INTRODUCTION
Global energy demand has increased swiftly in the last decade [1]-[3]. Air conditioning system has become the highest energy end-user in both residential and non-residential sectors [4],[5]. Issac and Vuuren [6]
modeled the global residential sector energy demand for air conditioning systems. Their results showed that the negative impact of climate change could rapidly elevate the energy demand by 72%. For instance, climate change would contribute to 50% increase in energy demand in South Asia [7]. Sensible heat, latent heat, room area, room location and room function are some of the factors affecting air conditioning systems [8]. Dew point temperature is also another parameter affecting cooling condition. In wet place, dew point temperature deviates a few degrees from the air temperature; however, this could reach to 60oC in dry place [9].
There have been few novel cooling technologies implemented for the sake of thermal comfort with potential benefits to the environment and air quality.
There was an increase of energy efficiency by 33%
through dehumidification and 20% increase in coefficient of performance with better compression technologies [7]. Intelligent air flow control can provide better indoor air quality while using the same amount of energy. Dehumidification technology is a key method to achieve a reduction in energy consumption.
A valid way to extract moisture from the air while using less energy consumption is by using the liquid based desiccant dehumidification [6].
Windows type can also affect thermal condition and heat transfer [10]. The common window types are categorized into three: single frame and window, double frames and double glass‚ and single frame and double glass. The second and third types are superior compared to the first type for the existence of
INVESTIGATION OF COOLING CONDITION OF A ROOM WITH AIR-CONDITIONING WORKING CONCURRENTLY WITH AIR
SUPPLY AND DISCHARGE SYSTEMS – A CASE STUDY
G. T. Chala1*, L. S. Joe2, M. I. N. Ma’arof2
1International College of Engineering and Management, Muscat, Oman
2INTI International University, Negeri Sembilan, Malaysia Email: [email protected]
ABSTRACT
This paper investigates cooling condition in the air-conditioned room working concurrently with air supply and discharge systems. Case studies were conducted for a kitchen located at INTI Subang, Malaysia. The study was conducted by measuring temperature, humidity, pressure and air velocity, followed by simulation studies using ANSYS-Fluent workbench to predict indoor thermal condition and other useful parameters. Simulation results were validated with experimental results, after which four case studies were performed to analyze their effects on human comforts and observe an optimal condition of each case. The four cases considered were: both windows and doors open (additional window) (Case 1), closed doors and windows open (additional windows) (Case 2), additional ventilation system (Case 3), and exhaust with half closed valve (Case 4). It was observed that Case 1 provided best indoor condition with the room temperature only at 296.4 K, which was 8.3 K lower than that of the original kitchen.
The peak temperature for this case was also found the smallest. The addition of electronic valve between the main ventilation pipe and the negative outlet reduced the room temperature less efficiently compared to Case 1.
Keywords: Air-conditioning, indoor temperature, humidity, air discharge, air inlet
air space between glass, which increases the thermal resistance and reduces the heat transfer rate. The single frame and double glass window were found to be best for cold area, followed by the double frames and double glass window because of the thermal performance. The heat transfer coefficient of single frames and double glass window was also observed to be smaller when compared with the other two window types [11].
In this modern society, building sector including commercial and residential sectors is consuming about 1/3 of the global energy [12]. Researchers are using user behavior models to forecast the energy consumption in the building sector [13]. Bin method was used to calculate the seasonal energy consumption of air conditions [14]. There was a total of 40%
energy reduction under mild-load conditions, while the reduction by individual operation duration was estimated at 20%. This shows that voluntary shutdown is required for energy saving. Time-related effect is much powerful than physical efficiency related effect when there is a reduction of individual operation duration by more than 26% during cooling season, indicating the importance of operation schedule to optimize the seasonal energy consumption [14].
Ning et al. [15] discussed the three types of heat gain from the building envelope: the wall conduction, solar heat through the windows and window conductions.
The change in outdoor temperature would change the indoor quality due to the heat transfer through the envelope. In order to weaken this effect, researcher came out with two types of methods: active and passive cooling. The former is by using more air conditioner to cool down inconvenient and costly places. The latter includes the use of phase change materials. It has to change the insulation and ventilation of the building envelope in order to reduce the cooling load and enhance the indoor air quality.
The use of air conditioning system varies according to the season. Some places use air conditioning the whole day regardless of time, especially in countries having four seasons in a year. This action would greatly increase the energy consumption in the building [16],[17]. Consequently, task/ambient conditioning system (TAC) was introduced for energy saving, flexible control over thermal environment and
enhance thermal comforts [15]. TAC system could maintain the indoor environment; however, it could leave the outside temperature to fluctuate out of comfort limit. Murray et al. [18] designed a dedicated outdoor air system comprising an enthalpy wheel, cooling coil and passive dehumidification component wheel. The system resulted in around 30% of the total cooling load and removed the needs for energy in re-heat process. The results from the questionnaire and field measurement showed that the cold situation of discomfort in fully air-conditioned building had a bit higher percentage than the mixed-mode building [19].
In mixed mode building, the mode of air conditioner is based on the occupants’ preferences. In this line, the occupants would switch on the air conditioner when prevailing mean outdoor temperature or daily mean temperature is close to 20oC. Zhang et al. [20]
reported the significant impacts of overcooling during summer and overheating during winter on workers’ productivity, mood and health. Changping et al. [21] discussed that mixing ventilation can create more chance for occupants to control their preferable environment condition, resulting in a more occupant satisfaction.
Prakash [22] conducted experimental and simulation studies to improve the indoor thermal comfort with different types of building roof. Their results clearly indicated that well insulated rooms prevent the heat gain from solar radiation. The optimum supply air temperature for good thermal comfort is around 299-300 K [23]. There have been limited studies available for the air conditioning system when it is operating concurrently with air inlet and air discharge systems.
The objective of the present work was, therefore, to investigate the cooling condition of a room with air- conditioning working concurrently with air inlet and discharge system. This highlights preferable cases to save energy and obtain optimal condition.
MATERIAL AND METHODS Experimental Setup
Figure 1 shows the schematic diagram of a kitchen room found at INTI Subang Culinary department, Malaysia.
Actual dimensions of the room were considered in the
simulation study. Twelve ceiling ducted supply grilles, nine heat sources, four doors, two ceiling ducted return grilles and one exhaust hood were available in the kitchen. The size of all the twelve ceiling ducted supply grilles and two ceiling ducted return grilles was 640 mm × 640 mm. Figure 2 depicts the equipment in the kitchen. Temperature, pressure and airflow rate were collected to observe the condition of the original
kitchen room. A digital thermo hygrometer was used to measure the relative humidity and temperature of the kitchen, and anemometer AVM-07 for the air velocity. A GM-320 infrared thermometer that could read temperature between 223 K and 603 K was also used to measure surface temperature. Moreover, temperatures of the kitchen were measured in two different ways: 1) measuring the temperature for every single air conditioning system, air supply system and heat generator systems, and 2) splitting the kitchen into four parts at which temperatures were taken at 2.585 m, 5.17 m, 7.755 m and 10.24 m from the start line at the bottom.
CFD Simulation
ANSYS-FLUENT 18.2 workbench was used to first simulate the original kitchen condition, which was validated using the experimental data. Supply air temperature and velocity, ambient temperature, indoor air velocity and relative humidity were considered in the simulation to evaluate the initial thermal condition of the specific room. Following the validation of the simulation results for the original kitchen, the indoor thermal comfort and air flow rate were simulated for four different cases. The purpose of the four different case studies is to predict the indoor thermal condition by using additional windows, additional ventilation system and also by opening and closing of the windows and doors. All the results of the four cases were compared with the original kitchen condition to inspect the optimum method of creating a comfortable condition. Case 1 referred to both windows and doors Open (additional windows);
Case 2: doors close and window open (additional windows); Case 3: with additional ventilation system;
and Case 4: exhaust with half close valve. The conservation equations for mass, momentum and energy were considered and discretized using the finite volume technique for the analysis.
Energy Equation
The energy equation is given as follows [24]:
––∂
∂t
(
ρE) + = ? (v→(ρE + p))
== ?
(
keff =T –∑
jhj J→J + (τ=eff ? v→))
+ Sh (1) where keff is the effective conductivity (k + kt)Figure 1 Schematic of the original kitchen
Figure 2 Labelling of equipment in a kitchen room
kt is the turbulent thermal conductivity J→J is the diff usion fl ux of species j
The first three terms on the right-hand side of Equation 1 represent energy transfer due to conduction, species diff usion, and viscous dissipation, respectively.
Sh includes the heat of chemical reaction, and any other volumetric heat sources defi ned.
In Equation 1 E = h – p
–ρ + v2
–2 (2)
w here sensible enthalpy h is defi ned for ideal gases as:
h=
∑
jYjhj (3)a nd for incompressible fl ows as:
h =
∑
jYjhj + p–ρ (4)
I n Equations 3 and 4, Yj is the mass fraction of species j and
hj =
∫
TTref Cp,j dT (5)where Tref is 298.15 K.
RESULTS AND DISCUSSION
Figures 3-6 show experimental results of temperature, humidity and air velocity for the air conditioning and air supply systems. The average temperatures at diff erent
Figure 3 Temperature profi le of air-conditioning and supply air systems of original kitchen (Experiment)
Figure 6 Total Temperature at every stove, grill, oven and fridge of original kitchen (Experiment)
Figure 5 Air velocity of all air-conditioning system and supply air system of original kitchen (Experiment) Figure 4 Humidity profi le of all air-conditioning system
and supply air system of original kitchen (Experiment)
points were between 296 K and 301 K. The relative humidity was between 57.9% and 70.9% RH. The air velocity at diff erent point was at an average value of 1.54 m/s to 3 m/s. Figure 6 shows total temperature at diff erent points. The temperature range was between 300 K and 370 K. Heat index table matching air temperature with relative humidity was also referred to observe the cooling condition. Both Ansys and
Temperature, K
301.0 300.5 300.0 299.5 299.0 298.5 298.0 297.5 297.0 296.5 296.0
Humidity, RH%
75 70 65 60 55
Air velocity, m/sTotal Temperature, K
1
Ref. Point
2 3 4 5 6 7 8 9 10 11 12 13
3.5 3 2.5 2 1.5
1 1
Ref. Point
2 3 4 5 6 7 8 9 10 11 12 13
Position Stove A
Stove B Stove C
Stove D Stove E
Stove F Grill Oven
Fridge 370
360 350 340 330 320 310 300
1
Ref. Point
2 3 4 5 6 7 8 9 10 11 12 13
experimental data revealed closer results in Kelvin.
The average temperature for all the 12 air-conditioning systems, 1 air supply system, 6 stoves, 1 grill, 1 oven and 1 fridge was higher than the desired values in the range between 295 K (22) and 300 K (27) [25]. Another two sets of data were taken every 15 minutes at similar four positions of 2.585 m, 5.17 m, 7.755 m and 10.34 m from the start line.
Figure 7 compares the temperature between every single one of the air conditioning system and the heat generators. It can be seen that the air conditioning system maintained its temperature at around 300 K while all heat generators generated hotter air within the range of 300 K ~ 360 K. Consequently, the air conditioning system with the temperature of 300 K was not suffi cient to off set the heat generated from the stove, grill and oven, reducing the cool air in the room.
Ideally relative humidity should be between 40%
and 60%. As humans perceive a low rate of heat transfer from the body, the body experiences greater distress of waste heat burden at a higher humidity than lower humidity, given equal temperature [26].
Low humidity is a common cause of nosebleeds and eye irritation [27].
Temperature, K
380 360 340 320 300 280 1
Ref. Point
2 3 4 5 6 7 8 9 10 11 12 13
Time
Temperature, K
306 305 304 303 302 301 300 299 298 297 296
12.45pm 1.00pm 1.15pm 1.30pm 1.45pm 2.00pm 3.00pm 3.15pm 3.30pm 3.45pm 4.00pm 4.15pm 4.30pm 4.45pm 5.00pm
Time
Humidity, %RH
80
75
70
65
60
55
12.45pm 1.00pm 1.15pm 1.30pm 1.45pm 2.00pm 3.00pm 3.15pm 3.30pm 3.45pm 4.00pm 4.15pm 4.30pm 4.45pm 5.00pm
Figure 7 Comparison of temperature between A/Cs and heat generators
Figures 8 and 9 show temperature and humidity over time for the fi rst set of experiments. The peak temperature was found to be 305.1 K between 1.45 pm ~ 4pm as most of the heat generators were turned on. The highest humidity level at the start was around 76.5% RH. The humidity started dropping a bit to 68.4% after one hour and until a certain percentage of around 58.6% RH, indicating dry environment during operation and an increase in the humidity level after the operation hours. The relative humidity measured in the kitchen was higher than the optimum value. A person feels comfortable within a wide range of humidity level depending on the temperature.
Figure 8 Trends of temperature over time (fi rst set of experiments)
Figure 9 Humidity vs Time (fi rst set of experiment) Figure 10 depicts pressure profi les over time in the original kitchen. It was approximated using ideal gas equation to observe the trends at diff erent points from the reference line. It was observed that pressure profi les
follow that of the temperature with minimal infl uence of air density on the pressure. Pressure was observed varying insignifi cantly.
Figure 14 compares total temperature between the air supply and discharge systems for the original kitchen, as observed from the simulation. The temperature ranges between 300 K and 310 K for the air supply system at diff erent points (Figure 14a), while it was between 300 K and 345 K for the exhaust system (Figure 14b). A relatively higher pressure at the air supply system was observed as opposed to that in the exhaust system, where there was higher temperature and as a result a reduced density of air dropped the pressure at the exhaust system, aff ecting
Average Temperature, K
304 302 300 298 296 294 1
Ref. Point
2 3 4 5 6 7 8 9 10 11 12 13 Time
Pressure, x105 Pa 1.075 1.070 1.065 1.060 1.055 1.050 1.045 1.040
12.45pm 1.00pm 1.15pm 1.30pm 1.45pm 2.00pm 3.00pm 3.15pm 3.30pm 3.45pm 4.00pm 4.15pm 4.30pm 4.45pm 5.00pm
Time
Temperature, K
306 305 304 303 302 301 300 299 298 297 296
12.45pm 1.00pm 1.15pm 1.30pm 1.45pm 2.00pm 3.00pm 3.15pm 3.30pm 3.45pm 4.00pm 4.15pm 4.30pm 4.45pm 5.00pm
Figure 12 Humidity vs Time (second set of experiments) Figure 13 compares simulation and experiment data.
Experimental and simulation results are quite similar, and the errors were below 5%, and this validates the
simulation results
Figure 13 Comparison between experiment data and simulation data
Time
12.45pm 1.00pm 1.15pm 1.30pm 1.45pm 2.00pm 3.00pm 3.15pm 3.30pm 3.45pm 4.00pm 4.15pm 4.30pm 4.45pm 5.00pm
306 305 304 303 302 301 300 299 298 297 296
Temperature, K
Figure 10 Pressure profi les over time (fi rst set of experiments)
Figure 11 Temperature vs Time (second set of experiments)
Figures 11 and 12 show temperature and humidity for the second set of experiments. The peak temperature (305.3 K) often occurred during the time between 2:00 pm and 3:45 pm at point 3. Humidity versus time was diff erent from that of the fi rst set, where the highest humidity level did not occur at the beginning.
Humidity was very low at the beginning, average around 63.1% RH, and it started to increase slowly to 87.1% RH.
the indoor condition. The inside pressure could be balanced with that of the outside by sucking the high temperature from the heat sources as the trapped hot air in the surroundings brings poor ventilations with an inefficient exhaust system. Moreover, common indicators of pressure influence include the suction pressure that makes exterior doors difficult to open, exhaust fans that do not work properly, drafts around doors and windows, poor indoor air quality, and infiltration of outdoor air.
Air distribution, temperature, pressure and air flow rate of the model kitchen with different cases were simulated and compared with the experimental results. Figure 15a shows air flow pattern in the air- conditioning and air supply systems of the original kitchen. Temperature contour is shown in Figure 16a. As can be seen from the temperature contour, the hottest place was at the center of the kitchen, under the exhaust hood, which was 382 K. It was default kitchen layout without any windows, so the only outlets for the air were the two return air grilles located at the back of the kitchen and four doors located at the side.
Figure 15b shows air flowrate for the Case 1. Figure 16b shows temperature contour for the Case 1. For this case, five windows were added at the left side and two windows at the back. They were kept open with the first and fourth doors open during the entire working period to check whether these seven windows would provide a better indoor air quality
and indoor thermal comfort. It was observed that there were slight differences at the center point when compared with the original kitchen. The highest temperature also reduced from 382 K to 365 K. The room temperature for the case 1 was around 296.4 K, and this was 8.3 K lower than the room temperature of the original kitchen.
Figure 15c shows the air flow pattern at all air- conditioning and air supply system for the Case 2.
Temperature contour of the Case 2 is shown in Figure 16c. There were some differences with five windows added at left hand side and kept them open during the entire working period. The doors were kept close for better indoor air quality and indoor thermal comfort. There were also slight differences at the center point as can be seen from the temperature contour. The hottest temperature (382 K) at the center point of the original kitchen (382 K) reduced to 378.8 K. Although the highest temperature was reduced, yet the size of the red area was bigger than the original kitchen layout. The room temperature reduced slightly when compared with the original kitchen. For this scenario, all the doors inside the kitchen were shut down, according to the air flow pattern from ANSYS. Subsequently, cool air and hot air used the windows as outlet. The five windows were found insufficient to cover whole kitchen, especially for heat generators located on the right-hand side.
Figure 15d shows air flow pattern at every air- conditioning and air supply systems for the Case 3.
(a) (b)
Figure 14 Total temperature of the original kitchen at: a) air supply system and b) exhaust system.
(e)
Figure 15 Air velocity at air-conditioning system and air supply system:
a) default b) Case 1, c) Case 2, d) Case 3 and e) Case 4
(a) (b)
(c) (d)
(e)
Figure 16 Temperature contour: a) default, b) Case 1, c) Case 2, d) Case 3 and e) Case 4
(a) (b)
(c) (d)
Temperature contour for this case is depicted in Figure 16d. With addition of four ventilation systems at the left side, two additional ventilation system at the back, temperature and humidity values were slightly different from the original kitchen. It was observed that temperature at the center point was different from the original kitchen.
There was no significant difference with addition of ventilation system in indoor thermal condition due to insufficient volume to overcome the heat in the kitchen.
Figure 15e shows air flow pattern for the Case 4.
Temperature contour is depicted in Figure 16e. An electronic valve, which was made half closed, was added inside the exhaust hood to investigate indoor air quality and indoor thermal comfort. Temperature contour showed that there were slight differences at the center point. The red area at the center point of the original kitchen (382 K) reduced to around 380.6 K. The temperature contour was found almost similar with the original kitchen.
However, a reduction in maximum temperature was observed. With the help of half close valve, it can be made sure that the hot air will not flow back from the external environment, but this also means that there may be a reduction of efficiency of the valves reducing its performances. Air would still flow through the doors instead of going through the half-closed valve installed at the middle of the kitchen. The electronic valve could limit the negative air supply depending on the situation. The valve can rotate up to 90 degrees and it could be set at every
10-degree interval, and this can either be done manually or automatically. It could be placed between the main ventilation pipes the negative outlet and the fan. In this way, air flow rate can be adjusted accordingly. This would balance the pressures inside and outside of the kitchen by removing the high temperature from the heat sources. The poor ventilation in the kitchen was due to the trapping of the hot air in the surrounding area causing ineffective heat rejections.
Figure 17 compares total temperature at the air conditioning and air supply system of original kitchen with that of the Case 1. It can be seen that the drop in temperature in Case 1 was significant when compared with that in the original kitchen. The overall temperature at each of the air-conditioning inlet is also lower than the default case. The installed windows for this case could help the exhaust hood to expel some of the hot air to the outdoor environment and reduce the temperature inside the kitchen.
Figure 18 depicts comparison results for different cases at a selected point. Case 1 provided the best indoor condition with the room temperature only at 296.4 K, which is 8.3 K lower than the original kitchen. The room temperature dropped to 304.6 K for the case two. The temperature contour of case three showed the room temperature of around 304.8 K, slight change with that of the original kitchen condition. There was a high pressure located between A/C 3 and air supply unit, indicating that the additional ventilation system installed at the side
Figure 17 Total temperature at air conditioning and air supply system: a) original kitchen and b) Case 1
(a) (b)
and back did not improve the indoor environment of the kitchen.
The valve inside the exhaust hood was made half- closed while keeping the other settings similar to the original kitchen. The addition of electronic valve between the main ventilation pipe and the negative outlet and the fan reduced the room temperature insignifi cantly. The half-closed valve installed inside the exhaust did not perform well in expelling the hot air. There was also some red spot under the exhaust.
According to the temperature contour of kitchen Case 1, the temperature around the whole kitchen mostly is around 296.4 K to 303.3 K, which is in the acceptable ranges.
CONCLUSION
An investigation on cooling condition of a room with air-conditioning working concurrently with air supply and discharge systems was carried out by identifying factors that could have significant influences. A field study was conducted for the kitchen available at INTI Subang Culinary Department.
Following the validation of the simulation results, four diff erent cases were simulated via ANSYS-Fluent 18.2 workbench. Based on the four cases studied, Case 1 appeared to be the best condition with enhanced indoor thermal condition and lowest peak temperature and reasonable pressure as opposed to the other three cases, with temperature around 296.4 K. The performance of windows and doors was also quite similar to a ventilation system in letting hot air out of the kitchen room. By adding windows at the
side and back of the kitchen, better circulation of air was observed as it served as a small exhaust system.
On the other hand, the inclusion of electronic valve between the main ventilation pipe and the negative outlet reduced the room temperature less effi ciently compared to Case 1.
ACKNOWLEDGMENTS
The authors would like to thank INTI Subang culinary department for their support during data collection.
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