Metro Train Simulation

Air Conditioning Calculation For Train Coach

Air Conditioning System Simulation for Train Coach :

An air conditioner is basically a heat pump; it pumps heat from the inside area of the coach (cold regions) to outside atmosphere (warm regions). Air Conditioner of 1 ton capacity removes heat equivalent to 3.517 kW heater or 12,000 BTU/hr. from coach area to atmosphere. The tonnage capacity required to control the coach temperature and humidity depends on several factors like atmosphere temperature and humidity, fresh air requirement per passenger, number of passengers in coach, power rating of electrical appliances used in coach, area of glass windows, area of doors and its opening duration and frequency, temperature setting of coach air conditioners, insulation of walls, infiltration losses etc. An oversized air conditioning system is not good because it may reduce the temperature much fast but can not control the moisture/humidity level inside the coach perfectly. The compressor of oversized air conditioning system may switch 'ON' and 'OFF' frequently, without running long enough to dehumidify the coach properly or cool the space uniformly. On the other side, an undersized air conditioning system will not be able to cool the coach properly in extremely hot weather and peak passengers loading conditions.
Required minimum tonnage capacity of air conditioning system mainly depends on the atmosphere temperature and humidity, passengers loading per coach, fresh air requirement per passenger and temperature setting of coach air conditioners. Average value of passengers loading, atmosphere temperature and humidity may be considered to avoid an oversized air conditioning system. Cooling capacity of air conditioning system reduces with increase in atmosphere temperature. Therefore, it is very important that the entered value of air conditioner units tonnage capacity in this simulation tool should be in reference to atmosphere temperature. Value of sensible heat ratio (SHR) is one of the controlling parameter which controls the humidity inside the coach. SHR is the ratio of sensible heat load to total heat load of the space. Required value SHR can be achieved by proper designing of evaporator unit, cooling tube temperature and air flow rate. The Coefficient of Performance (COP) of an air conditioning system reduces with increase in atmosphere temperature. Therefore, entered values of COP and tonnage capacity in this simulation tool should be in reference to the atmosphere temperature to minimize the error in results.

operation in day operation in night

#Note: Please read Para 'Limitations of online Air Conditioning Simulation tool'. This is an online demo simulation tool and results of online Air Conditioning simulation tool may have some error due to these limitations. However, our offline software tools do not have such limitations and provide much accurate simulation results. For detailed simulation of Building / Train HVAC system, write us.

Important facts regarding Air Conditioning System:

1. Psychrometry is the study of the properties of mixtures of air and water vapour. The relative amount of water vapour (humidity) in atmosphere can be calculated with the help of Dry Bulb Temperature (DBT) and Wet Bulb Temperature (WBT). DBT is the temperature of the moist air as measured by a standard thermometer or other temperature measuring instruments. A thermometer with a wetted wick is used to measure the wet bulb temperature. When the water from wetted wick is evaporated, required latent heat is taken from the mercury filled in the bulb and thus, wet bulb thermometer shows low temperature as compared to DBT. More difference between DBT and WBT indicates lower relative humidity of the surrounding air. At 100% humidity, DBT and WBT thermometers indicate same temperature reading. At a given temperature and pressure the dry air can only hold a certain maximum amount of moisture. When the moisture content is maximum for given air temperature, then the air is known as saturated air. If unsaturated moist air is cooled at constant pressure, then the temperature at which the moisture in the air begins to condense on the surface is known as dew-point temperature (DPT) of air. The value of relative humidity at dew point is 100%.

2. Water vapour holding capacity of air depends on its temperature and increases with increase in the air temperature. Relative humidity or % humidity is defined as the ratio of the actual amount of water vapour in the moist air at any temperature to the maximum amount of water vapour, that the air can hold at that particular temperature and pressure. Thus, if the temperature of a closed room is increased, the water vapour holding capacity of air will increase but the content of water vapour in the room air will remain same (because room is closed) which results into reduction in % humidity and vice-versa. The relative humidity is 100% if the air is saturated with water vapour. Absolute humidity is defined as the mass of water vapor present in a unit volume of moist air.

3. Thermal comfort is defined as “that condition of mind which expresses satisfaction with the thermal environment”. Important factors which affect thermal comfort are the age, sex, health, activity, clothing, air temperature, % humidity and air velocity. It should be noted that since so many factors are involved, many combinations of the above conditions provide comfort. A range of recommended temperature and humidity for comfort zone is given below:
Summer : 23°C - 26.6°C and 40 - 60% RH
Winter : 20°C - 24°C and 30 - 50% RH

4. The types of heat gain are sensible and latent. Sensible heat gain results in a rise of air temperature. Latent heat gain merely changes the state of substance i.e solid to liquid or liquid to gas without causing any change in its temperature. The proper design of an air conditioner system requires the determination of the total sensible and the latent heat gain inside the coach.

5. If compressors turn 'ON' and 'OFF' frequently, it indicates an oversized air conditioning system. A well designed air conditioning system has duty cycle of 80 to 90 % during peak loading conditions.

6. Cooling capacity of air conditioning system reduces with increase in atmosphere temperature.

7. Co-efficient of Performance (COP) of a air conditioning system is a ratio of useful heating or cooling provided to work required. Higher the COP, higher the efficiency of the equipment.
COP = Amount of heating or cooling provided by AC in Watts / Input electric power consumed by AC in Watts
Similarly, Energy Efficiency Ratio (EER) is the ratio of heating or cooling capacity (in BTU / hr) to the power input (in watts).
EER = (Amount of heating or cooling provided by AC in BTU/hr) / Input Electric power consumed by AC in Watts
Since, 1W = 3.412 BTU/hr; therefore, the relation between COP and EER may be written by the following expression:
EER = 3.412 * COP

8. Sensible Heat Ratio (SHR) is the ratio of sensible heat load to the total heat load i.e. (Sensible load + Latent load) of refrigerated space. Total sensible heat load and latent heat load are calculated for the coach to find out the value of SHR. Further, the design of air conditioner is finalised to achieve the required SHR.

9. Some useful unit conversions :
1 ton= 3.51 kW = 12000 BTU / hr
1 kWh = 3600 kilo-joule
1 BTU = 1.055 kilo-joule = 252.16 Calories
1 kilo-calories = 4.18 kilo-joule = 3.97 BTU
1 watt-hour = 860.4 calories = 3600 joules

Limitations of online AC System Simulation :

1. It is considered that two air conditioner units ( 1st and 2nd units ) are fitted at both ends of coach. Doors opening time at each station stop is considered 30 seconds.

2. Constant atmosphere temperature is considered during the complete simulation however the value of atmosphere temperature vary with time. Atmosphere pressure is considered 760 mmHg.

3. User can not input the value of sensible and latent heat of passenger, heat transfer coefficient of walls, roof, floor, windows etc.

4. The value of initial coach % humidity is considered equivalent to the % humidity of outside atmosphere.