Simulation on energy use for mechanical ventilation and air-conditioning (MVAC) systems in train compartments

Energy 25 (2000) 1–13 www.elsevier.com/locate/energy

Simulation on energy use for mechanical ventilation and airconditioning (MVAC) systems in train compartments W.K. Chow *, Philip C.H. Yu Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, PR China Received 31 March 1999

Abstract Unlike the conventional automotive, modem railway trains are designed with non-openable windows; and a mechanical ventilation and air-conditioning (MVAC) system is installed in each train compartment for better indoor air quality as well as to provide a thermally comfortable environment. The ventilation rate is no doubt a critical element in the design of a MVAC system, especially in Hong Kong where the daily passenger load is extremely heavy. Earlier studies illustrated that carbon dioxide can be controlled at 1000 ppm by increasing the ventilation rate to 25.2 m3 h⫺1; however, it will also lead to an increase in energy consumption. In this paper, the electrical energy consumption at various ventilation rates was studied, and the cost of maintaining a low carbon dioxide level was estimated. These provide solid information for the local railway companies to improve the air quality inside the train compartments.  2000 Elsevier Science Ltd. All rights reserved.

1. Introduction Daily passenger loading of the railway lines in Hong Kong (now the Hong Kong Special Administrative Region) is very heavy. Passengers might have to stay inside the train compartment for up to an hour [1] and some railway lines are constructed underground and some through tunnels. Most of the train compartments are made of metal with fixed windows under normal operation for energy conservation and safety reasons. This design is different from the train compartments 30 years ago with openable windows and natural roof ventilators. Since windows are normally closed, mechanical ventilation and air-conditioning (MVAC) systems are installed to supply fresh air and to provide a thermally comfortable environment. Those systems function well when the train is moving because the outside air can be driven into the compartment easily. * Corresponding author. Tel.: +852-2766-5111; fax: +852-2765-7198. E-mail address: [email protected] (W.K. Chow)

0360-5442/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 9 9 ) 0 0 0 6 1 - 4

2

W.K. Chow, P.C.H. Yu / Energy 25 (2000) 1–13

However, when the train is stationary due to regulation of train service or waiting for the signal to enter the station, fresh air can only be provided through the fans of the MVAC system. When the train is fully-crowded, the amount of fresh air supply from the ventilation system might not be sufficient. The incident of the delay in one of the railway lines in 1996 led to over 90 people experiencing discomfort and the normal operation of the railway line was disturbed for almost half a day [2–4]. Many people blamed the inadequacy of the MVAC design in the train compartment and actions to modify the design have been taken over the past two years. A preliminary discussion [5] based on a simple theory and existing ventilation design criteria was reported. In that study, the carbon dioxide level was identified as the main component for consideration. The well-mixed model was assumed to calculate the possible carbon dioxide level under different passenger loading and ventilation rates. Fresh air must be supplied to the train compartments to dilute the odours, carbon dioxide and water vapour given off by the passengers and other internal contaminants [6,7]. The design of the ventilation systems should keep air contamination or over-heating at an acceptable level in accordance with the ventilation standard. Also, air movement is important as reported by Chow and Fung [8] because higher air velocity is preferred under local conditions. The required ventilation rate per person Qp can be determined by plotting y against Qp (m3 s⫺1) as shown in Fig. 1: (1)

y⫽C/Co

Fig. 1.

Carbon dioxide level in train compartment.

W.K. Chow, P.C.H. Yu / Energy 25 (2000) 1–13

3

where C is the carbon dioxide level inside the train compartment, and Co is the outside atmospheric carbon dioxide level. It was suggested that the ventilation rate of up to 0.01 m3 s⫺1 per person is required to keep the indoor carbon dioxide level y at 10/3%, because the carbon dioxide level would increase significantly if the train compartment is crowded while the ventilation rate is kept constant. The situation would worsen when the train is stationary due to regulation of train service or waiting for the signal to enter the railway station, during which time, fresh air can only be driven into the train compartment by fan. If the fan is inadequate, the intake air flow rate might not be sufficiently high for the passengers because the loading of a ‘fully-crowded’ compartment can be 30% more than the design condition. Obviously, providing a higher ventilation rate will demand higher energy costs to operate the MVAC system. Exact energy usages are not known unless field measurement, energy auditing or energy simulations are carried out. It is expensive to do the actual measurement and this method is not practical in determining the design criteria. An alternative method appropriate at the initial stage of design is to use well-developed energy simulation packages [9]. With the advancement in computer technology, many energy simulation packages which can be run on personal computers are available in the market. For example, TRACE 600 [10–12] is commonly used in local consultant firms for cooling load estimation as well as building energy simulation. They are popular because only minimal training is required for engineers. There are not too many hardware constraints and the programs are becoming more and more user-friendly nowadays. Advanced computer packages like BLAST and DoE-II [13] are much more sophisticated but they are not yet popularly used in commercial building projects. The expected energy cost for running the MVAC system was simulated in this paper. The energy simulation program TRACE 600 [10–12] was selected as the simulation tool for studying the energy use in operating the MVAC system in the train compartments of a local railway line. 2. Selected train compartments A typical train compartment of one of the railway lines is considered. The dimension of one compartment is approximately 22 m long, 3 m wide and 2.5 m high, giving a space volume of about 165 m3. The maximum design capacity per compartment is 270 persons and the maximum number of compartments for each train is 12. However, according to a survey not disclosed publicly, the ‘fully-crowded’ loading is 350. The typical fresh air flow rate is 0.688 m3 s⫺1, giving 15 air changes per hour (ACH) per car. Thus, the fresh air supply during rush hours should be 1.96 l s⫺1 (7.07 m3 h⫺1) per person (i.e. 0.688×3600/350, or 0.04 ACH per person). The value, nearly 8.5 m3 h⫺1 per person, is able to keep the indoor carbon dioxide level down to 0.25%. A suggestion was made to keep the carbon dioxide level down to 0.1% (or 1000 ppm), which provides a higher air flow rate of 25.2 m3 h⫺1 per person (7×10⫺3 m3 s⫺1 per person or 0.15 ACH per person). Therefore, the typical fresh air supply of 7.07 m3 h⫺1 per person during rush hours is insufficient to bring the carbon dioxide level down to 1000 ppm unless a large amount of conditioned fresh air is introduced by increasing the fan size. However, the value recommended by ASHRAE [6] for vehicles, waiting rooms and platforms of transportation is 8.0 l s⫺1 (8×10⫺3 m3 s⫺1 or 28.8 m3 h⫺1) per person.

4

W.K. Chow, P.C.H. Yu / Energy 25 (2000) 1–13

Chow [5] recommended the value of 42.5 m3 h⫺1 (or 11.8×10⫺3 m3 s⫺1 or 0.26 ACH) per person in order that the carbon dioxide level y can be kept at 10/3% as illustrated in Fig. 1. Apparently, higher fresh air supply rate is desired when the carbon dioxide level increases as a result of increasing passenger loading. The three important figures are summarized below: 앫 Typical operation value at present: 1.96×10⫺3 m3 s⫺1 (or 0.04 ACH) per person. A higher level of carbon dioxide is expected but lower energy cost for operating the MVAC systems. 앫 Suggested value: 7×10⫺3 m3 s⫺1 (or 0.17 ACH) per person. It can lower the carbon dioxide level to 0.1%. 앫 Value recommended in earlier studies by Chow [5]: 11.8×10⫺3 m3 s⫺1 (or 0.26 ACH) per person. It came from a preliminary study on ventilation requirement which would give an ideal indoor carbon dioxide level y of 10/3%.

3. Energy simulations Heat gain is negated by the air-conditioning system according to the cooling load calculated and the suitable equipment selected. All of the factors contributing to heat gain in the train compartment were considered. For instance, climatic factors [14] affecting total solar radiation, outdoor air temperature, humidity and air velocity are the key parameters. The MVAC system must function properly on a design day because passengers staying inside are not able to leave the train so freely as in a building. A design day is a day which maximum dry-bulb and wet-bulb temperatures are obtained, and there is little or no haze in the air that results in maximum total solar radiation. The internal loads are mainly consisted of the people load based on the ‘fullycrowded’ condition, and the lighting load. Some common design criteria are listed below: 앫 Outdoor conditions Summer: 33.3°C dry bulb temperature and 95% relative humidity. Winter: 10°C dry bulb temperature and 40% relative humidity. 앫 Indoor conditions Summer: 22.5±2°C dry bulb temperature and 55%±5% relative humidity. Winter: both dry bulb temperature and relative humidity for winter conditions would vary but these were not considered in this study because heating load was not the interest. Energy use of the MVAC system in the train compartment was simulated using TRACE 600 [10– 12]. There are two air-conditioners in each compartment. The roof and the floor of the compartment is made of steel, whereas the walls are made of honeycomb aluminum alloy with doubleglazed windows. The estimated values of the key parameters are as follows: Window to wall ratio (WWR) 0.5 U-values 4 Wm⫺2 K ⫺1 for roof; 5 Wm⫺2 K ⫺1 for exposed floor; 6 Wm⫺2 K ⫺1 for

W.K. Chow, P.C.H. Yu / Energy 25 (2000) 1–13

Shading coefficient Indoor design temperature Type of air-conditioner Operating schedule Design passenger loading Heat gain from people (per person) Heat gain from internal lighting

5

wall; and 3.5 Wm⫺2 K ⫺1 for doubleglazing fenestration 0.82 23°C maximum value would be up to 26°C Single zone DX air-conditioner 05:00–24:00 daily 350 persons 80 W sensible; 80 W latent 1 Wft⫺2

The typical passenger flow pattern is shown in Fig. 2. Two modes of train operation were considered: 앫 Stationary mode (SM): the train is not moving.

Fig. 2. Typical passenger flow pattern.

6

W.K. Chow, P.C.H. Yu / Energy 25 (2000) 1–13

앫 Normal traveling mode (NM): the train is moving at a speed of 85 km h⫺1. Various ventilation rates, QT, from 0 to 150 ACH, were considered to study the impact on the energy consumption for operating the MVAC system using the energy simulation program TRACE 600 [10–12]. The QT values above 100 ACH were included so as to cover a wider range of ventilation rates for overcrowding conditions.

4. Overview of TRACE 600 The execution of TRACE 600 is divided into four phases: Load, Design, System, and Equipment [10–12]. 4.1. The Load phase The Load phase operates independently of the other three phases of Design, System and Equipment. However, the information generated will be used by those three phases. Only the Job and Load sections of the TRACE 600 Input can affect the load calculations. The purpose of the Load section is to generate building load profiles for use in both the Design and System phases. The load profiles created for the Design phase are based on the design weather conditions and the design daytype portion of the load schedules. The load profiles created for the System phase arc based on the typical weather of the daytypes: Monday, Weekday, Saturday, and Sunday. In addition, a Design daytype profile is generated using the design cooling weather and the Design daytype portion of the load utilization schedules. The non-design profiles are used in the System and Equipment phases to calculate the energy consumption. The design-based load profiles are used in the System and Equipment phases to calculate the demand peaks. 4.2. The Design phase The design supply air temperatures, coil capacities, fan air flows, and the required capacities of cooling/heating equipment are calculated in the Design phase of TRACE 600. For applications with known building design parameters, users can override some of the calculated values by entering the appropriate information. This gives the user a choice to simulate existing buildings with installed equipment that may not be sized according to the loads calculated in the Load phase. 4.3. The System phase In the System phase, the hourly internal and external loads for each room, the design conditions and operating characteristics of each system are combined to determine the hourly airside system loads. First, the response of the system equipment within each space at a particular hourly load is determined. For example, the response of a constant volume system is to change the supply air temperature, and the response of a variable volume system is to adjust the supply airflow to meet the space load requirements. Once this information is gathered, the return air condition and air quantity from each room are known. The return air mixture condition from all rooms is then

W.K. Chow, P.C.H. Yu / Energy 25 (2000) 1–13

7

calculated and any return air heat pickup is added. Finally, the return/outdoor air mixture condition at the central equipment is determined. Knowing the return air condition and the functioning of the equipment in response to the air-conditioning requirements in each room, the heating, cooling and humidification loads on each system are calculated. This procedure is repeated every one hour to produce a series of hourly system loads. 4.4. The Equipment phase The calculation sequence for equipment simulation begins with the reading of data files, then the necessary calculations are performed for each of the four primary equipment types : cooling equipment, heating equipment, air-handling equipment and cogeneration equipment. The data for each piece of primary equipment and all of the corresponding accessories are read from the equipment performance library. These include the full load consumption rate, part load curve coefficients and ambient modification coefficients where applicable. The data files of the System phase contain the hourly system demands for cooling, heating, humidification and air-moving for all months of the year. The wet bulb and dry bulb temperatures are read from the weather data files and are used to modify the performance of the equipment affected by ambient conditions. All of these data are then used to determine the energy consumption of the cooling, heating and air-moving equipment. 5. Field measurement Two sets of field data measured earlier were used to justify the results in this paper [15]. The measurement and computation methods are briefly described below: 5.1. Cooling capacity Each train compartment is served by two independent air-conditioning (A/C) units. In order to determine the actual cooling capacity and energy efficiency, two existing A/C units were set up in a workshop of the local railway authority for testing. The temperature and pressure along the refrigerant circuit were measured at six different points, including the compressor inlet and outlet, evaporator inlet and outlet, condenser inlet and outlet, and the expansion valves. By using the measured data to construct the refrigeration cycle on a T–S diagram, the cooling capacity and the coefficient of performance (COP) can be calculated. 5.2. Energy data Electricity is the main energy use in the local railway. Daily power consumption (in kWh) can be obtained from the energy meter in each station and daily mileage (in km) can be read off from the counter of each train. The value of the sum of kWh readings divided by the sum of mileage readings is recorded as the daily energy data. A monthly average is calculated based on the mean value of the daily energy data. However, these ‘raw’ data cannot be used to validate the simulation results.

8

W.K. Chow, P.C.H. Yu / Energy 25 (2000) 1–13

During winter time, for example in January, the energy used by the MVAC system of each train is insignificant (as only the circulating fan is operating); and the energy used to drive the train is hardly affected by any climatic factors. Therefore, the 24-month energy data was normalized based on the January data as its variation reflects only the variations in energy use of the MVAC system. 6. Results and validation The predicted peak cooling load CT (in kW), annual heat gain Ah (in kWh) and electrical energy consumption Ec (in kWh) for a typical train compartment under empty and fully-crowded conditions and in two different operation modes are shown in Table 1. The values of the peak cooling load CT under stationary mode (SM) of a crowded train compartment vary from 92 to 364 kW for ventilation rates of 0–150 ACH. The corresponding CT values for an empty compartment are lower and vary from 43 to 364 kW. When the train is in normal traveling mode (NM), the CT values are not much different from those predicted for the SM. This is because the additional amount of outside air driven into the train compartment under NM is very difficult to determine and therefore, this factor was ignored in the simulation. The actual cooling capacity provided by the two air-conditioners in each train compartment [15] is indicated by a horizontal line in Fig. 3(a). It intersects the SM–Empty peak cooling load curve at 15 ACH which agrees with the value of the typical fresh air flow rate mentioned in the previous section. The annual heat gain Ah for a train compartment under SM changes from 258,609 to 664,455 kWh for 0–150 ACH under crowded conditions; and it changes from 78,980 to 523,777 kWh under empty conditions. There is a big difference in Ah for an empty and a fully-crowded train. For example, Ah of a crowded train is 3.27 times that of an empty train at 0 ACH. It becomes 1.27 times if the ventilation rate is increased to 150 ACH. Similar variations arc observed for NM, where Ah varies from 255,107 to 636,391 kWh for a crowded train; and from 62,589 to 489,176 kWh for an empty train. The electrical energy consumption Ec, for operating the MVAC system is the most important factor as it is directly related to the actual running cost. This will be further discussed in the following section. The values of Ec are 42,284 to 103,967 kWh for 0–150 ACH in a crowded train compartment; and 18,007 to 86,565 kWh for an empty train compartment under SM. Under NM, it becomes 41,941 to 100,664 kWh for a crowded train and 15,872 to 83,035 kWh for an empty train. Taking 15 ACH ventilation rate as a reference, the monthly data obtained from simulation and the field measured data [15] were normalized with respect to the minimum value accordingly for easy comparison, which is shown graphically in Fig. 3(b). The Field Data curve fluctuates around the curves SM–Crowded and NM–Crowded; however, the trend of field data curve almost coincides with the NM–Crowded curve. 7. Discussion The electrical energy consumption Ec for operating the MVAC system under different ventilation rates can be visualized by plotting Ec against QT for crowded and empty trains under two

W.K. Chow, P.C.H. Yu / Energy 25 (2000) 1–13

9

Table 1 Results predicted by TRACE 600 Ventilation Stationary Mode (SM) rates QT (ACH)

0 5 10 15 20 30 50 70 100 120 150

Normal Moving Mode (NM)

Peak cooling load CT (kW)

Annual heat gain Ah (kWh)

Electrical consumption Peak cooling Ec (kWh) load CT (kW)

Crowded Empty

Crowded

Crowded

92 103 113 122 132 149 186 240 290 320 364

258,609 285,279 300,750 311,167 323,365 345,858 395,677 446,592 530,771 584,576 664,455

43 53 63 73 184 105 147 189 252 297 364

Empty 78,980 101,858 118,949 137,208 153,160 183,545 239,970 296,684 382,567 444,237 523,777

42,284 46,200 48,468 50,551 52,542 56,166 63,657 72,578 84,926 92,689 103,967

Empty 18,007 21,115 23,779 26,312 28,751 33,479 42,396 50,961 64,432 73,632 86,565

Annual heat gain AT (kWh)

Crowded Empty Crowded

Empty

93 103 112 122 131 149 185 239 290 319 363

62,589 84,057 102,152 119,046 135,613 163,484 218,110 270,034 357,414 414,885 489,176

42 52 62 73 84 105 147 189 252 296 364

255,107 268,678 280,001 292,060 302,716 323,978 370,277 417,480 501,090 557,111 636,391

Electrical consump Ec (kWh) Crowded 41,941 44,264 46,317 48,169 50,059 53,419 60,640 69,175 81,530 89,305 100,664

Empty 15,872 18,953 21,573 24,063 26,478 31,028 39,557 47,928 61,265 70,076 83,035

10

W.K. Chow, P.C.H. Yu / Energy 25 (2000) 1–13

Fig. 3. Comparison of simulation results with field measurement.

different operation modes as shown in Fig. 4. The values of Ec were normalized by referring to the electrical consumption Eco for an empty train under SM (i.e. 18,007 kWh). From Fig. 4, Ec can be as large as 5.8 times of Eco in a crowded train compartment under SM if the total ventilation rate is increased to 150 ACH. This energy impact ⌬Ec (in kWh) per unit increase in ventilation rate ⌬QT, (in ACH) can be expressed as an empirical relationship: ⌬Ec ⫽400 ⌬QT

(2)

W.K. Chow, P.C.H. Yu / Energy 25 (2000) 1–13

11

Fig. 4. Normalized electrical energy consumption.

For the present electricity cost of HK$1.00 (US$0.13) per kWh in Hong Kong, the fiscal implication will be a HK$400 (or US$50) penalty per train compartment for every unit increase in ventilation rate in ACH. Regarding the three figures concerned: the typical operation value at present of 1.96×10⫺3 m3 ⫺1 s (0.04 ACH) per person, the suggested value of 7×10⫺3 m3 s⫺1 (0.15 ACH) per person and 11.8×10⫺3 m3 s⫺1 (0.26 ACH) per person as recommended by Chow, QT will be 15, 59.5 and 91 ACH respectively under fully-crowded condition (i.e. 350 passengers). The corresponding electrical energy consumption can be obtained from Fig. 4. It is observed that the maximum value of Ec at 91 ACH is about 4.4 times of Eco for operating the MVAC system under crowded condition in SM. Substituting this value (⌬QT=91) into Eq. (2), the improvement work on ventilation will cost HK$36,400 (US$4670) maximum relative to the Eco. If the existing MVAC system is designed for a typical QT of 15 ACH, the annual cost will be HK$30,400 (US$3900) per train compartment at most based on the alternative of 91 ACH. The incremental electrical energy consumption will be around 1.57 times more or 57% increase. However, another challenge of the ventilation design and cooling load estimation for a train compartment is the difficulty in estimating the influx of hot and humid outside air as a result of frequent opening and closing of the sliding doors. The infiltration (or exfiltration) will upset the normal ventilation through the MVAC system, and hence introduce an uncertainty factor to the cooling load of the fully air-conditioned train compartment. Part of the studies had been discussed in the literature [16–18]. However, a practical approach to solve this problem is to build partition

12

W.K. Chow, P.C.H. Yu / Energy 25 (2000) 1–13

walls around the passenger platform with doors that open and close at the same time with the train doors.

8. Conclusion From a preliminary study [5] on the ventilation requirement in the train compartments of the railway lines in Hong Kong, carbon dioxide was taken as the indicator for indoor air quality. Based on a well-mixed model [5,19], carbon dioxide levels under different ventilation rates were found. The electrical energy consumption of operating the MVAC system to maintain the carbon dioxide level at certain values was studied using the energy simulation software package TRACE 600 [10–12]. The results were compared with the field measured data [15]. In order to maintain the indoor carbon dioxide level y at 10/3% as recommended by Chow [5], the electrical energy required to operate the MVAC system for each train compartment can be 4.4 times more than that without any ventilation (Eco). A rough estimate gives HK$30,400 (US$3900) or 57% increase in electricity cost for the MVAC system per train compartment per year, if the existing system is designed for a typical ventilation rate of 15 ACH. In fact, this is just a small percentage of the total electricity cost as the electrical energy required to run a railway line is far more than running the MVAC system alone. Nevertheless, it is worthwhile to spend more money on reducing the carbon dioxide level to provide passengers with a healthier train compartment. Note that increasing the fresh air supply might not be sufficient for maintaining a satisfactory level of carbon dioxide. A mixing factor might be included in Eq. (1). It requires advanced simulation technique to find out the air flow induced by diffusers [20,21] and the mixing process of carbon dioxide [8]. A similar concern of heat gain due to air infiltration during opening and closing of train doors should be addressed by further studies.

Acknowledgements The authors wish to thank the Trane C.D.S. Department for supporting this study.

References [1] Fung WY. Numerical studies on the air flow and assessment of thermal comfort indices related to draught in airconditioned and mechanical ventilated space. PhD thesis—Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, 1995. [2] Hong Kong Standard. MTRC accepts blame for chaos. 8 May 1996, Wednesday, p.1. [3] Hong Kong Standard. MTRC must make its import public. 8 May 1996, Wednesday, p. 12. [4] Ming Pao. MTRC ventilation system, 8 May 1996, Wednesday (in Chinese). [5] Chow WK. Ventilation studies in train compartment. Research Report. The Hong Kong Polytechnic University, Hong Kong, China, 1998. [6] ASHRAE Standard 62. Ventilation for acceptable indoor air quality. ASHRAE, Atlanta, USA, 1989.

W.K. Chow, P.C.H. Yu / Energy 25 (2000) 1–13

13

[7] ASHRAE handbook—fundamentals. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE), 1993. [8] Chow WK, Fung WY. Investigations of the subjective response to elevated air velocities: climate chamber studies. Energy Build 1994;20(3):187–92. [9] Chow WK, Fong SK. Simulation of energy use in a building with three weather files of Hong Kong. Energy Engng 1996;93(2):30–54. [10] TRACE 600 engineering manual. La Crosse, WI: The Trane Company, 1994. [11] TRACE 600 user’s manual. La Crosse, WI: The Trane Company, 1994. [12] TRACE 600 utility reference manual. La Crosse, WI: The Trane Company, 1994. [13] Chow WK, Chan KT, Yik Francis WH, Wu Tony PW, Ho CM, Leung SL. Building energy analysis simulation: experience of three programs in use. Build Serv Engng Res Technol 1994;15(3):157–64. [14] Kyle B. Hong Kong weather reviews. Hong Kong Meteo Soc Bull 1994;4(1):79–86. [15] An in-house testing on energy efficiency of the air-conditioning unit for passenger train compartment. Internal Technical Paper of a local railway company. Hong Kong, 1998. [16] Lawton EB, Howell RH. Energy savings using air curtains installed in high-traffic doorways. ASHRAE Trans 1995;101(2):136–43. [17] Hendrix WA, Henderson DR, Jackson HZ. Infiltration heat gains through cold storage from doorways. ASHRAE Trans 1989;95(2):1155–68. [18] Janssen JE, Pearman NA, Hill TJ. Calculating infiltration: an examination of handbook models. ASHRAE Trans 1980;86(2):751–64. [19] Chow WK. On the ventilation design in underground car parks. Tunn Under Space Technol 1995;10(2):225–46. [20] Li ZH, Zhivov AM, Zhang JS, Christiansen LL. Characteristics of diffuser air jets and airflow in the occupied regions of mechanically ventilated rooms—a literature review. ASHRAE Trans 1993;99(1):1119–27. [21] Zhivov AM. Theory and practice of air distribution with included jets. ASHRAE Trans 1993;99(1):1152–9.