Humidification Technology – Are You in the Know?
CAREL designs, manufactures and distributes humidification solutions for the industrial and commercial segments in the HVAC market. They have solutions for air-conditioning, refrigeration and heating, and humidification and evaporative cooling systems. Not constrained to just one humidification technology, Carel has multiple options to offer the best fit for new and retrofit applications.
A voltage is applied to metal electrodes immersed in common drinking water, heating the water (Joule effect) until boiling, producing steam. The quantity of steam produced is proportional to the electric current, which is in turn proportional to the level of water.
This electric current is measured by a current transformer which varies the level of water using a fill solenoid valve and drain pump. The current, and consequently the steam production, can be modulated. Due to evaporation, the level of water decreases and must therefore be topped up.
As steam does not carry mineral salts, the salt concentration increases until the water becomes potentially corrosive. The control software keeps the water concentration at the optimum value, creating the best compromise between water, energy economy, and humidifier life. Lime scale is deposited over time and covers part of the cylinder, which can easily be replaced or cleaned.
Compared to the complementary immersed element heater or gas humidifiers, immersed electrode humidifiers:
- Are less expensive to purchase;
- Operate on drinking water (not completely demineralised or softened);
- Require the periodical replacement (or cleaning) of the cylinder;
- Feature modulation suitable for comfort or industrial applications, without extreme requirements.
As one of the humidification solutions, immersed heater humidifiers heat the water until boiling, thus transferring heat from the hot element (electric heater) to the liquid. These appliances can operate on demineralised water, as they do not exploit the electrical conductivity of the water, and periodical maintenance can therefore be greatly reduced.
The resistive elements must always be completely immersed in the water, to avoid overheating, and consequently sensors are need to measure the water level. To precisely modulate the steam flow-rate, components (solid state relays) are used to apply power with a variable duty cycle.These characteristics make the heater units more complex than their counterparts with electrodes, yet independent of the characteristics of the water, and with much more precise modulation of the flow-rate. In addition, as heater humidifiers are intrinsically subject to overheating, the quality of design and the presence of safety systems are essential to ensure reliable service over time.
Immersed heater humidification, complementary to immersed electrode humidification, is becoming increasingly present in work specifications where:
- Humidity must be controlled precisely (museums, laboratories, cleanrooms);
- The quality of the water is not constant or is problematic (for example, aboard ships);
- Periodic maintenance needs to be minimised (using demineralised water).
Gas-fired humidifiers use a local burner/heat exchanger system to produce steam, combining the low-cost operation of the combustion process with the simplicity of a stand-alone appliance. The installation and maintenance for the gas assembly are similar to those encountered with residential boilers.
The heat exchanger is generally made from stainless steel or aluminium, and is contained in a tank. The tank is usually stainless steel and filled with water to a constant level. Inside the heat exchanger is the combustion assembly, generally featuring premix burners, metal fibre combustion head and flame sensors. The flame is modulated continuously by controlling the combustion air fan speed and consequently steam production.
Gas-fired humidifiers offer humidification solutions which are generally preferred over electrically powered humidification, when:
- There are high humidification loads (heavy-duty applications)
- The cost of gas is lower than electricity
- The user accepts a higher initial investment to obtain savings over time
Direct steam humidifiers are designed to receive pressurised steam from a centralised system and distribute dry steam directly into a duct or air handling unit. Advantages:
- Minimum absorption distance: The outlet of steam through continuous slits (rather than through nozzles) creates a thin layer of steam that flows uniformly from both sides of the distributor, creating a large surface of contact with the air. This allows a minimum absorption distance (typically half the distance of conventional systems)
- Minimum losses due to condensation: The distributors are coated with a layer of very high tech ceramic insulation which reduces up to 90% of the losses from condensate and the heating of the surrounding air
- No emission of droplets of condensate: The steam distributors trap and return any droplets of condensate back to the centre of the pipe, where these evaporate again.
Adiabatic humidifiers bring about the direct evaporation of water into the air without adding external energy, and therefore without raising the temperature. The heat required for evaporation is supplied by the humidified air, which consequently is cooled.
These devices create a large interface surface between the air and water while in the liquid state. A thin layer of saturated vapour forms, with a partial pressure equal to the saturation pressure at the temperature of the liquid. If this pressure is greater than the partial pressure of the water vapour present in the air (as occurs when the water temperature is higher than the dew point of the air) and the air is not saturated, there is a pressure gradient that brings about the progressive evaporation of the liquid, at the expense of the sensible heat of the water and the air.
This principle is used by atomizing humidifiers, which generate very small droplets whose diameters are in the order of millionths of a metre. These droplets have a huge surface area, meaning the water is rapidly evaporated. Every 1 kg of water split into droplets with a diameter of 10 micrometres has a surface area of 600 square metres!
The main advantages of atomizing (adiabatic) humidifiers are:
- Very low electricity consumption: high pressure systems need less than 4 Watts per kg/h of capacity, against the 750W of steam humidifiers
- High capacity: the capacity can range from just a few kg/h (for example, humiSonic has a minimum capacity of 0.5 kg/h) to thousands of kg/h (humiFog)
- Very low maintenance, in particular when supplied with demineralised water
In the past, adiabatic humidification solutions were not used in applications that required very high hygiene, mainly because they recirculated the water. Today, on the other hand, the more advanced adiabatic humidifiers have a high absorption efficiency (ratio between the water absorbed by the air and the amount sprayed) and therefore do not recirculate the water. Stagnation of water is avoided through periodical washing and emptying cycles, the use of demineralised water and bacteriostatic materials.
Adiabatic humidifiers are increasingly used for energy saving applications: direct or indirect evaporative cooling in air handling units, or alternatively directly into rooms, with very low electricity consumption bringing considerable cooling of the air using simple water.
Centrifugal humidifiers use a spinning disk to atomize the water and transform it into millions of very small droplets that. These are blown by a built-in fan and introduced into the environment, where they evaporate, humidifying and cooling the air.
These humidification systems are simple, economical and easy to maintain. Humidifier operation is controlled by an electronic board inside the humidifier. The board manages the normal operation of the appliance and also performs a tank washing cycle when starting the unit, and an emptying cycle at the end of the humidification request. This avoids having stagnated water inside the unit.
Each centrifugal humidifier must be connected to an electrical panel, supplied by CAREL, to manage one or two humidifiers in parallel. They can operate on both utility water and treated water.
A piezoelectric transducer, immersed in water, converts the applied electricity into high-frequency mechanical vibrations. The water, due to its mass, is not able to keep up with this mechanical oscillation and creates successive waves of compression and depression, during which the cavitated water boils at low temperature and pressure, creating a very fine mist.
- Very low power consumption
- Use of demineralised water which prevents problems due to bacteria and other contaminants.
- Humidification, due to the adiabatic effect, ensures simultaneous cooling of the air, with a consequent decrease in refrigerant compressor running time.
- Finely atomized water with a diameter of a few microns are easily and quickly absorbed by the air.
The operating principle of the compressed air and water atomizers involves using compressed air to atomize the water into very fine droplets. The droplets evaporate spontaneously in the air, humidifying and cooling it. Indeed, evaporation occurs by “absorbing” sensible heat from the air that, as a consequence, is cooled.
The system essentially consists of:
- The control cabinet, fitted with a electronic controller
- Special atomizing nozzles that can be installed in an AHU/duct or directly in the room being humidified/cooled
- Manifolds for installation in the duct
- UV lamp sanitation system and protection filters.
With all of the types of humidification solutions discussed, you can be sure an Access Sales Engineer has the answer for your new or retrofit project.
Read a case study on how one of Carel’s high efficiency humidification solutions was provided by an Access Sales Engineer, for Precision Paper Converters located in Kaukauna, WI.
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