Department of NFE
Daffodil International University
Md Sohel Rana
ID No- 163-34-562
Department of NFE (Evening)
Submitted Date .0.2018
Properties of Dry Air:
Is the specific heat, ratio of specific heats, dynamic viscosity, thermal conductivity, density and kinematic viscosity is the dry air properties, dry air properties temperatures at ranging 175 – 1900 K.
Air is a mixture of several constituent gases. The composition of air varies slightly, depending on the geographical location and altitude.
The apparent molecular weight of standard dry air is 28.9645. The gas constant for dry air, Ra, is computed as 8314.41/28.9645 = 287.055(m3Pa/kgk).
Specific Volume of Dry Air is the ideal gas laws can be used to determine the speci?c volume of dry air. Therefore, V’a =RaTa/pa.
Specific Heat of Dry Air is the 1 atm (101.325 kPa), the speci?c heat of dry air Cpa in a temperature range of -40 to 60 degree C varying from 0.997 to 1.022 kJ/(kg K) can be used.
Enthalpy is the heat content of dry air, is a relative term and requires selection of a reference point. Ha = 1.005(Ta-To)
Dry bulb temperature is the properties of dry air it measured by a thermometer.
Properties of Moist Air:
Is the dry bulb temperature, dew point temperature, wet bulb temperature, relative humidity, absolute temperature, vapor pressure, heat content, specific volume ; density.
Moisture in inappropriate amounts and places is very damaging to the useful life of food. Because of this, much effort is put into reducing the water content of dry foods in order to prolong their shelf lives. Once it is reduced to the desired level the product can then be packaged for storage. Unfortunately, merely reducing moisture content is not always sufficient. Environmental conditions can play a role as well.
There are four mechanisms by which environmental conditions may cause a moisture problem in your food storage:
1. – The air trapped in the container with the food may have held sufficient humidity to raise the moisture content of the food to undesirable levels.2. – Even if the water vapor content wasn’t too high, a falling temperature level may cause the trapped humidity to reach its dew point causing water to be squeezed out of the air to condense on your food much the same way as dew forms on your lawn on cool mornings after a warm, humid night.3. – The seal of the container may not be sufficiently tight enough to prevent moisture laden air from leaking in.4. – The packaging material itself may be porous to water vapor to one degree or another. All paper, wood and cardboard has this fault. Depending upon their particular physical properties some plastics do as well. Metal and glass containers have excellent barrier properties though their seals may not.