ASHRAE Journal - September 2012 - 21

+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 55-2010.

Figure 1: ASHRAE thermal sensation scale from Standard 55-2010. 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 702006, 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 T50 /L and is documented in the room air-distribution chapter of Handbook—Fundamentals. The T50 /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 113-2009. This is a method of test to measure air velocity and temperature within the occupied zone. 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 55-2010 definition of comfort.

Predicted Mean Vote & Predicted Percent Dissatisfied
All the methods for predicting occupant comfort outlined in Standard 55-2010 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 5.2.1.2 from Standard 55-2010). 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 Icl = cloth index, clo v = air velocity, m/s tr = mean radiant temperature, °C ta = ambient air temperature, °C Pw = 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).

Predict Occupant Comfort
Standard 55-2010 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 55-2010 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
September 2012

Metabolic Rate
Also included in the Standard 55-2010 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/m2).
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ASHRAE Journal - September 2012

Table of Contents for the Digital Edition of ASHRAE Journal - September 2012