humidification 2

Department of Chemical Engineering, U.E.T. Lahore

Chemical Engineering Department

University of Engineering & Technology, Lahore

Prof. Dr.G.M.Mamoor

Course: SHMTF

Simultaneous Heat and Mass Transfer

Humidification/Dehumidification

2Course: SHMTDepartment of Chemical Engineering , U.E.T. Lahore

Industrial Applications & Equipment


The Fume Scrubber

Throat pieces are used to assure close contact of gas and liquid streams.

Open area allows separation of the two phases. Nozzle capacities are normally 5-10 gal/min of

water Throat pieces capacity is 300 ft3/min of gas per

gallon per minute of liquid. Scrubbers are often applied to removing dust

from gas stream


Spray pond

Spray nozzle delivers 50 gals/min each. Pressure is about 7 psig. Located about 6ft above the pond surface. Spaced about every 4 ft. Pond is surrounded by a louvered fence about 12 ft

high to reduce wind losses. Typical water loading rates for spray ponds are about

500 Ib/hr ft2 of pond area. Advantage:

Low Cost.

Disadvantage: Low capacity for a given pond area. Large water losses.


Cooling Towers (Types)

Cooling towers are a very important part of many chemical plants. They represent a relatively inexpensive and dependable means of removing low grade heat from cooling water.

Cooling towers fall into two main sub-divisions:

1) Natural Draft.2) Mechanical Draft.

Natural draft :Designs use very large concrete chimneys to introduce air through the media. Due to the tremendous size of these towers (500 ft high and 400 ft in diameter at the base) they are generally used for water flowrates above 200,000 gal/min. The tower is built of reinforced concrete varying from 5 to 12in. Thick.Usually these types of towers are only used by utility power stations.


Natural Draft.Cooling Tower


Mechanical draft Cooling Towers Mechanical draft: This type of cooling towers are much more widely used.

These towers utilize large fans to force air through circulated water. The water falls downward over fill surfaces which help increase the contact time between the water and the air. This helps maximize heat transfer between the two.

Mechanical draft towers offer control of cooling rates in their fan diameter and speed of operation. These towers often contain several areas (each with their own fan) called cells.


Mechanical draft Cooling Towers


Types of Mechanical Draft Towers

Figure 2: Mechanical Draft Counterflow Tower Figure 3: Mechanical Draft Crossflow Tower

Mechanical Draft Counterflow Tower Mechanical Draft Crossflow Tower


Types of Mechanical Draft Towers


Natural Draft


Mechanical Draft Towers


Simplified diagram


Natural Draft


Natural draft towers in action


Component Description

Condenser pump & distribution systems

Hot water from a chilled water system is delivered to the top of the cooling tower by the condenser pump through distribution piping. The hot water is sprayed through nozzles onto the heat transfer media (fill) inside the cooling tower. Some towers feed the nozzles through pressurized piping; others use a water distribution basin and feed the nozzles by gravity.

Cooling loop system A cold-water collection basin at the base of the tower gathers cool water after it has passed through the heat transfer media. The cool water is pumped back to the condenser to complete the cooling water loop

Components of a cooling tower


Heat transfer media

Cooling towers use evaporation to release waste heat from a HVAC system. Hot water flowing from the condenser is slowed down and spread out in the heat transfer media (fill). A portion of the hot water is evaporated in the fill area, which cools the bulk water. Cooling tower fill is typically arranged in packs of thin corrugated plastic sheets or, alternately, as splash bars supported in a grid pattern.

Cooling tower fan system Large volumes of air flowing through the heat transfer media help increase the rate of evaporation and cooling capacity of the tower. This airflow is generated by fans powered by electric motors. The cooling tower fan size and airflow rate are selected for the desired cooling at the design conditions of hot water, cold water, water flow rate, and wet bulb air temperature.

HVAC cooling tower fans may be propeller type or squirrel cage blowers, depending on the tower design. Small fans may be connected directly to the driving motor, but most designs require an intermediate speed reduction provided by a power belt or reduction gears. The fan and drive system operates in conjunction with a starter and control unit that provides start/stop and speed control.

Components of a cooling Syatem


Components of a cooling tower Drift eliminators As cooling air moves through the

fill, small droplets of cooling water become entrained and can exit the cooling tower as carry-over or drift. Devices called drift eliminators are used to remove carry-over water droplets. Cooling tower drift becomes an annoyance when the droplets fall on people and surfaces downwind from the cooling tower. Efficient

drift eliminators remove virtually all of the entrained cooling water droplets from the

air stream.


Induced draft cooling tower


Cut- view of a tower


Cooling towers in action (induced draft)


Properties of humid air

• Humid air can be said to be a mixture of two ideal gases - dry air (d.a.) and water vapor. • The dry air can be treated as one component

since its composistion does not change in the temperature and pressure interval considered here.

• Dry air can be assumed to consist of:

Nitrogen: 78 % by volume, M=28 kg/kmol

Oxygen: 21 % by volume, M=32 kg/kmol

Argon: 1 % by volume, M=40 kg/kmol

• Mean molecular weight of dry air, M=29 kg/kmol


Amount of water in humid air

The amount of water in humid air can be

expressed in several ways;

• Relative humidity, Y

• Water content, xw

• Dew point

• Dry, and wet bulb temperature


Dew point

Aircool.gif

Cooling Air

When air is cooled, relative humidity increases as shown to the right. Cooling air that is 90oF/20% RH causes the relative humidity to increase until it reaches 100%; the temperature known as the dew point (43.6oF in this example). At the dew point, moisture begins to condense out of the air and is transformed from vapor to liquid.Most everyone has witnessed bathroom mirrors 'fogging' during a hot shower or iced-drink glasses 'sweating' on the outside. The cool surfaces are simply condensing moisture out of humid air.This aspect of psychrometrics is very important when it comes to analyzing indoor conditions as temperatures and humidity levels within our environments are very dynamic.


Relative Humidity

The amount of moisture in air can be expressed in a number of ways but the most common reference is relative humidity. This value, expressed as a percentage, represents the amount of moisture in the air relative to the amount of moisture air could hold at that temperature.

Therefore, the relative humidity at a given temperature will range from 0% (dry air) to 100% (fully saturated air). (When you exceed 100%, moisture starts falling out of the air and it rains.)


Relative Humidity

In other words, air at 70 degrees Fahrenheit (70oF) and 50% relative humidity (50% RH) is holding half of what it could hold when completely saturated (100% RH) as shown on the right.


PERCENT SATURATION

100 Y . Ma/Mb

Percent saturation = 100 Y/Ys

Ys . Ma / Mb

Y = mole ratio of condensable to non-condensable component

Ys = mole ratio present at saturation

Ma, Mb= Molecular weight.


The Humidity Chart


Example

100%

60%


Determination of bulk-gas phase temperature

V

A

P

O

R

P

H

A

S

E

E

N

T

H

A

L

P

Y

Hv

Temperature (Tl,Tv,or Ti) , 0F

Interface condition

locus

Operating Line

C(Tl,Hv)

F(Tv,Hv)

D

GH

I

J

L M N

O P Q

humidification 2

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