1、OR-05-8-2 Displacement with Induction: Conditioning Our Classrooms in Accordance with ANSIIASA S12.60 Kenneth J. Loudermiik Associate Member ASHRAE ABSTRACT Designers of HVACsystems serving educational facilities face considerable acoustical challenges in applying equip- ment that complies with the
2、acoustical levels specified in ANSI/ ASAStandardSl2.60-2002. Like allANSIstandards, the impo- sition of S12.60 is volunta y; therefore, its evolution as a regu- latory document is likely to he a gradualprocess. Howevec the lobby that led to its adoption over many industry concerns is strong and will
3、 likely speed its integration and the need for HVAC systems that comply with its mandated space noise levels. Displacement with induction ofers designers an oppor- tunity to economically design compliant systems usingproven current day technologies. INTRODUCTION ANSIIASA Standard S12.60-2002 was dev
4、eloped as a result of lobbies seeking to reduce noise levels in the nations classrooms. Its objective was to ensure that the educators speech could be clearly understood by almost all of the students within the classroom when delivered at levels that do not require electronic amplification nor cause
5、 undue vocal stress of the educator. The introduction of the standard in 2002 was made amid a great deal of industry controversy as it stip- ulated the maintenance of space noise levels not exceeding 35dBA (about NC27) in core learning areas. Conformance to the standard essentially precludes the use
6、 o unit ventilators and other packaged equipment within the classroom as suffi- cient insulation andor isolation of these noise sources cannot feasibly be accomplished (Baker 2003). Fan coils and heat pumps serving the classroom would also have to be located outside the space and ducted to allow for
7、 an appropriate level of attenuation prior to discharge within the space. It should be noted that the 2003 ASHME Handbook-Applications recommends that classrooms be designed for acoustical levels conforming to the standard. The maintained delivery of outside air in conformance to ANSUASHUE Standard
8、62-2001 is also a frequently debated issue in classroom design. Cost issues (construction and operational) associated with the delivery of significant outside air quantities result in widespread cases of aciual deliveries far below the mandated volume. A DOE study (Fischer and Bayer 2003) of ten sch
9、ools in Georgia indicated that only 20% of the schools maintained ventilation rates in accordance with Standard 62 (1 5 CFM or 25 m3/h per person). In five of the eight non-complying schools, actual ventilation rates were 6 CFM per person or less. Perhaps there is some correlation to the fact that G
10、eorgias average 1999 school construction cost (inclusive of land, building, furniture, etc.) of $75.63/ft2 (S8i4.00/m2) was also only 60% of the national average of $127.00/ft2 ($ 1366.00/m2) (Governors Commis- sion 2000). In 1994, a member of the Georgia Building and Mechanical Task Force requested
11、 a committee interpretation (Interpretation IC62- 1989-1 5) of ANSUASHUE Standard 62-1989 to allow the maintenance of ventilation rates of 7.5 CFM (12.5 m31h) per person. A favorable interpretation was rendered based on the allegation that the classrooms were (a) occupied or less than three hours co
12、ntinuously and (b) the classrooms are occupied less than half of the total time the mechanical system is running. Kenneth J. Loudermilk is vice president of technology and development at TROX USA, Alpharetta, Ga. 740 02005 ASHRAE. Almost all North American schools are served by mixed air diffusion s
13、ystems. Mixed systems (while operating in a cooling mode) introduce conditioned air at discharge velocities of 250 to 300 Ipm (1.25 to 1.5 ds) and supply air temperatures of about 55F (13C). They rely on relatively high discharge velocities to entrain room air and mix it thoroughly with the supply a
14、ir near the point of discharge. Residual air movement caused by this induction creates well-mixed conditions and uniform temperature and contamination levels throughout the space. This contaminant removal method is referred to as dilu- tion ventilation. It should be noted that the upper limit of ven
15、ti- lation effectiveness in a mixed system is 1 .O (although this is seldom obtainable). Space temperature control may be accom- plished by (a) varying the delivered air volume at a constant supply temperature, (b) varying the syupply air temperature at a constant air volume, or (c) varying both the
16、 supply air volume and temperature. Varying the supply air volume makes it almost impossible to maintain mandated outside airflow rates, while keeping the air volume constant often results in increased energy usage as these constant air volume deliveries (typically 2.5 to 3 times the space minimum v
17、entilation rate) must be maintained at all times, regardless of load. Variations in space airflow rates also result in proportional variations in space RH during humid operational periods. DISPLACEMENT AIR CONDITIONING Displacement air conditioning has been widely applied in Europe as a method of pr
18、oviding high levels of comfort and ventilation effectiveness. It relies on natural stratification to transport conditioned air through the space. Cool air at 63F to 68F (1 7C to 20C) is supplied at very low discharge veloc- ities (50 to 70 fpm 0.25 to 0.35 ds) from low sidewall or floor based outlet
19、s. The low velocity is not sufficient to create significant entrainment of room air, thus the supply air main- tains most of its thermal integrity as it falls and spreads across the floor. This air is confined to the lower extremities of the space by warmer (ambient) air above it. Figure 1 illustrat
20、es the operational principles of a displacement conditioning system. Occupants and electrical equipment in the classroom transfer heat to the ambient air by natural convection. This convective transfer (in the absence ofrandom velocity vectors) results in the formation of thermal plumes along the bo
21、und- aries of the heat sources, which rise through the upper parts of the space (gradually increasing in volume as they rise) until they either encounter equally warm air or reach the overhead return outlet. Cool air from the floor (drawn upward as the plume forms) passes over the boundaries of the
22、heat source, conditioning it and, in the case of the space occupants, provid- ing the source of inhaled respiration. Exhaled air is warmer than ambient and is thus conveyed with the rising thermal plume directly to the upper portion of the space where it can be easily removed. No horizontal transpor
23、t of the respiratory contaminants occurs in such a system. A number of European publications document design parameters for displacement conditioning systems. In 2002, a Figure I Principles of displacement conditioning. 5 E m E II m ._ O I o - . O m - m a E O0 O1 02 03 04 O5 OB O7 O8 OS Figure 2 Tem
24、perature and CO, gradients for displacement conditioning (applicable to rooms with 9 to 10) 2.7 to 3.0 m ceiling heights). European design guide for displacement ventilation was published (Skistad et al. 2002). In this guide, the principles of displacement and thermal plume theory are described in v
25、ery understandable detail and examples are provided of typical applications in non-industrial spaces. For a classroom with conventional (9 to 10 ft 2.7 to 3 m) ceiling height, a single sunlit exposure, and typical occupant and equipment loading, the guide suggests that the vertical temperature gradi
26、ents may be modeled by using a “50/50” rule as shown in Figure 2. This assumes that one half of the total space temperature differen- tial (TEXHAUST- TsuppLy) is dissipated in the supply air zone, which is bounded by the point where the air exits the diffuser to that at which it reaches the ankle le
27、vel of an occupant some 3 ft (1 m) away. The remaining gradient is relatively linear from the ankle level (4 in. O. 1 m) to the ceiling; therefore, the differential between the ankle and the mid-level (4 to 5 ft 1.4 to 1.5 m) of the space (referred to as the occupied zone) repre- ASH RAE Transaction
28、s: Symposia 741 sents 20% to 25% of the total supply to exhaust temperature differential. ANSUASHUE Standard 55- 1992 prescribes that this occupied zone gradient should not exceed 5F (2.8 K), so a supply to exhaust differential of 20F to 25F (1 1 to 13 K) would be allowable. However, a reasonable se
29、paration distance between the outlet and the nearest occupant would be recommended for such low (60F 16“C) room to supply temperature differentials. In cases where outlets are located within 3 ft (1 m) of space occupants, it is recommended that room to supply temperature differentials be limited to
30、no more than about 13F (7 K), resulting in a maximum supply to exhaust temperature differential of about 17F (9 K). Figure 2 also illustrates contaminant (CO,) distribution gradients associated with displacement ventilation systems. The single vertical passage of air through the space theoreti- call
31、y enables attainment of classroom ventilation effective- ness ratings as high as 1.5 to 1 .8 with displacement systems (Chen and Glicksman 2003). In addition, research shows that (due to the plume effect) contamination levels at the breathing level near space heat sources is only 20% that of ambient
32、 air at similar elevations when supply airflow rates of 2 (or more) air changes per hour are maintained (Etheridge and Sandberg 1996). This improvement in contamination removal coupled with inherently low acoustical levels (due to such low discharge velocities) has recently sparked considerable Nort
33、h American interest in displacement systems. DISPLACEMENT SYSTEMS IN NORTH AMERICAN CLIMATES Despite its significant advantages, the adoption of displacement conditioning in North America has been slowed due to capacity and dehumidification issues, some of which are real and others of which are over
34、stated. Supply of air at significantly lower (room to supply) air temperature differen- tials than those used with mixed systems concerns designers that displacement systems will require significantly greater (almost double) supply airflow rates. In fact (as shown above), the true capacity of the ai
35、r to remove heat from the space is proportional to the temperature differential between supply and return air. As vertical stratification in displacement systems typically creates return temperatures 5F to 6F (2.8 to 3.3 K) warmer than that at the mid-level of the classroom, the temperature differen
36、tial between supply and return is typi- cally 15F to 17F (8 to 9 K), so the airflow typically required is only 15% to 25% greater than that for mixed systems supplying air at a temperature differential of 20F (1 1 K). Dehumidification is yet another issue. Most North Amer- ican climates are sufficie
37、ntly humid that the outside air must be cooled to saturation (at 52F to 54F 11C to 12“CI) in order to sufficiently dehumidiQ the air to maintain recom- mended space humidity levels (50% to 60% according to the 2003 ASHRAE Handbook). The higher discharge tempera- tures used by displacement systems re
38、quires that this outside air be cooled to saturation, then somehow reheated prior to introduction to the space. This is commonly accomplished by bypassing a portion of recirculated return air around the cool- ing coil and mixing it with the saturated air mixture prior to delivery to the space. Altho
39、ugh this is reasonably accom- plished, it usually requires a custom air-handling unit (with more controls), increasing the central plant costs and making maintenance and operation of the unit more complex, neither of which is desirable for school applications. Such an air- handling unit configuratio
40、n is illustrated in Figure 3. Another issue regarding displacement systems is that they cannot reasonably be used for providing heat to the classroom. When conditioned air is discharged at temperatures greater than that of the room, it rises and bypasses most of the occu- pied level. Therefore, prop
41、er space ventilation requires that a I Figure 3 Typical air-handler conjguration for displacement ventilation. 742 ASHRAE Transactions: Symposia 1 I f I Chllled Water Return 1 Tsw = 64 to 68-F (18 to 20C) 1-1 II Figure 4 Displacement terminal with induction. separate (electric resistance or hydronic
42、) heating system be supplied to accomplish simultaneous heating while the displacement outlets continue to supply air at temperatures marginally below that of the room. Finally, displacement systems are capable of achieving the aforementioned ventilation effectiveness ratings (of I .5 to 1.8) only w
43、hen 100% outside air is supplied. A recent adden- dum to ASHRAE Standard 62 suggests that the ventilation effectiveness of displacement systems be assumed to be 1.2. The difference between this assigned value and the higher levels listed as achievable (Chen and Glicksman 2003) reflects the use of re
44、circulation of return air at the air-handling unit, which is common to most North American displacement systems operation. DISPLACEMENT WITH INDUCTION: A SOLUTION FOR HUMID CLIMATES The incorporation of induction nozzles within the displacement terminal affords an effective solution to the aforement
45、ioned issues of displacement conditioning in North American climates. Figure 4 illustrates a terminai fitted with a hydronic sensible cooling coil. Supply water to this coil should be maintained at least 1.5“F (i K) above the space dew point to avoid any condensation from forming during normal opera
46、tion of the system. In any case, a condensation tray should be provided to remove any condensation that might occur during start-up. Primary (100% outside) air is cooled and dehumidified at the central air-handling unit, then ducted to the terminal and injected through a series of nozzles to induce
47、room air (at an induction ratio of about two parts room air per part primary air) through the sensible cooling coil. The induced air is conditioned, then mixed with the primary air to affect a constant volume discharge mixture (about three times the primary air volume) at a temperature that varies b
48、etween 62F and 68F (i 7C and 20C). In most applications, space sensi- ble cooling loads can be satisfied using a primary airflow rate equal to (or very near) the mandated space ventilation rate. In the most extreme cases, the primary airflow rate may exceed the ventilation rate by 40% to 50% (althou
49、gh this airflow is still only about 40% of that required by a mixed system handling the same cooling load). Due to the reduced dehu- midified (primary) airflow rate, the outdoor air is cooled to saturation at a temperature (50F to 54F or 10C to 12C) slightly cooler than that associated with mixed systems. Space relative humidity levels will also tend to be slightly higher than those achieved by mixed systems (50% at design airflow); however, they remain constant (due to the constant volume primary airflow delivery) within the space RH recom
