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Matching the Sensible Heat Ratio of Air Conditioning Equipment with the Building Load SHR

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Matching the Sensible Heat Ratio of Air Conditioning Equipment with the Building Load SHR Matching the Sensible Heat Ratio of Air Conditioning Equipment with the Building Load SHR Final Report to: Airxchange November 12, 2003 Report prepared by: TIAX LLC Reference D5186 Notice: This report was commissioned by Airxchange on terms specifically limiting TIAX’s liability. Our conclusions are the results of the exercise of our best professional judgement, based in part upon materials and information provided to us by Airxchange and others. Use of this report by any third party for whatever purpose should not, and does not, absolve such third party from using due diligence in verifying the report’s contents. Any use which a third party makes of this document, or any reliance on it, or decisions to be made based on it, are the responsibility of such third party. TIAX accepts no duty of care or liability of any kind whatsoever to any such third party, and no responsibility for damages, if any, suffered by any third party as a result of decisions made, or not made, or actions taken, or not taken, based on this document. TIAX LLC Acorn Park • Cambridge, MA • 02140-2390 USA • +1 617 498 5000 www.tiax.biz Table of Contents TABLE OF CONTENTS............................................................................................................................ I LIST OF TABLES .....................................................................................................................................II LIST OF FIGURES .................................................................................................................................III 1. INTRODUCTION ......................................................................................................................... 1-1 1.1. THE MYTH OF THE DECLINING LATENT CAPACITY OF UNITARY AIR CONDITIONERS ................. 1-1 1.2. BUILDING HUMIDITY CONTROL PROBLEMS ARE NOT A MYTH .................................................... 1-2 1.3. THE RATIO OF SENSIBLE AND LATENT COOLING LOADS IN BUILDINGS HAS CHANGED.............. 1-3 1.4. OPTIONS FOR INCREASING THE LATENT CAPACITY ..................................................................... 1-5 2. THE EVOLVING SENSIBLE HEAT RATIO OF COMMERCIAL BUILDINGS................ 2-1 2.1. SENSIBLE AND LATENT COOLING LOADS .................................................................................... 2-2 2.2. CHANGING BUILDING ENERGY EFFICIENCY AND VENTILATION STANDARDS – ASHRAE STANDARD 90.1 AND ASHRAE STANDARD 62 .................................................................................... 2-3 2.2.1. Building Envelope ............................................................................................................. 2-4 2.2.2. Equipment Loads............................................................................................................... 2-5 2.2.3. Ventilation and Infiltration................................................................................................ 2-6 2.3. BUILDING COOLING LOAD CALCULATIONS FOR A RANGE OF CLIMATE ZONES ................................ 2-8 2.3.1. The Evolution of Envelope and Lighting Specifications Over Time.................................. 2-9 2.3.2. Analysis Results................................................................................................................. 2-9 3. THE SYSTEM SENSIBLE HEAT RATIO OF COMBINATIONS OF HVAC EQUIPMENT 3-1 3.1. SYSTEM SHR – GENERAL CASE.................................................................................................. 3-1 3.2. SYSTEM SHR – UNITARY AIR CONDITIONER COMBINED WITH AN AIR TO AIR ENTHALPY RECOVERY EXCHANGER ....................................................................................................................... 3-2 3.2.1. Example Calculation of Combined SHR for Unitary and ERV ......................................... 3-3 3.2.2. Illustrative Example – the Example Office Building of Section 2.3................................... 3-4 3.2.3. Sensitivity Analysis............................................................................................................ 3-6 3.2.3.1. Infiltration....................................................................................................................................3-6 3.2.3.2. ERV Latent Effectiveness < ERV Sensible Effectiveness ...........................................................3-8 3.2.4. Calculation of Unitary + ERV SHRs for the Examples in Section 3.2.2 ........................... 3-8 4. SUMMARY OF KEY FINDINGS ............................................................................................... 4-1 5. CONCLUSIONS............................................................................................................................ 5-1 6. REFERENCES .............................................................................................................................. 6-1 i List of Tables Table 2-1: Changing Minimum Outdoor Make-Up Air Ventilation Rates Per ASHRAE Standard 62 ............................................................................................................2-7 Table 2-2: Required CFM/ft2, Based on Estimated Maximum Occupancy and Required CFM/Person .................................................................................2-7 Table 2-3: Weighted Average Ventilation per Floorspace Required for Office Building ................................................................................................................................2-9 Table 2-4: Input Values of the Envelope ....................................................................2-11 Table 2-5: Design Conditions .....................................................................................2-11 Table 3-1: Combined SHR Calculation ........................................................................3-4 Table 3-2: Comparison of the Building Cooling Load SHR with the Combined Unitary + ERV SHR in Miami Climatic Conditions ................................................3-5 Table 3-3: Comparison of the Building Cooling Load SHR with the Combined Unitary + ERV SHR in Boston Climatic Conditions................................................3-5 Table 3-4: Sensitivity Analysis – Impact of Infiltration (at 0.5 Air Changes/Hour) ....3-7 Table 3-5: Sensitivity Analysis: ∈latent = ∈sens –0.05 (Unitary SHR = 0.75) ................3-8 Table 3-6: Ventilation Portion of Design Load, Corresponding ERV Net Recovery Capacity at ∈=0.7 ........................................................................................3-8 Table 3-7: Design Load and Unitary Capacity Net of the ERV NRC ..........................3-9 Table 3-8: Combined SHR (Unitary SHR = 0.75)........................................................3-9 ii List of Figures Figure 1-1: Airstream Temperature Passing Through Evaporator Coil.........................1-2 Figure 1-2: Indoor RH vs. Building Cooling Load SHR with Conventional Unitary Air Conditioning Equipment........................................................................................1-5 Figure 2-1: ASHRAE 90.1 Maximum Wall Assembly “U” Values, Over Time .........2-4 Figure 2-2: ASHRAE 90.1 Maximum Roof "U" Values Over Time ...........................2-5 Figure 2-3: ASHRAE 90.1 Maximum Solar Heat Gain Coefficient Over Time..........2-5 Figure 2-4: ASHRAE 90.1 Maximum Lighting Power Density (for Office Space) and Estimated Office Equipment Power Density .........................................................2-6 Figure 2-5: Model Office Building Configuration........................................................2-8 Figure 2-6: Area Plot of Basic Constituents of the Sensible and Latent Cooling Loads – Miami / Latent Design Day / 1st Floor North Zone / 1981-1989 ........................2-10 Figure 2-7: Area Plot of Basic Constituents of the Sensible and Latent Cooling Loads – Miami / Latent Design Day / 1st Floor North Zone / 1989-1999 ........................2-10 Figure 2-8: Building Load SHR from 1975 – Present, at Sensible Design Condition (High Outdoor Temperature) ...............................................................................2-12 Figure 2-9: Building Load SHR from 1975 – Present, at Latent Design Condition (High Outdoor Humidity)...............................................................................................2-12 Figure 2-10: Building Load SHR from 1975 – Present, at Shoulder or Part Load Condition (High Outdoor Humidity, Moderate Temperature ) ...........................2-13 Figure 3-1: Unitary Air Conditioner with a Factory Integrated AAHX ........................3-2 Figure 3-2: Comparison of the Building Cooling Load SHR with the Combined Unitary + ERV SHR in Miami and Boston Climatic Conditions .......................................3-6 Figure 3-3: Impact of Infiltration on the Match of the Building Cooling Load SHR with the Combined Unitary + ERV SHR (at the Latent Design Condition)..................3-7 iii Executive Summary In recent years, an increasing number of buildings and their occupants have experienced serious moisture problems including so-called “sick building syndrome”, mold growth, and occupant discomfort due to high humidity. These problems arise from both poor interior humidity control and liquid water sources such as plumbing leaks and rain water leakage. Moisture and mold problems can cause serious health problems among the occupants and reduce productivity. In extreme cases, buildings have been rendered uninhabitable. The financial consequences of these situations can be substantial
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