Humidification

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  • Topic: Temperature, Wet-bulb temperature, HVAC
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  • Published : January 22, 2013
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III. INTRODUCTION

Humidification is an operation involving transfer of materials between a pure liquid phase and a fixed gas, which is nearly insoluble in the liquid. The liquid contains only one component; hence there are no concentration gradients and no resistance to mass transfer. However, it involves both heat and mass transfer that are influencing each other. Humidification operations are mostly applied to air-water systems. To get data for a definite air-water mixture, a humidity chart is used. The most common is the Grosvenor chart which can be found in McCabe, Figure 19.2. (McCabe, 2001)

There are three ways of measuring humidity. The first one is the dew-point method where the temperature at which vapor begins to condense on a cooled, polished disk is directly measured. The second one is the psychometric method where the wet-bulb and dry-bulb temperature are determined simultaneously. From there temperatures, the humidity is located in the psychometric chart. The last method is the direct method where an analytical device is used. (McCabe, 2001)

According to Perry (1997), one of the oldest processes known is the cooling water process where water is cooled by exposing its surface to air. The heat transfer process involves latent heat (because a small portion of water is vaporized) and sensible heat (because there is a temperature difference between water and air. Approximately 80% of the heat transfer is due to latent heat while the remaining 20% due to sensible heat.

There are two types of cooling tower: natural draft and mechanical draft. The natural draft cooling tower depends on the movement of air due to the density difference between the cool inlet air and the warm exiting air. Mechanical draft cooling towers are called as such due to the use of mechanical fans. Under natural draft cooling tower is the hyperbolic type which is suited for very large cooling tower quantities. Mechanical draft cooling towers are futherly divided into two. The difference between the two types, induced and forced draft, is the location of the fans. For forced draft, fans are located at the bottom, while for induced draft, fans are located at the top of the tower. Systems for induced draft may be parallel or countercurrent. (Perry, 1997)

For the cooling tower heat transfer process, the most generally accepted theory is Merkel’s. His theory is based on the enthalpy potential difference as the driving force. It is assumed that each water particle is enveloped by a film of air and the enthalpy difference between the film and the surrounding air provides the driving force for the cooling process.

Figure 3-1. Water particle enveloped in a film of air.

Merkel’s equation evaluates the tower characteristic, KaV/L: KaVL=T2T1dTh'-h (Equation 3.1) Where:KaV/L = tower characteristic
K = mass transfer coefficient (lb water/h ft2)
a = contact area (ft2/ft3 tower volume)
V = active cooling volume (ft3/ft2 of plan area)
L = water rate (lb/h ft2)
T1 = hot water temperature (0F or 0C)
T2 = cold water temperature (0F or 0C)

h’ = enthalpy of saturated air at water temperature (Btu/lb) h= enthalpy of air stream (Btu/lb)
There are two methods to calculate for the tower characteristic. The first one is the Chebyshev’s method: KaVL=T2T1dTh'-h≅T1-T24x1∆h1+1∆h2+1∆h3+1∆h4 (Equation 3.2) Where: ∆h1= value of h’-h at T2 + 0.1(T1-T2)

∆h2= value of h’-h at T2 + 0.4(T1-T2)
∆h3= value of h’-h at T1 - 0.4(T1-T2)
∆h4= value of h’-h at T1 - 0.1(T1-T2)
The second one is the use of a nomograph prepared by Woods and Betts, which is a quicker but less accurate way. (Perry, 1997)

Figure 3-2. Cooling tower process balance.
The cooling tower process balance is illustrated on Figure 2.2. It can be seen that the water and air relationship, and the driving potential, exists in a counterflow manner. It is important to know that the heat removed from the...
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