When selecting an electric steam humidifier for either a residential or commercial/industrial application, one of the first choices you are likely to face will be whether to opt for a resistive or electrode humidifier(fr,ko,th). Each type is notably different in 3 main categories of efficient steam production:

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• Heating water in order to produce the desired steam;
• Controlling steam output, and
• Maintenance requirements

In part one of this article, we will begin the exploration of these differences with a brief overview and quick explanation of controlling steam output of electrode and resistive steam humidifiers.

Principles of Operation:

Electrode steam humidifiers bring water to a boiling point by passing an alternating current between electrodes within a disposable plastic cylinder (the defining feature) utilizing the ions dissolved in the water as the means of conduction. Potable water (which contains dissolved ions) will conduct electricity between the electrodes with substantial resistance, it will heat the water until the boiling point is reached and steam is produced. The capacity of the unit will depend on the size of the cylinder (volume of water), the surface area of the electrodes in contact with the water and the rated voltage and kW of the humidifier. A current transformer is used to monitor the current draw of the electrodes. It is the means of controlling the steam output which is accomplished by raising or lowering the water level inside the cylinder. As the cylinder fills with water, more of the electrode surfaces will be immersed allowing more current to pass between them thereby increasing the power consumed and total steam output. Reduction of steam output will occur by lowering the water level.

Mitigation of lime/scale:

Steam generated in an electrode cylinder is pure water (H2O). This means that all the ions from the supply water remain in the tank in the form of scale. The resulting build-up can be mitigated by flushing the cylinder periodically, however, over time the scale will accumulate to a point where the electrodes are coated and unable to generate the capacity required. The plastic tank will then have to be removed and discarded and, a new tank acquired and installed.

Resistive electric humidifiers use electric, self-cleaning, immersion heating elements to raise the temperature of the water to the boiling point within a permanent stainless steel evaporation chamber. Each of these immersion heaters has a specific power rating (KW) at a specific voltage. This radiant heating will raise the temperature of the water within the evaporation chamber to the boiling point, independent of the ion content of the water, thus permitting equal performance in purified (de-ionized or reverse osmosis) water or potable water. The capacity of steam generated is controlled by switching power over time to the resistive elements. This is accomplished in most cases with SSRs (solid state relays) with integrated zero voltage crossing (SCR control).

Mitigation of lime/scale: In potable water installations, the build-up of ions (scale) within the tank is mitigated with frequent flush cycles. Although the output of the humidifier will not be affected by the build-up of scale, the evaporation chamber will have to be removed and cleaned periodically. In pure water applications, these drain cycles may be greatly reduced or eliminated depending on the purity of the water, effectively increasing the efficiency of the humidifier and reducing the service required.

In next month's article, we will look at the total cost of ownership and the pros and cons of each on a per-application-basis.

Electrode boiler and resistive type electric steam humidifiers compared

Introduction

In this article I will explain the difference between electrode boiler and resistive type humidifiers.
They are both producing steam and use electricity as source of energy but they are based on two completely different principles of operation.
Knowing their differences and their limitations will help you to select the best technology for your application.

How electrode boiler humidifiers generate steam?

Electrode boiler humidifier uses electricity flowing through streams of water to create steam. The conductive properties of water are used to carry electric current. The current drawn and therefore steam production relies on water conductivity (typically between 125 and µS/cm) and also on the water level covering the electrodes surface.

Steam output and accuracy

Steam output of electrode boiler humidifier is a direct function of water conductivity and water level. The cleaner and purer the water is, the lower is its conductivity. Water conductivity contained in electrode boiler humidifier cylinder will increase as the humidifier is producing steam. Steam is free of minerals, salts or other substances, so all of those substances stay in the remaining water until a blow down is performed to renew water contents.

Another factor affecting steam output is the amount of scale or lime stone accumulated on the electrodes. Scale is the result of the precipitation of the minerals and salts contained in water. This process is absolutely inevitable for any electrode boiler humidifier as the use of tap water (city water) is required by its principle of operation.

Humidity output accuracy cannot be better than +/-5% RH for electrode boilers, furthermore the steam output will slowly decrease over the operation of the humidifier until the steam output cannot supply the demand. Then the cylinder has to be replaced.

For a resistive type humidifier, steam output is independent from water conductivity (or other conditions), also the possible scale build-up on the heating element will not affect the steam output of the humidifier. Humidity output accuracy for resistive type can be as low as +/-1% RH. This accuracy is maintained over the life of the humidifier.

Water quality

As already mentioned, an electrode boiler needs water conductivity (impurity) to be able to operate, so this type of humidifier cannot accept treated water such as RO or DI water.

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Reverse osmosis (RO) is a water purification technology that uses a semi permeable membrane to remove ions, molecules and larger particles from drinking water.
Deionized water (DI), often synonymous with demineralized water, is water that has almost all of its mineral ions removed. Deionization is a chemical process that uses specially manufactured ion-exchange resins.

Resistive type humidifiers can accept very pure water such as RO or DI water. However most manufacturers offering this type of humidifier will use poor materials and/or have limitations due to their water level sensing devices, so they will impose an added cost option for their humidifier able to operate with RO or DI water.

IER humidifier by steamOvap can accept any type of water from tap (city water) to RO water without additional option.

Service

Electrode boiler service consists in removing the 'consumable' plastic cylinder that is not able to produce enough steam anymore by a brand new one.

No recycling program is yet offered by manufacturers for this plastic and electrical spare part that will end in the waste.

In the contrary  steam cylinder of a resistive type humidifier is a pemanent part usually made of stainless steel. When tap water is used, this one needs to be serviced. The service consists in opening the cylinder and removing the accumulated scale inside.

This operation can be labour and time consuming and complicated thus requiring special skills and tools for most of the resistive type humidifiers on the market.

This is not the case with IER humidifier by steamOvap, thanks to its patent pending ''guide & lock'' system that allows you to open and clean the steam cylinder in a few minutes without needs of tools or special skills.

Cost of ownership

Initial cost to purchase an electrode boiler humidifier is lower than resistive type humidifier.

Most of the humidifier manufacturers are offering both technologies. They price electrode boiler humidifier much lower than their resistive type ones. Their pricing is done the same way printers manufacturers do with ink jet printers versus laser printers. Just like ink jet printers, electrode type humidifiers are cheaper to purchase, but as they will be used and operated the consumables will represent considerable costs:ink cartridges for printers or spare plastic cylinder for electrode boiler humidifiers.

Its low initial costs makes electrode boiler humidifier the contractors choice. Most of the time required future spares are not included in the initial budget. This is a very lucrative business for humidifier manufacturers and one of the reasons electrode boiler humidifier is their entry level product.

IER humidifier price is very competitive; furthermore no spare is required for the service of the humidifier. It is the only humidifier on the market offered with a 3 year warranty.

In any case, total cost of ownership is in favor of resistive type humidifier, after only 18 months in average.