ASHRAE Journal - August 2008 - (Page 44)

Value for Climate Location Specification Wall Assembly U-Value Btu/h · ft2 · °F Roof U-Value Btu/h · ft2 · °F Solar Heat Gain Coefficient Years 1975 – 1989 1989 – 1999 1999 – Present 1975 – 1989 1989 – 1999 1999 – Present 1975 – 1989 1989 – 1999 1999 – Present Atlanta 0.450 0.130 0.089 0.100 0.072 0.063 0.580 0.550 0.390 Albuquerque, N.M. 0.370 0.100 0.089 0.088 0.059 0.063 0.570 0.550 0.390 Boston 0.330 0.089 0.089 0.078 0.058 0.063 0.560 0.550 0.390 Fort Worth, Texas 0.425 0.150 0.089 0.100 0.058 0.063 0.590 0.550 0.250 Miami 0.400 1.000 0.089 0.100 0.074 0.034 0.560 0.550 0.250 Minneapolis 0.280 0.065 0.089 0.060 0.045 0.063 0.530 0.550 0.390 Washington 0.360 0.089 0.089 0.084 0.058 0.063 0.540 0.550 0.390 Table 2: Input values of the envelope. 1 0.9 Sensible Heat Ratio 0.8 0.7 0.6 0.5 0.4 1975 1980 1985 1990 1995 2000 2005 Albuquerque, N.M. 1 0.9 Sensible Heat Ratio 0.8 0.7 0.6 0.5 0.4 1975 1980 1985 1990 1995 2000 2005 Albuquerque, N.M. Fort Worth, Texas Minneapolis Boston Washington Atlanta Miami Fort Worth, Texas Minneapolis Boston Washington Atlanta Miami Figure 7: Building load SHR from 1975 to present, at sensible design condition (high outdoor temperature). Figure 8: Building load SHR from 1975 to present, at latent design condition (high outdoor humidity). outdoor conditions are summarized in Table 3 for each of the seven climate cities. Sensible Heat Ratio 1 0.9 0.8 0.7 0.6 0.5 0.4 1975 1980 1985 1990 1995 2000 2005 Albuquerque, N.M. Fort Worth, Texas Minneapolis Boston Washington Atlanta Miami SHR of HVAC: Options for Increasing Latent Capacity A variety of options exist for adjusting the latent capacity of the unitary air conditioning equipment that is used to condition the interior space of a significant portion of commercial building space. These include: • Overcooling part of the airstream to increase the latent fraction, e.g., coil bypass or reduced airflow through the coil; • Overcooling, as above, and reheating (undoing excess sensible cooling); • Active desiccants; and • Enthalpy recovery. Combined SHR of a Unitary AC with Air-to-Air ERV Figure 9: Building load SHR from 1975 to present, at shoulder or partload condition (high outdoor humidity, moderate temperature). The current level of building cooling load SHRs fall outside the range of SHRs that basic unitary air conditioners can provide. One approach to reducing the cooling equipment SHR is to combine a unitary air-conditioning system with another system or component that operates with a low SHR. Examples include dedicated mechanical dehumidifiers or desiccant dehumidifiers and enthalpy recovery ventilation systems. Unlike dehumidifi44 ASHRAE Journal cation systems that use new energy to reduce humidity, enthalpy recovery uses recovered energy from the building exhaust air as the energy source thereby increasing system efficiency and reducing peak electric demand (ARI Guideline V4 provides an accepted method for calculating the combined efficiency of a unitary air conditioner combined with an air-to-air enthalpy recovery exchanger.) The combined capacity and SHR of an ERV integrated with a unitary air conditioner can be calculated by adding the individual contributions to the latent and sensible cooling from ashrae.org August 2008 http://ashrae.org

Table of Contents Feed for the Digital Edition of ASHRAE Journal - August 2008

ASHRAE Journal - August 2008 Contents Commentary Industry News Letters Meetings and Shows Maintain to Sustain—Delivering ASHRAE’s Sustainability Promise Ultraviolet Germicidal Irradiation: Current Best Practices Improving Humidity Control With Energy Recovery Ventilation Single- or Two-Stage Compression Data Center Cooling: Using Wet-Bulb Economizers Building Sciences InfoCenter Practical Pointers Products Emerging Technologies Washington Report People Special Products Classified Advertising Advertising Index

ASHRAE Journal - August 2008

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