ASHRAE Journal - August 2011 - 77

even the smallest systems as summarized in Figure 1. Since the near universal requirement to use TER with a DOAS is relatively recent, and is not required for most all-air systems, many in the industry have not developed sufficient trust or familiarity with TER to make informed decisions. Many considerations exist; some are addressed at this link http://doas-radiant. psu.edu/DOAS_Enthalpy_Wheel_Issues. pdf. In this column, specific attention is focused on addressing EW control.

Marine (C)

Dry (B)

Moist (A) All Sizes Require TER

Warm-Humid Below White line
All of Alaska in Zone 7 except for the following boroughs in Zone 8: Bethel Northwest Arctic Dellingham Southeast Fairbanks Fairbanks N. Star Wade Hampton Nome Yukon-Koyukuk North Slope

First Misconception
TER to operate all the time in both analysis software and in real time. Hourly weather data for most geographic locations fall into five regions of the psychrometric chart as illustrated in Figure 2. In each region, the appropriate enthalpy based control action is defined in Table 1. If the EW is permitted to operate all the time, the consequences for each region are presented in Table 2. Table 2 shows that operating an EW in Kansas City, Mo., (KC) all the time for a 10,000 scfm (4719 L/s) OA system equipped with a balanced flow 70% effective (e) means that EW will consume 75,060 extra ton hours (TH) (263 908 kWh) of cooling per year. At 1 kW/ton and $0.15/kWh—this represents $11,260 of waste, and takes us far from net zero energy. The difference in energy use for this one example is large. Other errant EW control concepts exists, for example, DBT control, but they are not discussed in this column. An example of this EW control error during modeling is presented in Figure 3 for a U.S. Army Brigade Facility (office facility). The error shows up first at energy efficiency measure (EEM) Step 5 where an EW was introduced. The error carries all the way through to Step 13, and would lead one to falsely conclude that a DOAS uses more energy than allair systems. Two important items of EW control that were not discussed include: • Wheel cleaning when the EW is off. One way of cleaning is to energize the wheel once an hour for 1 min., resulting in about 20 airflow reversals in that minute per every hour for wheel cleaning.
August 2011

Zone 1 includes: Hawaii, Guam, Puerto Rico and the Virgin Islands

Climate Zone (60% TER Required Standard 189.1-2009) Design Airflow When >80% OA 1A, 2A, 3A, 4A, 5A, 6A, 7, 8 (Moist Eastern U.S. & Alaska) 6B 1B, 2B, 5C 3B, 3C, 4B, 4C, 5B ³0 cfm (All Sizes Require TER) ³1,500 cfm ³4,000 cfm ³5,000 cfm

Figure 1: U.S. locations and requirements for TER per ASHRAE Standards 90.1 and 189.1. • Frost prevention. There are many ways to do this. One method is discussed in the paper at this link: http://doasradiant.psu.edu/4428.pdf. A real time control is used to temper the OA just sufficient to assure condensation cannot occur in the EW whenever the OA temperature is below freezing. This is accomplished by computing the tangent to the saturation curve containing the exhaust air thermodynamic state point (TSP), and determining if the outdoor TSP is to the right or left of the tangent. If the TSP is to the left, and the OA DBT is below freezing, then the OA is sensibly heated enough to intersect the tangent curve. This control logic has been working superbly for more than 10 years in the field saving much unnecessary preheat energy. Related to EW control is the issue of airside free cooling (economizer). Of course, a DOAS generally does not supply as much airflow as an all-air system, limiting its ability in that area. DOAS free cooling can be maximized as follows: • In the illustration presented previously, it was assumed that the SA DBT could not drop below 48°F (9°C); however, it is not uncommon with just ventilation air that 48°F (9°C) is not cold enough to meet all the space sensible loads. In those cases, allowing colder OA expands the free cooling. • However, allowing the SA temperature to drop below 48°F (9°C) at the diffusers can be problematic. The OA can be tempered with heat removed from the spaces by the sensible terminal cooling equipment without losing free cooling. Such an example is presented at this link: http://doas. psu.edu/IAQ_summer_05.pdf. Radiant panels are discussed in this article, but any hydronic terminal unit can do the same be it fan coil, chilled beam, fan powered box, etc.

second Misconception
The DOAS supply air temperature must always be at a neutral temperature. For many, the conventional wisdom is to always cool the supply air to the
ASHRAE Journal 77



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
ASHRAE Journal - August 2011 - 59
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
ASHRAE Journal - August 2011 - 68
ASHRAE Journal - August 2011 - 69
ASHRAE Journal - August 2011 - 70
ASHRAE Journal - August 2011 - 71
ASHRAE Journal - August 2011 - 72
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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