Grilles and Diffusers engineering guidelines - Titus …

1 B7 Redefine your comfort zone | guidelinesBGRILLES AND DIFFUSERSBASIC PRINCIPLES OF AIR DISTRIBUTIONTHE GOAL OF AN AIR DIFFUSION SYSTEM: MAINTAINING COMFORTAn understanding of the principles of room air distribution helps in the selection, design, control and operation of air systems. The real evaluation of air distribution in a space, however, must answer the question: Are the occupants comfortable? In general, a person is thermally comfortable when body heat loss equals heat production without being conscious of any changes in the body s temperature regulating mechanisms. The human body heat loss to the environment can occur through the following: Radiation Convection Conduction Evaporation The comfort of an occupant is determined by both occupant variables and the conditions of the space. Occupant factors include activity level and metabolic rate (reported in Met units), as well as occupant clothing levels (reported in Clo units).

2 The factors that influence space comfort conditions include: Dry bulb and radiant temperatures Relative humidity Air velocity The design of the air distribution system should address the above factors so that the occupant s heat loss is maintained at a comfortable LIMITS SET BY ASHRAE HANDBOOK, ASHRAE STANDARD 55 AND ISO STANDARD 7730 For many years, it has been shown that individual comfort is maintained through the change in seasons when the following conditions are maintained in the occupied zone of a space: 1. Air temperature maintained between 73 - 77 F2. Relative humidity maintained less than 60%3. Maximum air motion in the occupied zone (6 to 6 vertical, within 1 of walls): 50 fpm cooling 30 fpm heating4. Ankle to head level, F standing & F seated maximum temperature gradientNote: Comfort standards state that no minimum air movement is necessary to maintain thermal comfort, provided the temperature is acceptable.

3 To maximize energy conservation, maintain proper temperatures at the lowest possible air previous conditions assume occupants are sedentary or slightly active individuals and appropriately dressed. Variations in clothing can have a strong effect on desired temperature levels, often creating circumstances where a single setpoint will not satisfy all individuals in a meeting the above criteria for comfort, the temperature of the space and the relative humidity is largely controlled by the mechanical equipment including chillers or package units, air handlers, room thermostat, and air terminal unit. The air motion in the occupied zone is a function of the discharge velocity, discharge temperature (and room load) and the pattern of the air diffusion device into the space. At today s relatively low (< 1 cfm / ) air delivery rates, and with properly selected Diffusers , room load (and resultant Dt) is often the strongest variable in setting room air 1.

4 Comfort Chart - Neck RegionFigure 2. Comfort Chart - Ankle RegionGrilles and Diffusers -20-6-4024 Feeling ofcoolness8020406010%20%30%10040%of warmthFeeling Ankle regionLocal Air Velocity, FPML ocal Air Temp. minus Ambient Temp.(TX - TA)10%-20-6-402440%Neck region2040608030%20%100of warmthcoolnessFeeling ofFeeling Local Air Velocity, FPML ocal Air Temp. minus Ambient Temp.(TX - TA)B8 Redefine your comfort zone | guidelinesBGRILLES AND DIFFUSERSCOMFORT: A FUNCTION OF ROOM AIR VELOCITYSome interesting relationships exist between room air motion and the feeling of occupant comfort. Figure 1 shows the effect of air motion on comfort. The charts show that the feeling of comfort is a function of the local room air velocity, local temperature and ambient temperature. Local temperature (Tx) (Figures 1 and 2) is the temperature at a given point in a space.

5 Ambient temperature (TA) is the desired room temperature and can be considered the thermostat basic criteria for room air distribution can be obtained from the curves shown in (Figure 1). The chart shows the equivalent feeling of comfort for varying room temperatures and velocities at the neck. The % curves indicate the number of people who would object to the temperature and velocity conditions. The same comfort perceptions are shown in (Figure 2) for the ankle region. If 20% objections or 80% acceptance at the same velocities are allowed between (Figure 1 and Figure 2), the temperature deviation allowed between the ankle and neck levels would be about 4 F (less than ASHRAE values of F).Table 1 shows the relationship between local velocities and temperatures on occupant comfort. As an example, at a local velocity of 80 fpm, the local temperature can be maintained at 75 F to reach an 80% comfort level in the space.

6 The same 80% comfort level can be maintained with local air velocity of 15 fpm and a local temperature reduced to 71 F. The lower portion of Table 1 shows the effect on comfort of room air velocity with local temperature remaining constant at 75 F. For example, with a local velocity of 30 fpm and a local temperature at 75 F, the comfort reaction is neutral. Increasing the velocity to 60 fpm results in the objective increasing to 10%. This phenomenon of feeling can be illustrated by using a ceiling fan. A person can be cooled without decreasing the actual temperature by turning on a ceiling fan. The fan, in effect, increases the local air velocity and increases the feeling of coolness. It is shown that a velocity change of 15 fpm produces approximately the same effect on comfort as a 1 F temperature change.

7 The dotted lines in (Figures 1, 2 and 3) show the division between the feeling or perception of heating and , the acceptable level of comfort for a space is considered to be at the point where 20% or less of the room occupants may object to the room conditions. This would indicate that the given condition is acceptable to 80% of the all have perceived this above change and these subjective responses to drafts (temperature difference and air velocity). In 1938 Houghten et. al. developed the curves shown in (Figures 1 and 2). Utilizing this data, the equation for effective draft temperature was 1: Effective draft temperature = ( tX - tC ) - ( Vx - 30 )where: = effective temperature tX = local air temperatures, F tC = ambient temperature (average room temperature or control temperature, F) Vx = local air velocity, fpmADPI (Air Diffusion Performance Index) was derived by Nevins and Ward.

8 The percentage of all local points in an occupied space where - 3< <+ 2 with the velocity less than 70 fpm results in a single comfort index for the space. In the parallelogram in (Figure 3), the left line is = - 3, the right line is = + 2 and the top line is 70 parallelogram in (Figure 3) shows these limits and all their points are within the overall 80% acceptance. This method of obtaining a single point comfort rating can be used to rate a system in the field. It will also be covered more thoroughly in the outlet selections section, and the ADPI section page B31. It is also expected that there is a strong correlation between ADPI and ventilation air mixing, or Air Change Effectiveness, and this issue is expected to be included in the upcoming revision to ASHRAE Standard 62 on indoor air 3. Comfort Chart with ADPI ParallelogramTable 1.

9 Effect of Velocity and Temperature on ComfortLocal Velocity fpmTemperature F(tX - tC)Percentage Objecting8075020%6074-120%4073-220%1571- 420%6075010%407505%30750 Neutral15753 WarmBasic Principles of Air Distribution (continued)10%-20-6-402440%Neck region2040608030%20%100of warmthcoolnessFeeling ofFeeling = -3F = +2 FLocal Air Temp. minus Ambient , F(tX - tC)B9 Redefine your comfort zone | guidelinesBGRILLES AND DIFFUSERSFANGER S COMFORT INDEXA third comfort index which is used frequently in reports to compare research results is Fanger s procedure which is the basis for ISO Standard 7730. Fanger s method determines the Predicted Mean Vote (PMV) and the Predicted Percentage of Dissatisfied (PPD) at each of a number of measuring points uniformly distributed throughout the occupied Fanger equation includes thermal parameters not considered in the ADPI (relative humidity, mean radiant temperature, clothing insulation, and activity levels).

10 The PMV approach has the advantage of providing a single number rating combining all comfort elements. The ASHRAE 55 and ISO 7730 Standards yield essentially the same space conditions for acceptability. ASHRAE 55 incorporates PMV in its current revision. From both ASHRAE and ISO standards, the estimated comfort of 80% of the individuals in a space can be plotted. From a program developed as part of the ASHRAE 55 review process, a consensus computer program was developed and published. This program has been used to plot data on a psychrometric chart for two sets of typically occurring conditions: Condition 1 (Executive): Met Rate = (Typical for office) Clothing Rate = (Shirt, tie, long pants, socks) Air Speed = 20 fpm (Typical interior office) Condition 1 (Clerical): Met Rate = (Sedentary) Clothing Rate = (Skirt, blouse, no socks) Air Speed = 20 fpm (Typical interior office)It can be seen from this graph that a single setpoint, such as 75 F, 50% RH is not likely to satisfy even 80% of all individuals in a COMFORT GUIDELINESMost published guidelines for comfort suggest the below conditions are maintained, adjusted for seasonal and occupational clothing and activity levels:HEATINGG enerally, during heating, local air velocities are low, often below 30 fpm.