Saturday, 7 July 2018

Effective Temperature | Comfort Index in Air Conditioning

Effective temperature (ET) is defined as the temperature of still or stagnant saturated air (that is air with 100% relative humidity (RH)) in the absence of radiation effects of surroundings so that the subject (person, equipment etc) would experience the same feeling of comfort as experienced in the actual unsaturated environment (that is RH<100%). It thus combines DBT and RH into a single index

There is another ET called New Effective Temperature (ET*) which is defined as the temperature of the air at 50% relative humidity so that a subject would experience the same feeling of comfort as experienced in the actual environment. Here, the effect of radiation and convection is considered.

Constant ET lines in a Psychrometric chart or in a vapor pressure vs DBT diagram is an inclined line towards the right like constant enthalpy lines or constant WBT lines. In this regard, at lower humidities, the DBT of the air can be higher for the same ET and for the same feeling of comfort. So at higher temperatures, the body will loose more heat in the form of latent heat (evaporation or perspiration). 

Important points to be noted for Effective Temperature

  • It is the best comfort index for still air condition with 30% to 70% of relative humidity.
  • It is not possible to generate a universal ET chart because it depends on activity and clothing.
  • Effective temperature is not a true comfort index as it does not consider the radiation effect of surrounding surfaces as well as air velocity.
  • To include the air velocity criterion, another index named Corrected Effective Temperature (CET) is introduced. CET will consider the effects of air velocity, radiation effects, DBT and RH.  

Friday, 6 July 2018

Winter Air Conditioning | Effects on DBT, WBT, DPT, RH, Specific Humidity, Enthalpy, Specific Volume

This article discusses the basics of Winter comfort air conditioning and its effects or possible changes on Dry Bulb Temperature (DBT), Wet Bulb Temperature (WBT), Dew Point Temperature (DPT), Enthalpy, Specific Volume, Relative Humidity (RH) and Specific Humidity or Humidity ratio with the help of a Psychrometric chart.

Winter Air Conditioning on Psychrometric Chart
Winter Air Conditioning - Option 1

Winter Air Conditioning
Winter Air Conditioning - Option 2


Effects of various parameters on Winter Air Conditioning

  • In winter comfort air conditioning, the dry bulb temperature (DBT) and specific humidity (also known as humidity ratio) will rise. This is because, in winter air conditioning, the outside air or the ambient air is cold and dry, which is well beyond our comfort level. The reason for the cold weather in winter is obvious. However, the dryness or low moisture content in the air is because of the reason that, as the temperature is low, the air molecules or bubbles will be smaller in size and it cannot hold more water or moisture in it. Hence by winter air conditioning, our motive is to increase the room temperature and humidity content in the air. So DBT and specific humidity increases.

  • In winter comfort air conditioning, the final relative humidity (RH) can be lower or higher than the initial value. This is because RH is dependent on both DBT and specific humidity. So the relative magnitude of DBT and specific humidity will decide whether RH will increase or not. 

  • In winter comfort air conditioning, both dry bulb temperature and wet bulb temperature will increase. The wet bulb temperature isotherms are depicted inclined in the Psychrometric chart and have an increasing trend towards the northeast (right of Psychrometric chart). So when DBT is increased or specific humidity is increased, then the wet bulb temperature lines move towards the right and hence WBT increases.

  • Enthalpy and specific volume lines also show the same trend as wet bulb temperature. That is in winter comfort air conditioning, enthalpy and specific volume will increase

  • In winter comfort air conditioning, DPT also increases. This is because, when specific humidity is increasing then obviously DPT will also increase. 

Saturday, 23 June 2018

Thermodynamic Properties of a System | Important Concepts

This article gives you an insight into important concepts related to the thermodynamic properties of a system. 


Introduction

Properties are also termed as state variables which are used to define or specify the state of a system. They are also known as point functions and are independent of the past history of the system or path of the process. Properties are exact differential. Properties of a thermodynamic system can be classified into intensive and extensive properties. Intensive properties are also called as intrinsic properties and are independent of mass. All specific properties are intensive properties. Some of the other examples of intensive properties are pressure, temperature, density, velocity, viscosity, thermal conductivity, molecular weight etc. Extensive properties are also known as extrinsic properties and they are dependent on mass. Some of the examples of extensive properties are volume, internal energy, entropy, enthalpy, Gibbs free energy, electric charge, magnetization etc. While deciding the type of property, we should not change the system under consideration.


Important Concepts

  • Important conditions to be fulfilled by the thermodynamic properties to specify the state of a system are, properties must be uniform within the system or throughout the system and it should be invariant with time at least for a temporary period. The uniformity of properties throughout the system indicates that there should not be any internal or external disbalance in the system. The system should not interact with its surroundings so that it is invariant with time. Such a condition or equilibrium can only be achieved by creating an isolating boundary between system and surroundings or by making system properties same as that of surroundings, in other words, by maintaining a dead state.

  • The above concept gives us an insight that when systems are not in a dead state, the state of the system can be defined only for an isolated system. For all systems, interacting with surroundings, they are not in equilibrium. Unfortunately for all our applications, the interaction between system and surroundings is our goal and from this only we are benefitted. So in order to tackle this, for all our thermodynamic studies, we consider intermediate states of the system while it is interacting with the surroundings to be in equilibrium in a limiting case which is known as quasistatic or reversible. So this way only we can specify the state of the system.

  • Even if system contracts to a point, intensive properties have a finite value. For example, we can define temperature at a point or pressure at a point.

  • However, when system contacts to a point, the value of an extensive property is zero. This is because the mass of the system tends to zero at a point. For example, we cannot define internal energy of a system at a point.

Effective Temperature | Comfort Index in Air Conditioning

Effective temperature (ET) is defined as the temperature of still or stagnant saturated air (that is air with 100% relative humidity (RH...