ASHRAE OR-05-8-3-2005 The Effect of Supply Air Systems on Kitchen Thermal Environment《对厨房热环境的供气系统的影响》.pdf

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1、OR-05-8-3 The Effect of Supply Air Systems on Kitchen Thermal Environment Andrey Livchak, PhD Member ASHRAE Derek Schrock Member ASHRAE ABSTRACT A commercial kitchen is a complicated environment where multiple components of a ventilation system including hood exhaust, conditioned air supply, and mak

2、eup air systems work together but not always in unison. That is why many kitchens are hot. A hot and uncomfortable kitchen contributes to productivity loss, employee turnover, and eventuallyprojt loss for the restaurant operator: Using thermal displacement venti- lation in kitchen environment allows

3、 for a reduction in space temperature without increasing the air-conditioning system capacity. Application of two systems (traditional mixing venti- lation system and thermal displacement ventilation system) is compared in a typical kitchen environment using computa- tional fluid dynamics (CFD) mode

4、ling. Often kitchen exhaust hoods are provided with untempered makeup air: It is not uncommon to hear the claim that this makeup air is exhausted through the hood without having any effect on kitchen space temperature. The validity of this claim is analyzed in this paper for two makeup air configura

5、tions using a combination of measured data and results from CFD models. Kitchen space temperature increase is calculated as a result of supplying unconditioned makeup air during the summer. INTRODUCTION It has been well documented that temperature affects productivity. If the temperature in the spac

6、e increases by 10F (5.5“C) above the comfort level, the productivity may drop as much as 30% (Wyon 1996). Take, for example, a well- designed and comfortable kitchen staffed with seven employ- ees. If the temperature in this kitchen increased by 10F (5.5“C) the manager would have to hire three more

7、people to do the same job. Zeqiang Sun Associate Member ASHRAE Besides the productivity losses, high temperature in a kitchen also contributes to a very high turnover rate. In the restaurant industry, on average, four persons per year will be hired and trained for the same job position. It is no won

8、der that the National Restaurant Association identified the single most critical issue facing the restaurant industry to be hiring and retaining a professional kitchen staff (National Restaurant Association 200 1). A modern commercial kitchen is often characterised by high heat loads and air change

9、rates. All cooking appliances release heat into the kitchen space in the form of convection and radiation. Kitchen hoods are designed to localize and Andrey Livchak is the director of research and development and Derek Schrock and Zeqiang Sun are engineers at Halton Company, Scotts- ville, Ky. 748 0

10、2005 ASHRAE. capture convective heat and cooking effluents rising from hot cooking surfaces. Radiation heat from cooking appliances under the hoods, as well as heat from other sources in the kitchen (lights, people, heat transfer through the building envelope, heat from other equipment installed apa

11、rt from the hoods), are transferred to the kitchen and have to be accounted for as load to the space. This load is used to properly size the air-conditioning system and achieve the desired temperature in the space. If this load is underestimated, the air- conditioning system will be undersized, and

12、it will lack the cooling capacity to reach design temperature in the space-the kitchen will be hot. All the air exhausted from the kitchen through the hoods has to be replaced with outside air. There are three sources of replacement air in an air-conditioned kitchen: 1. outside air portion of supply

13、 air delivered by the air-condi- tioning system transfer air from adjacent dining room a dedicated kitchen makeup air unit The first two sources deliver conditioned air (cooled in summer) and the third usually brings untempered (not cooled in summer) air into the kitchen. The amount of outside air t

14、hat has to be cooled is the primary factor affecting cooling capacity of the air-condition- ing system for a dominant part ofthe US with summer temper- atures above 78F (26C). The higher the cooling capacity requirement of the air-handling unit (AHU), the higher its operating costs and energy consum

15、ption. That is why it is a common practice to bring most of the replacement air in the kitchen through a dedicated makeup air unit that doesnt condition the air-a solution that is inexpensive and not always effective. It is not uncommon to see a hood manufac- turer claim that this makeup air can be

16、delivered close to the hood in such a way that this air is being exhausted through the hood and has no affect on kitchen space air temperature. A recent commercial kitchen ventilation study (CEC 2002) tested the impact of various makeup air systems on hood capture and containment. A few systems, inc

17、luding rear discharge (back drop plenum) and perforated perimeter supply (PPS), were identified as least affecting hood performance. Schrock (2002), studying thermal comfort in kitchen, discov- ered that outside air, hot and humid in summer, supplied through the back drop plenum ends up in the kitch

18、en, thus being the main reason for a hot kitchen. Vanstraten and Brown (2003) came to the same conclusion solving the makeup air problems for a family chain restaurant. Field practice and experiments have shown that to date there isnt a method of bringing unconditioned makeup air into a kitchen such

19、 that it doesnt have an effect on the temperature and humidity in the space. It is not a question ofwhether or not the space temperature will increase, but rather how much the air temperature in the kitchen would increase when uncondi- tioned makeup air is supplied into a kitchen on a hot summer day

20、. 2. 3. PERFORMANCE ANALYSIS OF THE PPS SYSTEM As discussed earlier, one makeup air system that is being recommended because it was found to have minimal impact on the hood performance is the PPS system that introduces air from a horizontal plenum (installed near the ceiling) in a downward direction

21、 (CEC 2002). It was found that PPS has little or no impact on the hood performance at 150 fpm (0.762 ds). However, one key parameter that has a large impact on how the PPS system performs in conjunction with the exhaust hood is the supply temperature of the air being introduced. For the CEC study, t

22、he air was introduced in a thermally neutral condition of 75F (23.9“C). To test the impact of the PPS system on the hood perfor- mance and kitchen comfort under different supply tempera- tures, four CFD scenarios were modeled using a commercial CFD code: 1. An exhaust-only style hood. 2. A PPS syste

23、m supplying air at 75F (23.9“C), which is similar to the configuration tested by Fisher. 3. A PPS system supplying air at 50F (10.0“C). 4. A PPS system supplying air at 100F (373C). For all of the PPS configurations, the supply air was intro- duced at a velocity of 150 fpm (0.762 ds) to match what F

24、isher tested in his experiments (CEC 2002). The appliance modeled in the CFD scenarios had a surface temperature of 600F (315.6“C) and the exhaust airflow was set to 248 cfd ft (384 l/(sm) of hood length. The makeup air (when used) was equivalent to 80% of the exhaust airflow. The three-dimensional

25、CFD model for these simulations contained approximately 520,000 elements. The turbulence model used for the simulation was RNG k-E, radiation was turned off, and a first order upwind advection scheme was utilized. The simulation was run in steady-state mode until the convergence criteria of a residu

26、al of 1 E-4 and a global imbal- ance of 0.3% were met. Results were generated for each of the scenarios showing the effect of the makeup air in the space and the comfort level in the kitchen. The exhaust-only system (see Figure 2) shows all of the convective heat from the appliance being captured by

27、 the hood, and the kitchen space air temperature is 75F (23.9“C). For this scenario, no untempered makeup air was introduced to the space. The results with the front PPS supplying 75F (23.9“C) air see Figure 31 are similar in terms of space temperature to the exhaust-only case. However, it is eviden

28、t that the plume inside the hood is being induced slightly by the air from the PPS system as it passes the front of the hood. The results with the front PPS supplying 50F ( 10.0“C) air (as shown in Figure 4) are more dramatic in terms of the space temperature in the kitchen. For this scenario, there

29、 would be cold air bowling downward directly onto the chef, which could cause discomfort. Cold supply air, as it acceler- ates toward the floor, entrains the plume out from the hood. ASH RAE Transactions: Symposia 749 Temperature Temperature (Plane 1) 95.0 88.8 82.5 76.3 70.0 FI Figure 2 CFD simulat

30、ion, case I-exhaust-only hood, Temperature (Plan 1) 95.0 83.7 72.5 61.3 50.0 FI Figure 4 CFD simulation, case 3-Front supply at 50F (I o. o “C) . (Plane i) 95.0 88.8 82.5 76.3 70.0 FI Figure 3 CFD simulation, case 2-Front Supply at 75F (23.9“C). Temperature (Plane 1) 95.0 88.8 82.5 76.3 70.0 FI Figu

31、re 5 CFD simulation, case 4-front supply at 100F (3 7.8“C). 750 ASHRAE Transactions: Symposia Figure 6 Space Temperature vs. room height for various PPS conJigurations. Part of this cold air also impinges on the hot cooking surface and compromises the performance of the cooking appliance. During the

32、 summer a PPS supply 100F (37.8“C) air would have an extremely negative impact on the comfort in the kitchen (see Figure 5). For this scenario, the upper body of the chef would be enveloped by hot, and potentially humid, outdoor air. A comparison of the space temperature as a function of height abov

33、e the floor for the various PPS configurations is presented in Figure 6. This shows that the largest difference in the temperature that the chef is exposed to is in the upper body between the heights of 60 and 70 inches (1.5 to 1.8 m) from the floor. The sampling location for the space temperatures

34、was 1 foot (0.305 m) from the front edge of the griddle to represent where a chef would stand. CALCULATING THE EFFECT OF UNCONDITIONED MAKEUP AIR DELIVERY AND HOOD SPILLAGE ON KITCHEN SPACE TEMPERATURE An energy balance equation (see Equation 1) can be used to calculate the supply airflow and supply

35、 air temperature needed to achieve a desired kitchen design air temperature. This equation assumes that hood is capturing and the untem- pered summer makeup air is captured by the hood without entering kitchen space. where m, = supply airflow, lb/h kg/s cp tr = design kitchen space air temperature,

36、“F “CI tS = supply air temperature, “F “CI m, = transfer airflow, lb/h kg/s = specific heat of air, Btu/(lb.“F) J/(kg.K) ttr = transfer air temperature, “F “CI Qhg = total design heat gain to the space, Btu/h W If the hoods are not sized properly they will spill effluents and convective heat Qsp int

37、o the kitchen; also, in summer, the untempered outside air, delivered through the conventional makeup systems, will add heat to the kitchen Qma. With the added heat gain to the space from Qsp and Q, the source of cooling in the kitchen remains unchanged-it is the same amount of transfer and supply a

38、ir delivered to the space at the same temperature. The new kitchen space air temperature I,! as a result of additional heat gain to the space can be calculated from the following set of equations: 1 1 ms cp(r - tS) + m, Cp(tr - ttr) = Qhg + Qsp + Qma (2) (4) 1 Qma = Mma cp(t,a-tr) where QSp = heat g

39、ain to the space due to the hood spillage- convective heat escaping from the hood, W em, = heat gain to the space from untempered makeup air-hot in summer outside air entering the kitchen space, Btu/h W mma = makeup airflow, lb/h kg/s msp = amount of hot air spilling fiom under the hood, Ib/h = kitc

40、hen space air temperature as a result of additional heat gains to the space, “F “CI = temperature of convective airflow escaping from the hood, “F “CI system, equal to or above the outside air temperature in summer. OF “CI kdsl 1 r tsP toa = temperature of air, supplied through the makeup Solving th

41、e system of equations 1 to 4 allows calculating the new kitchen space air temperature as a result of additional heat gains to the space from hood spillage and untempered makeup air. (5) where X, X, Xsp, Xm, = corresponding ratios of m, mtn msp, m, to the total hood exhaust airflow mhood Equation 5 a

42、ssumes fully mixed conditions -mixing air distribution is used and air temperature tf. is uniform through- out the space. . Usually the air intake for a makeup system is located on the roof where temperature is several degrees above ambient. ASHRAE Transactions: Symposia 75 1 Example 1 A restaurant

43、kitchen air-conditioning system is designed to maintain a space air temperature t,. of 76F (24.4“C). The design is based on an assumption that hoods are not spilling and the untempered makeup air has no effect on kitchen heat gain. Hoods are delivered with the makeup air system sized for 80% of the

44、total hood exhaust, X, = 0.8. The makeup air is untempered in summer and, contrary to common perception, enters the kitchen space before being exhausted through the hood. Transfer airflow amounts to 10% of hood exhaust, X, = O. 1. A standard rooftop packaged air conditioner is used with 25% of outsi

45、de air, with the total supply airflow of 40% of hood exhaust (1 0% outside and 30% return air), Xs = 0.4. Lets calculate the resulting kitchen air temperature t: when the outside air temperature is 96F (35.5“C). In that case temper- ature of air, delivered through the makeup air system, will be at l

46、east tma = 98F (36.7“C). It is not uncommon to see even higher t, since the air intake of the makeup system is on the roof. Using Equation 5, i (0.4 + 0.1). 76 + 0.8 .98 = 89.50F I, = O. 1 + 0.4 + 0.8 i t, = (0.4 + 0.1). 24.4 + 0.8 .36.7 = 32.00c 0.1 + 0.4 + 0.8 that is, 13.5“F (7.6“C) above the des

47、ign temperature. Example 2 Lets use example 1 and assume that hoods are not captur- ing and spilling 20% of convective heat back into kitchen Xsp = 0.2 at tsp = 100F (373C). The resulting kitchen air temper- ature will be i (0.4 + 0.1) . 76 + 0.2 . 100 + 0.8 . 98 = t, = 0.1 + 0.4 + 0.2 + 0.8 1 (0.4

48、+ 0.1) 24.4 + 0.2 . 37.8 + 0.8 . 36.7 = 32.70c 1, = 0.1 + 0.4 + 0.2 + 0.8 that is, 14.9“F (8.3“C) above the design temperature. The effect of spillage alone, considering there is no untempered makeup air, will result in 8F (4.4“C) temperature rise in the kitchen. Productivity Gain Due to Improved Th

49、ermal Comfort High temperature in the kitchen causes thermal discom- fort of employees, leading to productivity loss. As demon- strated in the examples above, it is not uncommon to see temperatures in the kitchen 10F (5.5“C) and more above the comfort level. Such a high temperature may result in produc- tivity loss of 30% (Wyon 1996). According to 2003 Restuu- rant Industry Operations Report, an average fll service restaurant spends 33% of their sales on salaries, wages, and benefits and has a before tax profit of oni