How the humidity hazard to machines is explained to a layman?

 by  Maryam Hussain

How does air become humid and dry?

When the air remains in contact with a water surface then it gets the vapor from water because of the continuous evaporation of water in the air above it. The extent of the evaporation rate is a function of the vapor pressure of water at the present temperature of the water.

When the quantity of vapors of the water in the air is such that the partial pressure of vapors in the air equals the vapor pressure of the water at its existing temperature then an equilibrium is said to be reached between the two pressures and the gas is said to be saturated with this vapor. It is not possible for the air to contain a greater stable concentration of water vapor, because as soon as the vapor pressure of the water is exceeded by the partial pressure of the water vapor, condensation takes place. When the air contains water vapor then it is called moist air or humid air.

When the air contains water vapor in such proportions that its partial pressure is less than the vapor pressure of the water at the existing temperature, then this mixture is called partially saturated or partially humid. The relative saturation of such a mixture may be defined as the percentage ratio of the partial pressure of the water vapor to the vapor pressure of the water at the existing temperature.

The relative humidity and the relative saturation are the function of both the composition of the mixture and its temperature as well as of the nature of the vapor. Relative saturation is a generally used term for a vapor-gas system that is replaced with relative humidity when it is a water-air system.

Illustration-1:

Moist air at 100 kPa and 25 ^oC was cooled to 18 ^oC by passing it through over a water tube heat exchanger. Assuming that exit air is saturated without condensation, what was the initial water vapor content, and compute the initial relative humidity? What can be concluded from the solution? Take required input data from steam tables.

Solution:

The required data is

Vapor Pressure at 18 ^oC:  P^s = 2.063 kPa (abs), Vapor Pressure at 25 ^oC:  P^s = 3.166 kPa (abs)

At 18 ^oC, relative humidity = 1.0 due to saturation condition or R.H = 100 %

So, Partial pressure of water vapor in air = vapor pressure of water vapor = 2.063 kPa

At 25 ^oC, relative humidity = R.H                                           

                                                 =  0.651

Or                                            =  65.1  %

Conclusion:

It can be concluded that high-temperature air can accommodate more moisture as long as it is not cooled.

What are some industrial and domestic hazards of environmental humidity?

1)    Humidity effects on the combustion of a gas turbine:

A gas turbine is one of the internal combustion engines. A chemical reaction, named “Combustion” is used for energy production in power plants, gas turbines, and engines. Combustion involves a chemical transformation between a substance or substances called fuels and other chemicals called oxidizers, both of these being reactants.

The oxygen required for a given combustion reaction comes from the air; on a volume & dry air basis, air contains about 78% Nitrogen gas in the form of diatomic N₂ molecules, about 1 % of rare & noble gases such as helium, argon, & 21 % Oxygen gas in the form of a diatomic O₂ molecule.  

For a gas turbine, combustion depends on the type of fuel used and the atmospheric conditions of air because the inlet of air to the gas turbine engine is from the atmosphere, and any change in temperature, humidity, or pressure of the inlet air will affect the mass flow rate and resultantly the power produced by the gas turbine.

2)    Why do humans feel hot during humid hot months?

The human body is sensitive to small changes in external temperatures. The temperature of the human body is normally around 37°C, and it attempts to maintain this temperature irrespective of the temperature of the surroundings of the human body.

If the surroundings are at a higher temperature, sweating will allow heat to be lost by evaporation & removal caused by air movement over the skin. On the other hand, if the surroundings are at a lower temperature than the human body temperature, shivering causes internal muscular activity and generates body heat.

At higher temperatures w.r.t human body temperature, the body has more and more difficulty in maintaining its natural temperature unless sweating occurs and therefore this lost water must be replaced by drinking. If the air surrounding the human body has high humidity, evaporation of the sweat from the body's skin cannot take place and the body begins to overheat. This leads to heart strain and may also cause heat stroke.

3)    Corrosion of appliances, equipment, and machinery:

Of all the failure mechanisms, corrosion is probably the most expensive. For the past 60 years, most developed countries have spent somewhere between 3 and 4% of their GDP on corrosion prevention, treatment, and correction. For most of the world, the cost is similar; however, the warmer and more humid countries are more burdened & affected by the consequences of corrosion. 

As an illustration, let us consider a gate valve of a process. The function of gate valve is to supply and interrupt the flow of fluid. During an attempt to close the valve, the operator reported that the gate valve is jammed. Maintenance was called on and it was revealed that it jammed due to corrosion. Later it was found that humid environment causes corrosion of the shaft disk or seat; these are the parts of the valve that directly affect the flow. Therefore, it is due to humidity-induced corrosion. We could periodically lubricate and operate the valve to avoid the corrosive effect, but the power to eliminate the humid environment is not ours.

4)    Degradation in electrical insulation of motors:

Outdoor storage and usage of electric motors without special precautions is not recommended because variations in ambient temperature and humidity can cause condensation, producing rust upon unprotected metal parts and a deterioration of the electrical insulation. Therefore, the electrical motor may get instantaneous tripping upon start on a high value of electrical current; this protection is usually called thermal overload protection.

Whenever the motor temperature is equal to or below the room temperature, and coincidently relative humidity of the environment is above 60 percent, water vapor can condense on and within the machine to promote rapid deterioration. In order to prevent condensation, the motor’s space heaters are required to be operated.

What is the dehumidification of a gas or air?

When a stream carrying moisture is cooled to a temperature below the dew point temperature of the air mixture, some condensation of the water vapor would occur. This process is called dehumidification. Dehumidifiers recirculate air while removing the moisture from the air by a refrigeration process. The operational cost of dehumidifiers is high due to high electrical costs.