1. HCB 3-Chap 20A: Room Air Distribution 1 Chap 20A: ROOM AIR DISTRIBUTION METHODS Agami Reddy (July 2016) Common types of diffusers and grilles Basic concepts and purpose Room air distribution concepts - Free jet behavior, throw, spread, Conda effect Classification of air distribution methods Fully mixed air distribution - Concepts of EDT, ADPI, NC - Design guidelines and procedure Underfloor air distribution (UFAD) Displacement ventilation

2. HCB 3-Chap 20A: Room Air Distribution 2 Diffusers and grilles -Ceiling mounted -Connected to distribution system above ceiling -Can be square or round -Can have diffuser vanes -Can have asymmetric throw Registers are grilles with adjustable dampers FIGURE 20.9 Common types of registers and diffusers FIGURE 20.10 Cross-sectional sketch of ceiling diffuser.

3. HCB 3-Chap 20A: Room Air Distribution 3 Simplified Residential Design

4. HCB 3-Chap 20A: Room Air Distribution 4 Purpose of Room Air Distribution -Want to create proper temperature and humidity throughout room by creating air motion -Need to avoid draft, gusts, stagnant air pockets and noise DESIGN CHOICES -Selection of type and location of diffusers -Selection and location of return air grilles -Size of such devices to deliver required air -Pressure loss criteria -Noise criteria

5. HCB 3-Chap 20A: Room Air Distribution 5 Basic Flow Patterns The location of air distribution devices in the room has a large effect on air flow patterns High wall location For good air distribution: -ceiling outlet location for cooling - floor or sill location under window for heating

6. HCB 3-Chap 20A: Room Air Distribution 6 Figure shows how a jet of air issuing from a duct outlet on the vertical wall behaves upon entering a room. The diagram shows that an isothermal jet (i.e., the temperature is the same as that of the room air) slowly diffuses into the surrounding still air. There are no buoyancy effects and so the centerline of the jet remains horizontal. The throw is the distance which the jet travels before the velocity reduces to a predefined value (usually, 50 ft/min or 0.1m/s, 100 ft/min, 150 ft/min or 200 ft/min as per ASHRAE Standard 70). Spread: The jet of primary airstream diverges (average of about 22 o ) due to entrainment of room air as a result of air friction. This gradually reduces the jet velocity while increasing the jet diameter. We need to understand behavior of air jets in rooms

7. HCB 3-Chap 20A: Room Air Distribution 7 Zone 2: a transition zone whose length is dependent on the type of outlet, its aspect ratio, initial flow turbulence,… Zone 3: the zone of greatest importance in terms of room air distribution is where fully turbulent flow is established, with the length being from 25 to 100 diameters of the outlet; Zone 4: the zone where the jet velocity (and temperature) degrade precipitously and lose noticeable form. A lower threshold of 50 ft/min is usually assumed since people tend to sense air movement above that value. Traditionally the full length of air jets can be divided into four zones: Zone 1: a short core zone closest to the outlet face wherein the velocity (and temperature for a non-isothermal jet) remain essentially constant;

8. HCB 3-Chap 20A: Room Air Distribution 8 Coanda Effect When a jet is projected close to (less than about a foot or 0.3 m) and roughly parallel to a room surface (either ceiling or vertical wall). The entrainment is limited to one side only, and the jet tends to “stick ” to the surface (in this figure, the ceiling) because of the negative pressure created between the jet and the surface. Though the velocity is lower as compared to the free jet case, the throw of the jet is increased while the drop for horizontal jets is reduced. This phenomenon can be used to advantage by the designer while selecting diffuser sizes and their placement since the supply air under the ceiling can be made to spread out further than otherwise, thereby having to specify fewer diffusers for the space.

9. HCB 3-Chap 20A: Room Air Distribution 9 Classification of air diffusion methods Typical occupied spaces can be viewed as consisting of: -occupied zone where occupants normally reside. The head, or specifically the nose height is taken to be 67 in (1.7 m) for standing occupants (which could be altered depending on type of space occupancy). - unoccupied zone: higher spaces and the volume which includes a perimeter strip of 3.3 ft ( 1 m) from any exterior wall or windows plus that 1 ft (0.3 m) of any internal wall is assumed to be unoccupied.

10. HCB 3-Chap 20A: Room Air Distribution 10 Room air distribution strategies are classified into three types: Fully mixed systems: room air fully mixed - little or no thermal/pollutant stratif. vertically -achieved by supplying large amounts of conditioned air into the room either from overhead or underfloor -conditioning of the space achieved by diluting space air with the supply air. Partially mixed systems: this arises when the occupied space is fully mixed and maintained at a condition distinctly different from the unoccupied zone. The unoccupied volume is usually stratified into three zones whose relative lengths may vary. Most underfloor air distribution systems are examples of this type. Fully stratified systems: the space is fully stratified vertically, i.e., there is a distinct thermal gradient which arises naturally by distributing the air into the room using a technique called displacement ventilation, FIGURE 20.6 Vertical temperature stratification profiles in a room caused by different room air distribution methods

11. HCB 3-Chap 20A: Room Air Distribution 11 Fully Mixed Room Distribution Systems Proper room air distribution is an interplay of three types of forces. (a)how the heated/cooled jet pumped into the room by the supply fan through the diffusers interacts with the room air (or in case of natural ventilation, the opening of windows and doors). (b)the interaction of the buoyancy or natural convection effects caused by temperature differences between room air and room surface temperatures (such as heat losses through windows in winter causing the window pane to be cooler than other room surfaces). (c) influence due to furniture and partitions in rooms

12. HCB 3-Chap 20A: Room Air Distribution 12 Some designers use general guidelines for first deciding on diffuser locations based on buoyancy considerations. -For example, in locations requiring more heating than cooling (say HDD > 2,000 o F-days), floor or sill diffusers located under windows is the better choice, -For locations requiring more cooling than heating (say, HDD < 2,000 o F-days), ceiling distribution is preferred. -Supply airflow rates in excess of 3 cfm/ft 2 (values in typical office buildings are at most 1.0-1.5 cfm/ft 2 which translates usually to 5-15 ACH) require special care in their air distribution system design. - For that matter, clean rooms or computer server rooms may require upto 8-10 cfm/ft 2.

13. HCB 3-Chap 20A: Room Air Distribution 13 Figure illustrates the manner in which air introduced into a room from a floor perimeter diffuser fans across the room with a thermostat set at “T”. The contour for the air distribution is drawn for air velocity of 50 ft/min which is the acceptable value for human comfort. Note the Coanda effect Note the large difference during heating Fig. 20.17 Airflow patterns and vertical thermal stratification for a floor- perimeter diffuser under heating and cooling modes

14. HCB 3-Chap 20A: Room Air Distribution 14 Ceiling diffusers are more common in commercial spaces since the ductwork can be hidden behind the false ceiling Under cooling mode, Coanda effect helps to better spread the jet across the ceiling, and cold air drops due to buoyancy forces. The room is well mixed and there is no stratification in the occupied zone of the room. However, under heating mode, the buoyancy force is so dominant that the supply air does not mix well with the room air- very pronounced thermal stratification. This would be unacceptable for occupant comfort, and so a separate perimeter heating system may have to be specified. Figure 20.8 Air flow patterns created from a ceiling diffuser under cooling and heating modes.

15. HCB 3-Chap 20A: Room Air Distribution 15 For Commercial Buildings Effective draft temperature (EDT) is a standard measure of the effective temperature difference between any point in an occupied space and the control conditions: The empirical constant M = 7.0 o C.s/m (0.07 o F.min/ft) when dry bulb temperatures are in o C ( o F) and velocities in m/s (ft/min). It has been found empirically that a majority of people involved in sedentary activities are comfortable when EDT is: between -1.7 o and 1.1 o C (-3 o and 2 o F) when the air velocity < 0.36 m/s (70 ft/min).

16. 16 Air Distribution Performance Index (ADPI) Estimate of % of people who will be comfortable as per EDT criteria It is a statistical property: ADPI = 100% implies that 0% of people objecting ADPI is function of local velocity and DBT (not of RH or MRT) ADPI used primarily to select THROW of diffuser People tolerate higher velocities and lower temperatures at ankle level Heavy heating and cooling loads tend to lower ADPI FIGURE 20.12 Effect of room air velocity on occupant comfort for ankle and neck regions

17. HCB 3-Chap 20A: Room Air Distribution 17 For a quiet room the NC value should be low (15- 30), for offices (35-40), while for a factory it can be high (45-60). Noise Criteria Noise is produced by the air diffuser and ductwork -It can be objectionable -Method to evaluate noise level is the Noise Criteria (NC) -Single measure which aggregates several frequency levels -Manufacturers provide NC ratings However, some amount of noise is desirable both psychologically and thermally. Occupants feel reassured that conditioned air is being supplied to the room when they hear the noise from the distribution systems, and tend to complain less. An oversized diffuser will be so quiet that almost no noise is heard.

18. HCB 3-Chap 20A: Room Air Distribution 18 Air distribution system design procedure The design process does not lead to an unique solution, and so numerous possible good design solutions are possible. These feasible solutions must all meet certain performance criteria, from which personal preferences of the designer would dictate final selection. a)For the given room, determine the room air volumetric flow rate at summer and winter design conditions. b)Select diffuser type, approx. number and placement based on the room configuration. For example, for a square room, one could use a single ceiling circular diffuser or if the room is large, use four uniformly placed diffusers. c)For a rectangular room of aspect ratio close to 2, one would assume two locations, and so on. d)Determine the characteristic length L depending on type of diffuser and their characteristics from Table 20.2

19. HCB 3-Chap 20A: Room Air Distribution 19 Diffuser Type Approximate Air Loading per Floor Space L/(s.m 2 ) [cfm/ft 2 ] Characteristic Length L High sidewall grille 0.6 – 1.2 [3-6] Distance to wall perpendicular to jet Circular ceiling diffuser 0.9 – 5.0 [5-25] Distance to closest wall or midway to nearest ceiling diffuser Sill grille0.8 – 2.0 [4-10]Length of room in direction of jet Table 20.2. Common Diffuser Types and their Characteristic Lengths

20. HCB 3-Chap 20A: Room Air Distribution 20 e) Determine the value of diffuser throw T 0.25 (or T 50 ) which maximizes ADPI for each diffuser using manufacturer catalog tables (Table 20.3) Often the optimum value cannot be met, and so the last two columns in the table give the acceptable range of values for the throw ratio which will yield an ADPI value greater than the value listed in the last but one column. f) Select the appropriate diffuser for each location from the manufacturers’ catalogs based on the flow rate and throw determined earlier. Table 20.4 is applicable for round diffusers, while similar tables for other diffuser types can also be found in the literature. g) Check using tables such as Table 20.4 whether the corresponding pressure drops and NC values are satisfactory under both peak design conditions. Iterate if necessary.

21. HCB 3-Chap 20A: Room Air Distribution 21 Terminal Device Room Load W/m 2 (Btu/hr.ft 2 ) T 0.25 /L (T 50 /L) Max. ADPI* Maximum ADPI For ADPI Greater Than Range of T 0.25 /L High sidewall grilles 250 (80) 190 (60) 125 (40) 65 (20) 1.8 1.6 1.5 68 72 78 85 - 70 80 - 1.5-2.2 1.2-2.3 1.0-1.9 Circular ceiling grilles 250 (80) 190 (60) 125 (40) 65 (20) 0.8 76 83 88 93 70 80 90 0.7-1.3 0.7-1.2 0.5-1.5 0.7-1.3 Sill grille, straight vanes 250 (80) 190 (60) 125 (40) 65 (20) 1.7 1.3 0.9 61 72 86 95 60 70 80 90 1.5-1.7 1.4-1.7 1.2-1.8 0.8-1.3 Table 20.3 Diffuser Selection Guidelines(ASHRAE Fundamentals, 2013) * T 0.25 = Throw to 0.25 m/s velocity, T 50 = Throw to 50 ft/min velocity

22. HCB 3-Chap 20A: Room Air Distribution 22 SizeNeck velocity, ft/min5006007008009001000 8” Total pressure, in. WG Flow rate, cfm Throw, ft NC 0.052 175 4 15 0.075 210 5 21 0.101 245 6 26 0.130 280 7 31 0.166 315 8 34 0.205 350 9 37 10” Total pressure, in. WG Flow rate, cfm Throw, ft NC 0.043 270 5 11 0.062 330 6 17 0.084 380 7 21 0.108 435 8 26 0.138 490 9 30 0.170 545 10 33 12” Total pressure, in. WG Flow rate, cfm Throw, ft NC 0.042 390 6 11 0.060 470 7 17 0.081 550 8 22 0.105 630 10 26 0.134 705 11 30 0.166 785 12 33 14”Total pressure, in. WG Flow rate, cfm Throw, ft NC 0.061 530 8 18 0.087 635 9 23 0.118 745 11 28 0.152 850 12 32 0.194 955 14 36 0.240 1060 15 40 Table 20.4 Typical Circular ceiling Diffuser Catalog Data

23. HCB 3-Chap 20A: Room Air Distribution 23

24. HCB 3-Chap 20A: Room Air Distribution 24 c) The characteristic length L = 25/2 = 12.5 ft d) From the third column of Table 20.3, the optimum value for T 50 /L for circular diffusers in 0.8, irrespective of the room loading. e) The throw to 50 ft/min for maximum ADPI should be = 12.5 x 0.8 = 10.0 ft. For the room whose loading is 20 Btu/(hr-ft 2 ), the maximum ADPI from Table 20.3 is about 93. f) The air flow rate through each of the two diffuser = (25 ft x 25 ft).(1.02 cfm/ft 2 ) = 636.55 cfm. Since a throw of 10 ft is desired, we find from Table 20.4, that we could use a 12” size with a neck velocity 800 ft/min and NC = 26, or a 14” size with a neck velocity of 66 ft/min with NC=23. The NC values of both these selections are acceptable for a quiet office setting. The pressure drops for the two sizes are 0.105 in-WG and 0.087 in-WG, and, the designer could opt to go with the one with the smaller pressure drop.

25. HCB 3-Chap 20A: Room Air Distribution 25 Underfloor Air Distribution (UFAD) The fully mixed air delivery system is an excellent system to meet the thermal needs of the space. Also, UFAD generally superior in terms of contaminant removal and occupant comfort. Tends to be quieter and more energy efficient (not in all cases though) Only amount of air needed to meet the minimum ventilation codes of the space is passed (typically 2.5-3 ACH as compared to 5-15 ACH for traditional fully-mixed HVAC distribution systems), and further can operate with higher supply air temperatures (60 – 65 o F)

26. HCB 3-Chap 20A: Room Air Distribution 26 UFAD becoming increasingly popular is America and Europe, especially for cooling the space. Conditioned air is supplied vertically into the occupied space from the underfloor plenum (an open airway created between a structural concrete slab and the underside of a raised floor which is slightly pressurized. No hard connections to the diffusers as in ducted overhead distribution systems. Depending on the throw of the floor diffusers from the underfloor plenum, the room could be made to behave either as a fully mixed space, or as a partially stratified space FIGURE 20.13 A typical UFAD space with representative air temperatures and a phantom line separating the occupied and unoccupied zones

27. HCB 3-Chap 20A: Room Air Distribution 27 Four of the common types of diffusers are shown : - Variable area square VAV directional diffuser that automatically vary the diffuser opening ratio - Perimeter linear bar grills - Interior swirl diffusers with vertical throw - Horizontal discharge swirl diffusers Air can be delivered through a variety of supply outlets typically located at floor level, or integrated as part of the office furniture and partitions. Return grilles are located at ceiling level, or at least above the occupied zone. FIGURE 20.14 Four different types of UFAD diffusers

28. HCB 3-Chap 20A: Room Air Distribution 28 Typical Office with UFAD and Typical UFAD Diffuser ( from Trox)

29. HCB 3-Chap 20A: Room Air Distribution 29 Fig. 20.15 Sketch of a partially mixed room with under floor air distribution showing different zones and air flow patterns Often, the UFAD design involves one diffuser per occupant whose flow can be adjusted by the occupant as needed. Stratification is affected by the air velocity. -If airflow is too great, then there is little stratification, and much of the energy savings benefits of UFAD are lost. -If airflow is too low, the resulting stratification will create a greater temperature difference between the ankle and the head which will be unacceptable if it exceeds 4-5 o F. The design intent is to maintain this temperature differential by suitably adjusting the supply air velocity. Clear zone: adjacent zone SH- stratification height 50 fpm occupant comfort velocity

30. HCB 3-Chap 20A: Room Air Distribution 30 Fig. 20.16 Effect of room air flow on thermal stratification using swirl diffusers Profiles measured in a room with total heat load of 5.2 W/ft 2 and supply temperature of 64 o F (18 o C), and room air flows of 1.0, 0.6 and 0.3 cfm/ft 2 (5.2, 3.0 and 1.5 L/s-m 2 ). -Note that thermal gradient is less the higher the air velocity. -At highest flow rate, the temp. diff. Between head-to-foot is about 1.3 o F: over- ventilated space. -Lowest air flow rate, diff close to 7 o F, exceeding the limit of a 5 o F differential -Intermediate flow rate of 0.6 cfm/ft 2 < 4 o F differential which would be acceptable.

31. HCB 3-Chap 20A: Room Air Distribution 31 Fig. 20.17 A typical UFAD space showing the supply plenum and the various heat gains from the space as well as the return air plenum from the floor underneath - UFAD systems are best suited for general open office areas, conference rooms, exhibit spaces, clean rooms, computer rooms, public lobbies. -Areas such as laboratories, child care centers, storage rooms are nor suitable -Construction costs higher, raised floor itself is an additional cost. Benefits: -reduced energy costs due to higher supply temperature, -more hours of economizer operation, -less fan energy, -offers occupants better control of the space, -can be varied easily to accommodate changes in space layout due to remodeling or churn

32. 32 Displacement Ventilation Distribution Ventilation air is supplied close to the floor at very low velocities of 0.15 – 0.25 m/s (50-75 fpm or less) at relatively high supply temperatures 18-20 o C (65 o F) for space temperatures at 23-24 o C. The cool air tends to hug the floor and form a pool of cold air because of its negative buoyancy. FIGURE 20.19 Schematic of a displacement ventilation system showing the two zones and the creation of the thermal plume around the heat source HCB 3-Chap 20A: Room Air Distribution

33. 33 Displacement Ventilation Distribution At specific locations in the room where the cool air comes in contact with heat sources, it replaces the air around the heat source and rising air produces a vertical airflow pattern near each heat source – thermal plume Originally developed for industrial applications- now adapted to commercial buildings with high ceilings (10 ft) HCB 3-Chap 20A: Room Air Distribution

34. 34 Displacement Ventilation Fig. 20.20 Typical Displacement Ventilation Diffusers Warm contaminated air accumulates in a fully mixed space close to the ceiling (this upper zone is outside the occupied zone) and is exhausted near the ceiling. Thus, the DV system is essentially a one pass system with buoyancy assisted mechanical ventilation where the overall floor-to-ceiling air flow pattern is created by natural buoyancy, while the removal of the warm contaminated air is by mechanical exhaust. The low velocity is achieved by passing the supply air through large opening diffusers usually configured as large-area floor pedestals or low side-wall

35. HCB 3-Chap 20A: Room Air Distribution 35 In an ideal displacement ventilation system, all the occupants are exposed to clean cool ventilation air. Once contaminated and heated, the air rises upwards in a plume. However, the air velocities of the plumes are very low, and so even small air currents set up by occupant movement inside the room or door opening/closing can cause the plume to be disrupted and cause undesirable mixing. As for UFAD, only sensible loads are being met by the supply air stream. Displacement ventilation systems aim to minimize mixing of supply air with room air, instead maintaining conditions in the occupied zone as close as possible to that of the conditioned supply air, leading to an improved air change effectiveness. Lights and other heat generating equipment are usually located above the occupied space, and so the supply air stream does not have to meet these loads. Energy for conditioning supply air stream < required for the fully mixed system

36. HCB 3-Chap 20A: Room Air Distribution 36 Several advantages: - Smaller ventilation systems compared to conventional all-air systems - Energy saving benefits similar to UFAD (lower fan energy, higher supply temperature, more economizer hours over year for suitable locations), -Better air quality, and such systems can be used to achieve better smoke control - Low noise (good for classrooms) - Can be used in conjunction with DOAS where required minimum ventilation provided at all times

37. HCB 3-Chap 20A: Room Air Distribution 37 Disadvantages: -Floor space available for occupants is reduced since diffusers cannot be obstructed and occupants cannot be seated near diffusers nor in the adjacent zone -Precise temperature and airflow control is needed to establish and maintain operating conditions -The air pollution in the upstream portion can become very concentrated, and in case they do fall back into the occupied zone, can cause severe occupant discomfort -Limited heating and cooling capacity (meant for low room loads, about 30 W/m 2 ), and so an additional separate system may be needed.

38. HCB 3-Chap 20A: Room Air Distribution 38 Outcomes -Understanding of the purpose and desirable traits of room air distribution -Familiarity with the various design criteria to be met -Understanding of the behavior of isothermal air jets in rooms -Understanding of the Coanda effect -Familiarity with the different types of air diffusion methods -Understanding basic concepts of air jets in rooms: EDT, ADPI, and NC -Familiarity with the design process for fully mixed air distribution systems -Familiarity with the operating principle behind UFAD, behavior of jets, typical system designs and pros and cons -Understanding the operating principle of displacement ventilation, behavior of supply air streams, components and pros and cons