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.  

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 - Option 1

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.