2011

ASHRAE Journal - August 2011
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
2011–12 Presidential Address: Sustaining ASHRAE Through Leadership
Retrocommissioning Older Buildings
Liquid Refrigerant Pumping in Industrial Refrigeration Systems
Technology Award Case Studies:
Hospital Upgrade: Heat Recovery, Geothermal Save Energy
Cool Weather Savings: Using Hybrid Refrigeration Systems for Chiller Retrofit
Special Section
InfoCenter
Standing Columns
Solar NZEB Project
Emerging Technologies
Special Products
IAQ Applications
Washington Report
People
Products
Classified Advertising
Advertisers Index
ASHRAE Journal - August 2011 - Intro
ASHRAE Journal - August 2011 - ASHRAE Journal - August 2011
ASHRAE Journal - August 2011 - Cover2
ASHRAE Journal - August 2011 - 1
ASHRAE Journal - August 2011 - 2
ASHRAE Journal - August 2011 - Contents
ASHRAE Journal - August 2011 - Commentary
ASHRAE Journal - August 2011 - 5
ASHRAE Journal - August 2011 - Industry News
ASHRAE Journal - August 2011 - 7
ASHRAE Journal - August 2011 - 8
ASHRAE Journal - August 2011 - 9
ASHRAE Journal - August 2011 - Letters
ASHRAE Journal - August 2011 - 11
ASHRAE Journal - August 2011 - 12
ASHRAE Journal - August 2011 - 13
ASHRAE Journal - August 2011 - 14
ASHRAE Journal - August 2011 - 15
ASHRAE Journal - August 2011 - Meetings and Shows
ASHRAE Journal - August 2011 - 17
ASHRAE Journal - August 2011 - 2011–12 Presidential Address: Sustaining ASHRAE Through Leadership
ASHRAE Journal - August 2011 - 19
ASHRAE Journal - August 2011 - 20
ASHRAE Journal - August 2011 - 21
ASHRAE Journal - August 2011 - 22
ASHRAE Journal - August 2011 - 23
ASHRAE Journal - August 2011 - 24
ASHRAE Journal - August 2011 - 25
ASHRAE Journal - August 2011 - Retrocommissioning Older Buildings
ASHRAE Journal - August 2011 - 27
ASHRAE Journal - August 2011 - 28
ASHRAE Journal - August 2011 - 29
ASHRAE Journal - August 2011 - 30
ASHRAE Journal - August 2011 - 31
ASHRAE Journal - August 2011 - 32
ASHRAE Journal - August 2011 - 33
ASHRAE Journal - August 2011 - 34
ASHRAE Journal - August 2011 - 35
ASHRAE Journal - August 2011 - Liquid Refrigerant Pumping in Industrial Refrigeration Systems
ASHRAE Journal - August 2011 - 37
ASHRAE Journal - August 2011 - 38
ASHRAE Journal - August 2011 - 39
ASHRAE Journal - August 2011 - 40
ASHRAE Journal - August 2011 - 41
ASHRAE Journal - August 2011 - 42
ASHRAE Journal - August 2011 - 43
ASHRAE Journal - August 2011 - Hospital Upgrade: Heat Recovery, Geothermal Save Energy
ASHRAE Journal - August 2011 - 45
ASHRAE Journal - August 2011 - 46
ASHRAE Journal - August 2011 - 47
ASHRAE Journal - August 2011 - Cool Weather Savings: Using Hybrid Refrigeration Systems for Chiller Retrofit
ASHRAE Journal - August 2011 - 49
ASHRAE Journal - August 2011 - 50
ASHRAE Journal - August 2011 - 51
ASHRAE Journal - August 2011 - InfoCenter
ASHRAE Journal - August 2011 - 53
ASHRAE Journal - August 2011 - 54
ASHRAE Journal - August 2011 - 55
ASHRAE Journal - August 2011 - 56
ASHRAE Journal - August 2011 - 57
ASHRAE Journal - August 2011 - 58
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ASHRAE Journal - August 2011 - 60
ASHRAE Journal - August 2011 - Solar NZEB Project
ASHRAE Journal - August 2011 - 62
ASHRAE Journal - August 2011 - 63
ASHRAE Journal - August 2011 - 64
ASHRAE Journal - August 2011 - 65
ASHRAE Journal - August 2011 - Emerging Technologies
ASHRAE Journal - August 2011 - 67
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ASHRAE Journal - August 2011 - 70
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ASHRAE Journal - August 2011 - 73
ASHRAE Journal - August 2011 - 74
ASHRAE Journal - August 2011 - Special Products
ASHRAE Journal - August 2011 - IAQ Applications
ASHRAE Journal - August 2011 - 77
ASHRAE Journal - August 2011 - 78
ASHRAE Journal - August 2011 - 79
ASHRAE Journal - August 2011 - Washington Report
ASHRAE Journal - August 2011 - 81
ASHRAE Journal - August 2011 - People
ASHRAE Journal - August 2011 - 83
ASHRAE Journal - August 2011 - Products
ASHRAE Journal - August 2011 - 85
ASHRAE Journal - August 2011 - Classified Advertising
ASHRAE Journal - August 2011 - 87
ASHRAE Journal - August 2011 - Advertisers Index
ASHRAE Journal - August 2011 - Cover3
ASHRAE Journal - August 2011 - Cover4
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