Humidification System Design Explained | Air Washers, Direct Systems and Efficiency Tips

Complete Guide to Designing Humidification Plants for Textile & Industrial Applications Part-1

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This article explains the essential design considerations, operational principles, and different types of humidification systems with a focus on air washer technologies. It provides engineers, technicians, and plant designers with a clear understanding of how to select and design the most suitable humidification solution.

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Introduction to the Design of Industrial Humidification Plants

Key Considerations in Designing a Humidification Plant

• Designing of any system requires optimizing the running cost and capital cost, while designing a humidification plant, the designer should consider the following.
• The type and the industry to which the plant is designed.
• The location of the plant with regard to the place to be humidified.
• The selection of equipment to optimize power spending.
• Flexibility to meet the range of requirement.

In the subsequent paragraph we are discussing with regard to the type of humidification plant, the principle of operation and various system available.

Overview of Central Humidification Systems

• In the central humidification system the dust laden hot air from the department and from the machined is drawn through a net work of underground trenches. The air is filtered in the exhaust plant.

• According to the requirement of the department the exhaust air is vented out or re-cycled back through the system. 
• The fresh air is drawn through the fresh air filter and it is mixed with the return air, when required, by operating the exhaust, return and fresh air dampers. 
• After blending, the air is passed through the air washer where it is saturated with water vapour, before discharged into the department.
• Water evaporates into air at all temperature. The rate of evaporation depends on the following factors.

Factors Affecting Evaporation:

1. Ambient pressure,
2. Ambient temperature,
3. Percentage of saturation,
4. Inter surfacial area available for evaporation.

In an air washer water is evaporated by increasing the inter surfacial area depending on the air washers are classified as below: 

1. Cell Type Air Washer,
2. Pulverizing Type Air Washer,
3. Spray Type Air Washer,

Cell type air washer🌬️

• In a cell type air washer, cells made of wood shavings, fibres (natural / synthetic) is drenched in water. Hence the entire surface area of the wood shavings or the fiber is available for the evaporation of water. 
• The advantages of this type of air washer is that the horse power required to increase the area of contact between air and moisture is very low. However the system has following disadvantages when used in a textile mill.

1. The wood shaving or the fibres in the cell type air washer increase the resistance to the air flow. Hence the pressure drop in the system is very high, which consequently increases the horse power required for the fans or decreases the air volume discharged by the fan. We have observed the total system horse power with the above resistance have only increased to maintain the given air volume of the system.
2. Even through the inlet air is filtered through a wire mesh it contains micro duct which is deposited in the cells and the micro fiber being biological material, they deteriorate very fast and cause fungus growth. This creates a bad stink inside the department.
3. The cell type air washer will have to be dismantled every 3 months and the filler will have to be dried in sun before repacking them in the cells with additional material. This increases maintenance work on the plant and the mill having number of plant will have a separate drying yard for this purpose. With additional material packed into the cell after every maintenance the system resistance increases making the system energy expensive.
4. As the cells are packed manually the porosity of the cells are nit uniform hence the air volume passing through the cell at different are not uniform. In a point where the air volume or the velocity is high, absorbs more moisture and the cell become dry in that area. The above will result in the air washer discharging air with different RH conditions. Hence the overall saturation efficiency of the air washer is much lower.
5. Due to the above disadvantages this air washer is not recommended for textile application. However they are being widely used for engineering industries such as machine shops where the impurities for the atmosphere is non fibrous.

Due to the above disadvantages this air washer is not recommended for textile application. However they are being widely used for engineering industries such as machine shops where the impurities for the atmosphere is non fibrous.

Pulverizing Type Air Washer⚙️ 

• In pulverizing type air washer water is beaten to micron size particles by mechanical pulveriser. 
• When the stream passes through the micron sized water droplets rapid evaporation takes place, because of increased inter surficial area. 
• Fan of the humidification system is generally used to pulverize the water with the centrifugal force.

• This method of evaporation of water is very ideal for smaller sized humidification plants. Semi-central type of humidification plants use this technique. 
• However, in large plants we observe water carryover into the department with this type of system. 
• In stray cases we have also observed ionization of air leading to yam fly in department processing synthetic fibres.

• We have also observed the dissolved salts are liberated in the course of evaporation and they are getting deposited over the travelers and other moving parts of the machines.

Spray Type Air Washer💧 

• In this type of air washer, water is pressurized with high pressure pump. The pressure head developed is converted into velocity head by a set of nozzles. 
• Water at high velocity is developed into a conical discharged by the large vortex angle of the spray.

• Water splits onto micron sized particles as the pumping volume is very high, large number of such particles are created in the air washer. 
• They have very large inter surfacial area and evaporates instantaneously. 
• In this process as the water become air borne, it offers minimum resistance for the air flow. 
• The total resistance in the spray type air washer varies from 4 to 6mm of water column.

• In the spray type air washer there is direct contact between air and water and there is no third media involved. Because of the length of the spray, any particle of air is in touch with the moisture for longer period. 
• Hence saturation efficiency of this air washer is very high and it is also consistent. Because of the above advantages of the spray type air washer the impurities in the air is also scrubbed to a great extent. 
• Due to the above characteristics the spray type air washer is universally accepted for textile application.
• The performance of the air washer is assessed by its saturation efficiency and the pressure drop. 

The saturation efficiency of the air washer is:

📘 Formula:

ESAT (%) = (Entry Air Dry Bulb − Leaving Air Dry Bulb) / (Entry Air Dry Bulb − Entry Air Wet Bulb) × 100

• Entry Air Dry Bulb (DBT in): Temperature of air entering the system (normal air temperature)
• Leaving Air Dry Bulb (DBT out): Temperature of air after cooling
• Entry Air Wet Bulb (WBT in): Lowest possible temperature air can reach by evaporation (depends on humidity)

Example:

If Entry DBT = 35°C,

If Leaving DBT = 28°C,

If Entry WBT = 25°C,

Then:

ESAT = (35 − 28) / (35 − 25) × 100 = 7/10 × 100 = 70%

👉 System efficiency = 70%

• The saturation efficiency of air washer vary with the different type of equipment’s, design and other features of the air washer.
• The air washer or the humidification plant capacity deteriorates with the fluff checking and scale formation. 
• The scales formed by the dissolved salts from the water forms a hard coating which is reinforced by the fibrous cotton lint. 
• The above not only blocks the air flow but also increases the surface friction of the air. 

The efficiency due to reduction in air volume on account of the blockade is:

📘 Formula: 

Eq (%) = (Actual Air Volume / Design Air Volume) × 100

• Actual Air Volume: The real airflow measured at the site (using instruments like anemometer or flow hood). 
• Design Air Volume: The airflow value specified in the HVAC design or drawings. 
• × 100: Converts the ratio into a percentage.

Example:

If Actual Air Volume = 900 CFM 

If Design Air Volume = 1000 CFM 

Then: Eq = (900 / 1000) × 100 = 90%

This means airflow is 10% lower than design

Direct Humidification System🌫️ 

• In this type of humidification system clean water from an over head tank is sprayed directly into the department with compressed air into micron size droplets. 
• These droplets evaporate into the department. As there is no fresh air draft into the system, there is no adiabatic cooling. 

• In this system we observe steep increase in the absolute humidity inside the department combined with a slight reduction in the temperature. The water used in this type of humidification system should be soft and demineralised.
• This type of humidification system is used in few installation in European countries where low dry bulb temperature and department where in high relative humidity is required.

Depending on the location of then the fan humidification system, they are also classified as below.

1. Suck – through system,
2. Blow or push – through system,

Humidification System Configuration Types

SUCK THROUGH SYSTEM

• In this type of humidification plants the supply air fan is located after the spray chamber. Air is humidified before being drawn through the fan. As the humidified air will have to pass through the fan structural, which are made of mild steel, corrosion takes place. 
• When the fan structural become weak fan failure takes place. In stuck through system depending on the number of supply air ducts, independent fans are provided for independent ducts at the duct mouth.

BLOW THROUGH SYSTEM

• In the blow through system fan is located before the air washer and always the fan handles dry air. This system is generally accepted for textile applications. From the above paragraphs, it is possible to select the type of system the based on the industry.
• With regard to the location of the humidification plant it is necessary that the air either through ducting for the supply to the department or exhausted out from the department should travel the minimum distance.
• With regard to the selection of various equipment’s that the humidification plant, the same is discussed in different chapter. While designing equipment, the flexibility in operation should be bone in mind for optimized working.

Plant efficiency:

Air washer efficiency should be maintained at 95%.
n = ( DBb – Dba / DBb- WBb ) x 100
where

• DBb = dry bulb temperature of air after air washer in C,
• DBb = Dry bulb temperature of air before air washer in C,
• WBb = wet bulb temperature of air before air washer in C,
• n = air washer efficiency of spray chamber in %

Air washer pump:
Automatic pump operation ON/OFF spray water pressure should be 2 to 3 kg/cm2.
Use softened water.

Conclusion

A well-designed humidification plant ensures stable environmental conditions, reduced energy consumption, and reliable performance. From cell-type and pulverizing systems to highly efficient spray-type air washers, each method has benefits and limitations.

Understanding airflow, saturation efficiency, system layout, and maintenance requirements is essential for selecting the right humidification plant for industrial applications.

For designers and engineers, this guide serves as a foundation for choosing a humidification system that meets operational needs while optimizing long-term efficiency.

About the Author – Insight Control System

Insight Control System provides technical education and practical guidance in the fields of Building Automation Systems (BAS), HVAC controls, PLC programming, sensors, and industrial automation. 

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3. Understanding Wet and Dry Bulb Thermometers & Relative Humidity Calculation
4. Humidity and Temperature in Textiles Industries
5. How to Calculate Humidity and Temperature in a Humidification Plant Using Hygrometers

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