IMPORTANCE OF RH AND TEMPERATURE:
The atmospheric conditions with respect to temperature and humidity play very important part in the
manufacutring process of textile yarns and fabrics.
The properties like dimensions, weight, tensile
strength, elastic recovery, electrical resistance, rigidity etc. of all textile fibre whether natural or
synthetic are influenced by Moisture Regain.
Moisture regain is the ratio of the moisture to the bone-dry
weight of the material expressed as a
Many properties of textile materials vary considerably with
moisture regain, which in turn is affected
by the ambient Relative Humidity (RH) and Temparature. If a dry textile material is placed in a room with
a particualr set of ambient conditions, it absorbs moisture and in course of time, attains an equilibrium.
Some physical properties of textile materials which is
affected by RH is given below:
- Strength of COTTON goes up when R.H.% goes up
- Strength of VISCOSE goes down when R.H.% goes up
- Elongation %ge goes up with increased R.H.% for most textile fibres
- the tendency for generation of static electricity due to friction decreases as RH goes up
- At higher levels of RH , there is also a tendency of the fibres to stick together
Temparature alone does not have a great effect on the fibres.
However the temperature dictates the amount
of moisture the air will hold in suspension and , therefore, temperature and humidity must be considered together.
psychrometrics is the study of the thermodynamic properties of air and water vapour mixture or simply
the study of solubility of moisture in air at different temperatures , the associated heat contents
and the method of controlling the thermal properties of air. There are various properties of moist air, they are
- Dry bulb temperature
- wet bulb temperature
- dew point temperature
- relative humidity
- specific voulme
- enthalpy etc.-
DRY BULB TEMPERATURE:
This is the temperature of air-moisture mixture as registered by an ordinary thermometer.
WET BULB TEMPERATURE:
It is the temperature of air-moisture mixture as registered by a thermometer where the Bulb is covered
with the wetted wick.
DEW POINT TEMPERATURE:
This is the temperature of air at which moisture starts condensing when air is cooled.
This is the weight of water vapour present in unit weight of dry air.
This is the ratio of the mass of water vapour to the mass of dry air with which the water vapour is associated
to form the moist air. Relative humidity is a measure of how thirsty the air is at a given temparature.
At 100%, the air is completely saturated. At 50%, the air holds one-half of what it could hold if saturated
at the same temperature. The thirstier the air, the lower the percentage and the more it can rob fibres of moisture.
It is the volume per unit weight of air.
It is the total heat contained in unit weight of air, taking the heat content of dry air at 0 degree
centigrade. Enthalpy includes both the sensible heat and latent heat contained in the air.
SENSIBLE HEAT AND LATENT HEAT:
Sensible Heat is any heat that raises the temperature but not the moisture content of the substance.
This is our regular and familiar every day heat. Because it raises the temperature it can be detected by the senses, and this in fact, is why it is called Sensible Heat.
Latent Heat is the tricky one. When we talk of Latent Heat we mean 'Latent Heat of Vaporisation'.
It is that heat required to transform a liquid to vapour. Take water for example. Water can be
heated to its boiling point of 100oC. If more heat is added at this point the temperature of the
water does not increase. The water continues to boil and becomes steam. So where does all the heat go? Well, the heat goes into changing the water into steam. The latent heat of vaporisation in this instance is the heat required to change water from liquid at 100oC to vapour at the same temperature.
TYPICAL AIR-CONDITIONING PROCESSES:
SENSIBLE COOLING / HEATING:
Involving a sensible change in the temperature of air with the specific temperature of air with the specific humidity
or moisture content of air remaining the same. This process is shown as a horzontal line in Psychrometric chart
as no moisutre has been added or removed from the air and the humidity ratio remains the same.
The heat required to bring this change is shown below
H = G(h2-h1)
H = (Q/V)(h2-h1)
H is the rate of heat flow, kcal/h
G is the mass rate of flow of air, kg/h
Q is the volume rate of flow of air, meter cube / h
h1,h2 are the enthalpy before and after heating, kcal/kg
V is specific volume of air, meter cube/ kg
COOLING AND DEHUMIDIFICATION:
This is a process invoving reduction in both the dry bulb temperature and the specific humidity.
If air is cooled to temperature below its dewpoint, condensation of moisture occurs. This condensation
continues as long as the air is being further cooled. By noting the enthalpy of air before and after cooling,
we can determine the heat to be extracted or the tonnage of refrigeration required for cooling air continuously.
COOLING AND HUMDIFICATION:
This is a process involving reduction in DRY BULB Temperature and increase in specific humidity.
HEATING AND DEHUMIDIFICATION:
This is a process where there is an increase in DRY BULB temperature and reduction in speccific humidity.
This is a process where there is only an increase in specific humidity. This is a process of steam injection.
HEATING AND HUMIDIFICATION:
This is the process where there is an increase in both DRY BULB temperature and specific humidity.
This is a process of cooling and humdification but with no change in the enthalpy of air during the process.
This is the process through an air-washer using recirculated water for spraying. This is the most commonly
used humidification system in a textile mill.
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