HUMIDIFICATION - 2
ADIABATIC SATURATION OR EVAPORATIVE COOLING:
In this process air comes in direct contact with water in the air washer.There is heat and mass transfer
between air and water. The humidity ratio of air increases. If the time of contact is sufficient, the
air gets saturated. Latent heat of evaporation required for conversion of water into water vapor is taken from
the remaining water.When equilibrium conditions are reached, water cools down to the wet bulb temperature
of the air.In general it is assumed that, the wet bulb temperature and before and after the process is the same.
If the air washer is ideal, the dry bulb temperature and wet bulb temperature of the air would be eqaual.
If a process is adiabatic, heat is neither added or removed from the system
Dry bulb temperature of the air goes down in the process and the effect of cooling is due to the evaporation
of some part of the water. That is why it is called EVAPORATIVE COOLING.
The sensible heat is decreased as the temperature goes down but the latent heat goes up as water vapour
is added to the air.The latent heat required by the water which is evaporated in the air is drawn from the
sensible heat of the same air.Thus it is transformation of sensible heat to latent heat. During this process
the enthalpy of air remains the same.
If humidity ratios of saturated air and of the air before saturation is known, then the difference between
the two would be the amount of water vapour absorbed by unit weight of dry air.
The amount of water sprayed in the air-washer to maintain misty condition can be as much as 200 times the
quantity of water absorbed by the air during summer time.
AIR CONDITIONING PROCESS FOR THE TEXTILE INDUSTRY:
Air is drawn in and is passed through the air washer, it gets saturated adiabatically. Since it is not
saturated 100%, the dry bulb temperature of the saturated air will be 1 degree greater than WBT.
When this air is admitted into the conditioned space, it gets heated due to the heat load of the room.
During this heating process the air does not lose or gain any moisture as latent heat load is absent.
The air displaces an equal amount of air in the room which is pushed outside the room.
If we know the heat load of the room, we can easily calculate the rate of flow of air, G, which is the air
circulation rate necessary to give the required relative humidity, from the following formula.
G = H(h2-h2)
G-mass flow rate of dry air, KG/h
H-total heat of air,Kcal/h
h1-enthalpy of supply air, Kcal/kg
h2-enthalpy of outgoing air,Kcal/kg
The air circulation rate is generally expressed in cubic meters per hour and not in terms of mass
flow rate. (h2-h1) can be calculated from the initial and final temperatures. Therefore
H = (Q/V)* Cp * (DB2-DB1)
Q-rate of air flow,metercube/h
Cp- specific heat of air
V-specific volume of air,metercube/kg
DB1- supply air DBT,degree centigrade
DB2- leaving air DBT,degree centigrade
However in practice, the air washer does not continuously supply air of 100% RH. The efficiency of air
washer falls. It is considered satisfactory, if the difference between DBT and WBT of air after the air washer is 1 degree centigrade.
The following equation can be used for practical purposes.
(DB2-DB1) = ((3.39 H)/Q)+0.52
Once the relative humidity to be maintained is decided, the quantity (DB2-DB1) is fixed. In other words,
once the inside relative humidity is fixed, the minimum dry bulb temperature in the condition space
is determined by the wet bulb temp. of the outside air. It is not possible to go below this DBT unless
refrigeration is used.
Why refrigeration is required?
Let us assume that WBT of outside temp is 35 degrees. If the RH% to be maintained in the department is 60%,
then DBT of the conditioned space should be 43.5 degrees. Whatever we do , we cannot reduce this temperature
as long as we are maintaining a RH OF 60%. Underthis circumstance, refrigeration plant is required to
bring down the WBT of the air inside, so that 60% RH can be maintained at lower DBT depending on
the refrigeration capacity.
Humidification system without chilling helps to maintain only the RH% without much difficulty. They can be classified generally as either unitary or central station. Central system is the most widely used sytem in the textile industry. The systems principal components are
- Air moving devices- fans
- mixing devices for air and washer- i.e Air washers
Air moving devices are always broken into two halves, 1. Return Air fans and 2.Supply Air fans.
The return air fans return the air to the plant room from where it may circulated or exhausted in the mill
The supply air fans- supply air to the mill from the plant room.
Air washer is a device for intimately mxing water and air. The intimate contact between these two elements is best brought about- for this application- by drawing air through a spray chamber in which atomized water is kept in transit.
The following components are a must in a Humidification system
- Return Air and Supply Air fans
- Air washer
- Return Air floor grills
- Return Air trenches
- Exhaust damper
- Fresh air damper
- Supply air ducts and grills
- face and bypass dampers on the air washer
- Automation control for damper operation to maintain conditions
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