Designing Air-Distribution Systems To Maximize Comfort

Transcription

1 Designing Air-Distribution Systems To Maximize Comfort By David A. John, P.E., Member ASHRAE An air-distribution system that provides occupant thermal comfort can be a complicated system to predict and analyze. Providing comfort depends on variables from the obvious thermal conditions in a space, which include radiant temperature, air speed, air temperature and humidity, to the less obvious occupant metabolic rate and even choice in clothing. A system can be successfully designed by understanding what makes us comfortable and selecting the proper air-distribution products and layout. This article discusses how HVAC designers can select, size, and place outlets using methods described in the room air-distribution chapters in the 2009 ASHRAE Handbook Fundamentals to maximize occupant thermal comfort as defined in ASHRAE Standard , Thermal Environmental Conditions for Human Occupancy. Standard addresses factors that determine human comfort in a space. Until recently, the air diffuser performance index (ADPI) as outlined in Standard 113, Appendix B, was a method to predict occupant comfort. The cognizant committee for ASHRAE Standard , Method of Testing for Room Air Diffusion, changed the language used in the ASHRAE Handbook to indicate ADPI is a measure of predicted room air thermal mixing, not a direct measure of occupant comfort. This article attempts to define the space comfort as defined by Standard , noting that this does not tell a designer how to select or space airdistribution devices. Using ADPI, a designer can select, size, and space outlets but can only measure the thermal mixing, not the level of occupant comfort. Predicting and Quantifying Comfort The purpose of Standard is to indicate the combination of indoor thermal environmental factors and personal factors that will produce thermal environmental conditions acceptable to a majority of space occupants. The variables that define comfort in the standard are: metabolic rate, clothing insulation, air temperature, radiant temperature, air speed, and humidity. About the Author David A. John, P.E., is general manager, vice president of A.D.E. Engineered Solutions of Florida, Inc. 20 ASHRAE Journal ashrae.org September 2012

2 This article was published in ASHRAE Journal, September Copyright 2012 ASHRAE. Posted at This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit +3 Hot +2 Warm +1 Slightly Warm 0 Neutral 1 Slightly Cool 2 Cool 3 Cold Figure 2: Predicted percentage dissatisfied based on predicted mean vote from Standard Figure 1: ASHRAE thermal sensation scale from Standard The thermal comfort chapter (Chapter 9) in the 2009 ASHRAE Handbook Fundamentals details the science behind comfort and describes these variables in detail. The comfort obtained in a space is most often determined by the outlets selected, their size and how they are placed in that space. In an overhead forced air system (most commonly specified in U.S. office buildings), the outlet manufacturers catalog data is used to properly select and space outlets. In most cases, the outlet performance was tested per ASHRAE Standard , Method of Testing the Performance of Air Outlets and Air Inlets. To properly design an air-distribution system to maximize comfort, the designer can use the outlet performance from the catalog data to maximize mixing and minimize temperature gradients in the occupied zone. A method that can be used to predict mixing in the occupied zone is using the ratio of the throw distance at 50 fpm (0.25 m/s) to the horizontal length of the zone. This ratio is referred to as T 50 /L and is documented in the room air-distribution chapter of Handbook Fundamentals. The T 50 /L ration can be used to predict the resulting air-distribution performance index, giving the designer a fairly good estimate of the mixing within a zone. The test method to measure the ADPI for a space is Standard This is a method of test to measure air velocity and temperature within the occupied zone. Predict Occupant Comfort Standard has been updated from the 2004 standard with provisions that allow elevated air speed to broadly offset the need to cool the air in warm conditions. This can be applied to natural ventilation applications, and to conventional overhead (and other) air-distribution systems up to an air speed of 150 fpm (0.8 m/s) with no local control, and up to 240 fpm (1.2 m/s) with local control. Standard also includes graphical and computer methods for determining comfort. The two methods include the combination of air temperature and mean radiant temperature as well as humidity, air speed, metabolic rate, and clothing insulation. The ASHRAE Thermal Comfort Tool, Version 2, can be used to calculate comfort conditions and is available from the ASHRAE bookstore. The graphical method can be used by designers to meet the Standard definition of comfort. Predicted Mean Vote & Predicted Percent Dissatisfied All the methods for predicting occupant comfort outlined in Standard use the ASHRAE thermal sensation scale that was developed to quantify people s thermal sensations. The predicted mean vote (PMV) index predicts the mean response of a large group of people according to the ASHRAE thermal sensation scale shown in Figure 1 (Figure from Standard ). The PMV model predicts steadystate comfort responses. The predicted mean vote/predicted percent dissatisfied (PMV/PPD) model is widely used and accepted for design and field assessment of comfort conditions. ISO Standard 7730 includes computer code to calculate PMV and PPD for a wide range of parameters. The equation to manually obtain the predicted mean vote for a space was developed by P. Ole Fanger (Chapter 9, Handbook Fundamentals) and is fairly complex to solve. The equation includes the following variables: M = metabolic rate, met I cl = cloth index, clo v = air velocity, m/s t r = mean radiant temperature, C t a = ambient air temperature, C P w = vapor pressure of water in ambient air, Pa A PPD of 10% corresponds to the PMV range of ±0.5, and even with a PMV = 0, about 5% of the people are dissatisfied. The PMV model defines comfort with +3, +2, 2 or 3 results, which indicate discomfort (Figure 2). Metabolic Rate Also included in the Standard definition of comfort is the metabolic rate for the occupants. In the standard, the unit used to express the metabolic rate is the met, which is defined as the metabolic rate of a sedentary person who is seated and quiet (1 met = 58.1 W/m 2 ). September 2012 ASHRAE Journal 21

3 Figure 3: Graphic comfort zone method from Standard The metabolic rates ranging from 1.0 to 1.3 is typical of an office worker in near sedentary physical activity such as working at a desk in a seated position. More information on metabolic rate can be found in the Standard Normative Appendix A activity levels. Also, a detailed discussion of metabolic rate can be found in Chapter 9, Handbook Fundamentals. Clothing Insulation Clothing insulation is measured in units of clo. As a reference, the 0.5 clo is typical for an office environment in the summer and 1.0 clo is typical for the office environment in the winter. The Normative Appendix B, Clothing Insulation, in the standard is a good reference for calculating different clo values for occupants. Clothing insulation is discussed in detail in Chapter 9, Handbook Fundamentals. Radiant Temperature Asymmetry Occupant comfort is also affected by the thermal radiation field around the body, which can cause discomfort. The radiant temperature asymmetry is caused by factors such as hot or cold surfaces or direct sunlight. The radiant temperature may differ from the dry-bulb temperature in a space. Standard does include allowable radiant temperature asymmetry for a space. Also, Chapter 9, Handbook Fundamentals, is a good reference. Standard lists four methods for evaluating comfort: 1. Graphic Comfort Zone Method for Typical Indoor Environments; 2. Computer Model Method for General Indoor Applications; 3. Graphical Elevated Air Speed Method; and 4. Standard Effective Temperature (SET) Model. All four methods include human factors that determine comfort such as metabolic rate and clothing insulation, and thermal factors such as space temperature, air velocity, humidity, and radiant temperature. 1. Graphic Comfort Zone Method The graphical method for predicting comfort in Standard assumes the occupants metabolic rate is between 1.0 and 1.3 met and the clothing worn is between 0.5 and 1.0 clo (typical for an office). This method predicts comfort for an acceptance level of 80%. This is based on a 10% PMV- 22 ASHRAE Journal ashrae.org September 2012

4 Figure 4 (left): Graphical elevated air speed method. Figure 5 (right): Standard effective temperature (SET) method. PPD index plus an additional 10% dissatisfaction that may occur from local thermal discomfort. Air speeds are not greater than 40 fpm (0.20 m/s). The method includes two areas of comfort: one for clothing insulation of 0.5 clo and one for 1.0 clo (Figure 3). Added to Standard is prediction of comfort using elevated air speeds. The graphical method includes Figure 4 to calculate the required air speed for applications with both mean and radiant temperatures. This figure Diffuser Type High Sidewall Grille Circular Ceiling Pattern Diffuser Sill Grille Ceiling Slot Diffuser Light Troffer Diffusers Cross-Flow Pattern Ceiling Diffusers allows for elevated air speeds of more than 150 fpm (0.76 m/s). 2. Computer Model Method The computer model method predicts the PMV and PPD for a given space. The standard includes computer code (Normative Appendix D) that assumes an average metabolic rate between 1.0 and 2.0 met, and where clo values of the occupants are 1.5 or less. Characteristic Length L Distance to Wall Perpendicular to Jet Distance to Closest Wall or Intersecting Air Jet Length of Room in Direction of Jet Flow Distance to Wall or Midplane Between Outlets Distance to Midplane Between Outlets Plus Distance from Ceiling to Top of Occupied Zone Distance to Wall or Midplane Between Outlets Table 1: Characteristic room length for several diffusers from 2009 Handbook Fundamentals. ASHRAE Thermal Comfort Tool The ASHRAE Thermal Comfort Tool is a convenient method to predict PMV/PPV for a space. The tool allows designers to predict PMV based on standard conditions, elevated air speeds, and adaptive method. The program allows users to input air temperature, air speed, humidity ratio, mean radiant temperature, activity level (that converts to met) and clothing (that converts to clo). The program output shows PMV and PPD, as well as indicates whether the selection complies with Standard Graphical Elevated Air Speed Method Included in Standard is the elevated air speed method that allows for higher space temperatures and room air velocities. The comfort level obtained depends on whether or not the occupants have local control of air speed. This method applies to a lightly clothed person with clothing insulation between 0.5 and 0.7 clo who is engaged in near sedentary physical activity with a metabolic rate between 1.0 and 1.3 met. 4. Standard Effective Temperature (SET) Model This method in Standard uses a thermophysiological simulation of the human body and skin heat loss to predict the occupant s comfort level. This model enables air velocity effects on thermal comfort to be related across a wide range of air temperatures, radiant temperatures, and humidity ratios (Figure 5). Selecting Outlets to Maximize Comfort Standard can be used by designers to predict the comfort level of a space based on PMV, but the standard does not indicate to designers where to locate air-distribution devices. For an overhead forced air system, the tools available to the designer to maximize the occupant comfort level is the manufacturers outlet performance obtained per Standard , and the T 50 /L ratio to predict the ADPI. Method of Testing for Room Air Diffusion A method to calculate the ADPI value in a space with overhead mixing air distribution operating in cooling is outlined in Standard , which defines a repeatable method of testing steady-state air diffusion performance of an air-distribution system in occupied zones of building spaces. The standard is based on air velocity and air temperature distributions at specified cooling loads and operating conditions. The standard can be applied to 24 ASHRAE Journal ashrae.org September 2012

5 furnished and unfurnished spaces, actual or laboratory conditions, with or without occupants. The standard is not applicable to naturally ventilated building space. Appendix B of Standard states that the test procedures can be used to generate the air distribution performance index for a space. The ADPI is a single-number rating of the air diffusion performance of a system of diffusers, as installed in a defined space, for a specified supply air delivery rate and space load. ADPI is based only on air speed and effective draft temperature and is not directly related to the wet-bulb temperature or relative humidity. Wet-bulb temperature, humidity, and similar effects (such as mean radiant temperature) should be accounted for according to Standard The ADPI method requires calculating the effective draft temperature at multiple points. The effective draft temperature is: f n = t acn t ac 0.07 (v an 30) F where f n = effective draft temperature at test point n t acn = corrected temperature at test point n t ac = average test zone temperature v an = time-averaged speed at test point n ADPI = Number of test points that meet effective draft temperature criteria ( 3 F and +2 F) % Total number of test points ADPI is for traditional overhead airdistribution systems under cooling operation only. The results can be used as an indicator of occupant comfort in a space. A high percentage of people will be comfortable under sedentary conditions where the effective draft temperature is between 3 F and +2 F with an air speed of less than or equal to 70 fpm ( 1.7 C and +1.1 C with an air speed less than or equal to 0.35 m/s). The ADPI is the percent of test points that meet these criteria. Using T 50 /L to Select Outlets A designer can use the ratio of T 50 /L to predict the level of mixing in a zone Terminal Device High Sidewall Grilles Circular Ceiling Diffusers Sill Grille, Straight Vanes Sill Grille, Spread Vanes Ceiling Slot Diffusers (for T 100 /L) Light Troffer Diffusers Cross-Flow Pattern Diffusers Room Load, Btu/h ft 2 T 50 /L for Maximum ADPI Maximum ADPI For ADPI Greater Than Range of T 50 /L to to to 1.9 < to to to to to 1.7 < to to to to to to to to to to to < < < to to to to 3.4 Table 2: Air diffusion performance index (ADPI) selection guide from 2009 Handbook Fundamentals. (this would not apply to naturally ventilated space, or high velocity, high temperature designs). This method can be used to select the type of outlet, the size, and throw distance and spacing to maximize the performance of an outlet. T 50 is the manufacturers cataloged throw data to 50 fpm (0.25 m/s) and L is the characteristic length of the space being evaluated. L is defined per Table 1 (Page 24) dependent of the outlet type and layout. By determining the value of T 50 from a manufacturer s catalog, and measuring the characteristic length L from the projects plans, the ratio can be determined and the predicted ADPI value can be estimated from Table 2. Conclusion Designers can select design conditions using Standard based on clothing, metabolic rate, humidity, radiant temperature, and air speed. For an overhead forced air system, the outlet can be selected, sized, and placed using data obtained using the testing procedures as outline in Standard and the T 50 /L method prescribed in Chapter 20, Space Air Diffusion, in the 2009 ASHRAE Handbook Fundamentals, to predict ADPI values. By selecting and placing outlets to achieve ADPI values of 80% or more, the design can predict a well thermally mixed system. The resulting level of comfort can be predicted and measured using one of the four methods prescribed in Standard ASHRAE Journal ashrae.org September 2012