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Air humidification: humidifier maintenance

Humidifier maintenance

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The maintenance of humidifiers can be considered in terms of two completely different aspects: a purely functional one, with the purpose of maintaining the technical-operational characteristics of the appliance over time, and the hygiene-health aspect, concerning the safety of the users in relation to the spread of pathogens. The latter aspect, for some types of humidifiers, is without doubt the more complex and will therefore be examined in detail in the following chapter.

This chapter consequently focuses on purely technical maintenance – essential for preventing malfunctions or the premature replacement of costly components – involving periodical inspection, checks, and the replacement of faulty or depleted items.

The most common reasons for the malfunctioning of humidifiers are due to the effect of the solids contained in the supply water: while following evaporation the water changes state from liquid to vapour and consequently leaves the appliance, the substances contained in the water remain in solution, creating, above certain concentrations, deposits and scale.

The various types of appliances will be looked at individually with the distinction made between liquid appliances differentiated according to whether they feature a collection sump or operate on once-through water, and steam appliances.

Maintenance of liquid humidifiers with collection sumps The presence of a water collection sump increases the risk of the formation of deposits, as the salts dissolved in the water are not discharged during the cycle. Consequently, draining is required, which, according to the appliance, may be continuous and regular, or alternatively intermittent.

The maintenance requirements involve, as well as the actual sump itself, both the parts where the water is released (see the next chapter) and the water circuit as a whole, including all the parts that over time tend to be covered in lime-scale until becoming blocked, even partially, causing malfunctions to the moving parts (pump, floats, valves).

  1. Adiabatic washers: As will be seen in the next chapter, for this type of appliance the maintenance procedures are particularly demanding in terms of the hygiene-health aspects.

    As regards the technical-functional aspects, it is important to ensure that the draining process is adequate for the mineral content; in particular the temporary hardness, to prevent concentrations that cause precipitation; the collection sump, the pump circuit, the drain valve and the spray nozzles must be periodically emptied and chemically cleaned.

    The mist eliminator should also be visually inspected, cleaned, and replaced if necessary.

  2. Wetted media humidifiers with recirculation: Wetted media humidifiers with recirculation require, similarly to adiabatic washers, the need for suitable draining and the periodical emptying of the sumps. The same care must be paid to the mist eliminators, with visual inspections, cleaning, and replacement where necessary.

    Specific problems are created by the evaporative media itself: on one hand it is susceptible to deposits of the minerals contained in the water that evaporates, on the other it tends to trap the impurities from the air, progressively blocking the pathways.

    This accumulation of material, as well as reducing the evaporating capacity, increases the pressure drop opposing the air as it passes through the media.

    The holes in the device that supplies the water onto the media must be checked regularly so as to prevent even partial blockages from causing irregular or no water supply. The media should be washed periodically, and when that is no longer sufficient, it should be replaced.

  3. Centrifugal humidifiers with bottom sumps: These devices do not require special maintenance operations other than normal periodical checking and cleaning of sumps, air filters and solenoid valves.

    To prevent excessive concentrations in these appliances, periodical draining cycles should be performed at suitable intervals.

    Considering the limited content of the sumps, these may be performed automatically each time the appliance is stopped by opening drain solenoid valves when they are fitted.

    When restarting an appliance after an extended period of inactivity, the operation of the mechanical components should be quickly checked (free rotation of the disk and movement of the level switch in the sump).

  4. Ultrasound humidifiers: Ultrasound humidifiers must be supplied with demineralised water, otherwise violent localised cavitation will cause the depositing of a film of lime-scale on the surface of the resonant ceramic plates and consequently reduce their working life.

    The more advanced appliances feature automatic draining of the bottom sump at regular intervals to prevent the concentration of dissolved salts over time.

    The replacement of the resonant plates, which have a normal lifespan of around 5,000 – 10,000 hours with demineralised water, is the only periodical operation required of any economic significance (the cost is often more than 30% of the overall cost of the appliance).

  5. 5. Rotating drum humidifiers: The deposits of inorganic (salts and lime-scale) and organic substances (mould, bacteria) contained in the water accumulate on the rotating drum during operation, closing the pores and rapidly reducing the humidification capacity. As these are generally low-cost appliances, without automatic draining, maintenance needs to be performed quite frequently, mainly involving cleaning and, very often, replacement of the rotating drum itself, as well as functional testing of the sump level controller.

Maintenance of liquid humidifiers with once-through water

These appliances are not affected by the concentration of substances contained in the supply water as this is completely discharged. Nonetheless, lime-scale may form on the surfaces where evaporation takes place.

  1. Atomising humidifiers with pressurised water. These appliances are made up of the supply water pump assembly and the distribution system with atomising nozzles.

    The maintenance operations required on the pump assembly involve checking the water filters (the frequency of this check obviously depends on the quantity of suspended particles in the water), checking the oil level in the pump, with planned oil changes every 2 000-3 000 hours, and the replacement of the gaskets on the pistons and the valves every 5 000 hours because the characteristics of the water - which is also filtered - are not modified so there is no risk of lime-scale deposits inside the pump assembly.

    As regards the nozzles, the characteristics of the supply water play an important part as even slight quantities of lime-scale may modify the minute cross-section of the opening affecting performance in terms of both the flow-rate and the size of the droplets in the spray.

    The cleaning of the nozzles, involving dismantling and descaling using appropriate substances is an operation that must be performed at intervals that depend greatly on the characteristics of the supply water.

    Nonetheless, these operations become redundant - or at least reduced to part of the general maintenance performed annually - if the humidifier is supplied with demineralised water produced by reverse osmosis.

    This practice is also recommended to prevent the introduction of fine dust into the air handling unit and/or the humidified rooms and to minimise the periodical cleaning of the mist eliminator.

    The microprocessor control systems fitted on the more complete models automatically calculate the correct frequency of the routine operations, and signal the need for special maintenance operations following abnormal situations detected by the sensors on the unit.

     

  2. Atomising humidifiers with compressed air. As well as the routine maintenance required on the air compressor, which may be a dedicated unit or alternatively shared with other utilities, the maintenance operations are quite simple and involve the filters, the pressure reducers and the control valves for the two fluids, located on the control unit.

    The nozzles on some systems feature a self-cleaning needle that automatically removes any particles of lime-scale whenever supply is shut off: this feature allows the intervals between the cleaning operations to be extended, even without using water treated by reverse osmosis. The latter is recommended for the same reasons described for the pressurised water appliances (see Figure 13.2).

    A special shutdown cycle, involving the blowing of compressed air only through the nozzles after having shut off the water supply, removes the remaining water from the nozzles and reduces the accumulation of solids and micro-organisms.

  3. Horizontal axis centrifugal humidifiers. Centrifugal humidifiers with once-through water require checks to be performed that involve the operation of the water supply system, the free rotation of the disk and the absence of deposits in the parts wetted by the flow of water.

    Often a high percentage of the supply water does not evaporate and is trapped by the mist eliminator. This must be frequently inspected and, if necessary, cleaned to eliminate any solid and organic deposits.

  4. Wetted media humidifiers without water recirculation. The absence of the recirculation sump significantly reduces the proliferation of bacteria inside the appliance as the wetted media is continuously washed with clean water.

    Nonetheless, special care must be taken to ensure the minimum pressure and flow-rate values of the supply water, as well as the correct operation of the constant flow valves, to prevent the presence of any potentially dry areas that would lead to the formation of lime-scale deposits.

The remaining maintenance requirements are similar to those for the same type of appliance with recirculation. As the flow-rate of water introduced is many times higher than the amount effectively evaporated, the use of demineralised water – despite being advantageous – is not common, for strictly economic reasons.

Maintenance of steam humidifiers

In the case of steam humidifiers, the maintenance issues are mainly functional in nature, as the risk of bacterial contamination, examined in the next chapter, is virtually non-existent.

  1. Centralised steam distributors: These appliances, connected to a source of steam produced centrally, have limited maintenance requirements, as most of the operations relate to the centralised boiler that, in many cases, is shared by many other utilities.

    The specific operations involve periodical inspections of the control valve and the steam distributors, as well as the cleaning or the replacement of the filter in the steam supply line.

  2. Immersed electrode humidifiers: As well as the periodical inspections of the steam distributor and the supply and drain solenoid valves, the maintenance of these appliances simply consists of the cleaning or the replacement of the steam production cylinder containing the immersed electrodes.

    This operation may be required for the following main reasons:
    • the formation of solid deposits on the surface of the electrodes – mostly due to the temporary hardness of the supply water – which, usually being dielectric, prevent the passage of current; even though the level of the water is automatically adjusted to compensate for this, over time the phenomenon affects the entire active surface of the electrodes;

    • the filling of the boiler with voluminous solid scale that, being trapped by the bottom filter, occupies an increasing amount of space until eventually preventing the correct flow of water into and out of the cylinder;

    • the formation of foam due to the presence of surfactants in the water; as well as causing the anomalous operation of the steam production process, the foam often covers the electrodes with a thin film of non-conductive substances that prevents the passage of current, even when the quantity of solids deposited is quite thin and consequently in a relatively short time;

    • The corrosion of the metallic electrodes; this phenomenon, even if infrequent, occurs when the water, rich in aggressive chemical elements such as chlorides, is concentrated excessively due to defective or insufficient draining; in the more significant cases, electric discharges may occur between the electrodes, which further amplifies the phenomenon, as they cause metallic particles to dissolve in the water, making it even more conductive;

    • sometimes a poor electrical conductor and carries only a modest quantity of current, representing an electrical resistance; in general, these deposits are cooled by the surrounding water, however in rare cases these may remain partially exposed (such as when there is only a minimum request in operation with variable steam production) and possibly damage the walls of the boiler.

    The operating mode of the appliance also has a significant influence on the duration of the boiler: in the case of intermittent operation, the frequency between two production cycles may be reduced significantly as excess water is introduced during the thermal transients, often with successive draining for adjustment, and with this a greater quantity of salts destined to precipitate. From this point of view, the choice of an appliance with excessive capacity generally represents a disadvantage.

    The following actions can be taken to resolve the problems described:
    1. Check the suitability of the water for use in immersed electrode humidifiers, according to the indications provided by the manufacturer (if necessary using the electrical conductivity meter installed on the more advanced units).

    2. Visually check the boiler every month.

    3. When necessary, perform maintenance on the boiler (required when signalled in the better appliances by the self-diagnostic checks) at intervals that are obviously strictly related to the characteristics of the water used, and that typically vary from 500 to 4 000 hours, even if in some case they may reach 10 000 hours. The average value is around 2 000 hours.

    The maintenance operations involve:
    1. The replacement of the boiler, a simple and very rapid operation, with a cost equal to around 10% of the purchase price of the appliance.

    2. The cleaning and the removal of the bottom filters and the electrodes for replacement or alternatively for chemical cleaning in a weak acid solution, in the case of the openable versions supplied by some manufacturers (see Figure 13.7); the dismantling and the cleaning of the electrodes takes considerable time and is economically feasible only when labour costs are low.

    Remembering that the operating principle of the immersed electrode humidifier is based on the electrical conductivity of the water due to the presence of dissolved salts, it is obvious that these appliances cannot use demineralised water.

    Only if the water is very hard is it recommended to use a mixture (generally 50%) of demineralised water with untreated water.

    Softened water should not be used, as being rich in chlorides it tends to corrode the electrodes.

  3. Heater humidifiers: In these appliances too the main problem involves the concentration of salts inside the boiler, even if this can be alleviated by partial draining cycles at set intervals, defined either:

    • During installation, according to the characteristics of the supply water.

    • In the more complete units fitted with special sensors, depending on the continuous measurement of the conductivity of the water (and therefore proportional to the concentration of salts).

    • The formation of solid precipitate progressively fills the boiler with flakes, and, more harmfully, covers the resistive elements with a thermally insulating crust, causing overheating and consequently possible serious faults.

    In this regard, one manufacturer proposes a patented system for the continuous measurement of the temperature inside the elements, consequently signalling the need for cleaning in conditions of absolute safety.

    Periodical maintenance requirements include the emptying of the boiler and the descaling of the heaters and the other functional components, specifically the solenoid valve and the level controller, by mechanical and/or chemical means after immersion for half an hour in a weak acid solution (for example, acetic acid at 20%).

    To assist the elimination of debris, the parts inside the boiler must be easy to access, and the boiler itself must be easily removable from the humidifier to allow it to be moved to the most convenient position.

    Some appliances feature a special adherent plastic lining on the inside walls of the boiler, which can be removed to eliminate the deposits in one simple step.

    Furthermore, some heaters are fitted inside flat metallic plates, lined with a special non-stick film, to impede the formation of scale on the surface and assist its spontaneous separation both during operation and when cleaning: in this case, a jet of water is usually sufficient to detach the lime-scale deposits following treatment with an acidic solution.

    The maintenance intervals obviously depend on the characteristics of the supply water, and may vary from a few hundred to several thousand operating hours. For heater humidifiers, treatment of the water is very advantageous.

    As the electrical conductivity has no influence on the functionality of the process, the appliances can be supplied with demineralised water, allowing the maintenance intervals to be extended to an annual frequency.

    The same is true for softened water, even if this requires a higher draining rate to prevent an excessive concentration of chlorides and any consequent negative effects; it should also be noted that sometimes the softening process introduces surfactants into the water, and these affect the correct operation of the appliance.

  4. Gas humidifiers: The maintenance issues for gas humidifiers are essentially similar to those just examined for the electric heater appliances. In addition, as regards the gas burner section, there is naturally the need to check combustion each year.

  5. Steam-water exchangers: For these appliances, the maintenance operations involve the combination of the typical actions required for steam distributors (checking the steam flow-rate) and the electric heater humidifiers as regards the sump, the supply, the level and the draining of the water, and the heat exchange coil.

    Nonetheless, it should be stressed that, due to the generally very complex shape of the coil, descaling operations are often rather difficult.