It is well known that dehumidification of indoor air is desirable when the relative humidity approaches or exceeds 70%. Maintaining the relative humidity below this level avoids indoor growth of micro-organisms and is desirable for human comfort. For this purpose, air conditioning systems used in hot humid climats are designed to enable removal of large amounts of water from the air, whilst also providing air cooling.

This invention aims to provide a simplified air treatment device for reducing the relative humidity and the absoute humidity of air. The invention is directed to a device which is particularly suited for air treatment of cold moist environments, and which provides air dehumidification without requiring the capability of cooling.

Moisture-absorbing filters are known which cause condensation of moisture from the air to be treated. A known filter of this type comprises a filter material impregnated with a hygroscopic salt. US 4,402,717 discloses apparatus for removing moisture from air and which comprises a corrugated paper structure which is coated with an absorbent. The air for treatment passes in one direction through a first portion of the filter, and heated regeneration air passes through a second portion of the filter in the opposite direction. The filter is rotatable to enable continuous air dehumidification and filter regeneration.

The present invention aims to provide a simplified indoor air treatment device, and which, in particular, avoids the need for exchange of indoor air with outdoor air during air treatment.

According to the invention, there is provided an indoor air treatment

device for air dehumidification comprising an air treatment section having a fan, a heating means and a filter impregnated with a hygroscopic salt arranged in series between an inlet and an outlet of the air treatment section, the device further comprising an indoor and an outdoor air intake, each coupled though an inlet selector valve to the inlet of the air treatment section, and an indoor and an outdoor air expulsion port, each coupled through an outlet selector valve to the outlet of the air treatment section.

The device of the invention has a simple construction and enables indoor air dehumidification without exchange of air between indoors and outdoors.

Preferably, therefore, the device has two modes of operation: a first, dehumidification mode, in which the inlet selector valve couples the indoor air intake to the inlet, and the outlet selector valve couples the indoor air expulsion port to the outlet; and a second, regeneration mode in which the inlet selector valve couples the outdoor air intake to the inlet and the outlet selector valve couples the outdoor air expulsion port to the outlet, the heating means being activated to regenerate the impregnated filter during the second mode of operation. During regeneration, moisture is evaporated from the filter and released into the outdoor air.

These two modes of operation enable indoor air dehumidification and filter regeneration without exchange of air between indoors and outdoors, and using a single fan for both operations.

The filter preferably comprises a corrugated hydrophillic cellulose paper (crepe paper) of high porosity. The filter may be impregnated with hygroscopic salts such as magnesium chloride, lithium chloride, calcium chloride or mixtures of these.

The filter is preferably relative thin along the direction of air flow, which avoids a high pressure drop across the filter in use of the device, giving rise to the possibility of using a low-power and low-noise fan. In use, the filter is preferably arranged such that air flows through the filter substantially vertically.

When the hygroscopic salt in the filter has absorbed a substantial amount of moisture, this arrangement reduces both sagging of the filter structure and

leaking of the salt solution from the filter under the influence of gravity. Furthermore, the performance of the filter remains reliable, despite any possible uneven distribution of the impregnated salt solution across the filter thickness caused by gravity.

The invention will now be described by way of example with reference to and as shown in the accompanying drawings, in which: Figure 1 shows an air treatment device according to the invention; and Figure 2 shows an example of the equilibrium relationship between the air relative humidity at 22°C and the salt solution concentration within the paper walls of the filter.

The device of the invention uses a filter impregnated with a moisture absorbing hygroscopic substance. This substance may comprise either a hygroscopic dry salt or a hygroscopic aqueous salt solution. Moisture absorption, which is caused by condensation of water vapour from the air, is accompanied by condensative air heating. However, to enable a simple mechanical construction, the device of the invention does not provide for air cooling, and consequently the air treatment device of the invention is particularly suited for air treatment in enclosed moist cold environments, rather than for general air treatment in warm humid climats. For example, the invention is particularly suited for garages, bathrooms, bedrooms and cellas, which may have temperatures below 20°C. In these cases, the condensative heating filter of the invention serves as an alternative to ventilation with outdoor air. It may be desirable to avoid this ventilation if the outdoor air itself is very moist, polluted or cold.

The air treatment device 2 of the invention shown in Figure 1 comprises an air treatment section 4 having an inlet 6 and an outlet 8. Between the inlet 6 and the outlet 8 there are arranged an air heater 10, a condensative filter 12 and a fan 14. The air heater 10 comprises a conventional electric heater, and many possible designs will be apparent to those skilled in the art.

The condensative filter 12 comprises a corrugated paper structure which

is impregnated with either hygroscopic dry salt crystals or with a water- absorbing salt solution, which is contained within the paper walls. The filter may be impregnated with hygroscopic salts such as magnesium chloride, lithium chloride, calcium chloride or mixtures of these. The salt solution remains contained within the paper walls by means of capillary forces. The filter is preferably corrugated to give a paper wall volume of at least 35% of the total filter volume, to provide a large storage volume inside the filter for retaining the impregnated hygroscopic salt and the moisture absorbed into the salts.

The filter preferably comprises a corrugated hydrophillic cellulose paper (crepe paper) of high porosity. Alternatively, filters made from other organic fibres or from inorganic fibres, such as glass, may be employed.

The larger the amount of hygroscopic dry salt that has been impregnated into the filter, the more water can be absorbed from the air stream at a given relative humidity. However, the total volume of salt solution inside the filter cannot exceed the volume of the paper walls within the filter, otherwise filter flooding occurs. Filter flooding results in a loss of salt from the filter, giving air flow obstruction and a decrease in the filter efficiency. When the volume of the paper walls within the filter is completely filled with salt solution, the filter has reached the point of saturation. It is the quantity of dry salts introduced within the filter walls that dictates the concentration of the salt solution in the filter walls at the point of filter saturation.

For an airflow of approximately 200-300m3/hour, the volume of the filter may be approximately 4 litres, and in one possible example this is obtained with a filter face area of 0.08m2 and a depth of 50mm. Preferably, the effective channel diameter produced by the corrugations amounts to approximately 1 mm.

This design gives rise to the possible presence of 1.1 litres of magnesium chloride salt solution at the point of filter saturation, while the air emerging from the filter outlet is very close to equilibrium with the salt solution inside the filter (i. e. at the same temperature and thermodynamic water activity). At equilibrium, the relative humidity of the exiting air is directly related to the composition of the salt solution within the filter walls, and thus provides an indicator of the salt solution concentration, and can thereby be used as an indicator for the time

when regeneration is required, as described below.

Figure 2 schematically shows the equilibrium relationship between the relative humidity of air (at a temperature of 22°C) passing through the filter and the concentration of the magnesium chloride salt solution within the filter.

A constant amount of salt is impregnated into the filter, which remains in place. This avoids the need for a salt solution reservoir and a pumping system, which have previously been proposed. The amount of impregnated hygroscopic salt in the filter is chosen such that when the volume of the filter walls is saturated with salt solution, the relative humidity of air (at ambient temperature) in equilibrium with the filter remains below 60%.

Under these conditions, the filter remains able to withdraw moisture from air at ambient relative humidity that exceeds 60% up to the moment of filter saturation with salt solution.

The filter may be impregnated with salt by spraying the filter with a concentrated hygroscopic salt solution, and subsequently drying the filter.

With the 1.1 Litre filter volume described above, 880g of magnesium chloride (MgCI2. 6H2O) salt may, for example, be introduced into the filter. At filter saturation, namely with 1.1 Litres of solution stored within the filter, a concentration by weight of approximately 63% is obtained (8809 of MgCI2. 6H2O salt and 520g of water giving rise to 1.1 Litres of solution at 22°C). From the graph of Figure 2, it is possible to determine that if the air is in equilibrium with the salt solution, then the relative humidity of the air will be 60%. A higher initial quantity of salt will give rise to a higher salt solution concentration at saturation, and a corresponding lower relative humidity of air in equilibrium with the impregnated filter.

At increasing amounts of moisture absorbed into the filter, some sagging of the salt solution under gravity may occur, causing the solution to sink to the lower part of the filter. In the arrangement shown in Figure 1, the vertical height of the filter 12 is of the order of 50 mm, whereas the diameter is of the order of 300 mm when an airstream of about 300 m3/hour is to be treated effectively. By disposing the smaller dimension (50 mm) vertically, the gravitational pressure drop from the top to the bottom of the filter is kept to a minimum so as to reduce

sagging of the salt solution.

An inlet valve 16 is arranged at the inlet 6 of the air treatment section 4, and selectively couples the inlet 6 to an indoor air intake 20 or an outdoor air intake 22. The barrier 24 shown in Figure 1 schematically illustrates the divide between indoors and outdoors. An outlet valve 26 is provided at the outlet 8 of the air treatment section 4 and selectively couples the outlet 8 to an indoor air expulsion port 28 or an outdoor air expulsion port 30.

The air treatment device shown in Figure 1 has two modes of operation.

In a first mode, the indoor air is treated to reduce its relative and absolute humidity. Optimum relative humidity for comfort is within the range 30% to 60%, so that the air treatment device is operated in the first mode once the indoor relative humidity reaches a selected maximum level, typically above 60% and below 70%. This first mode of operation is represented in Figure 1, in which the inlet valve 16 couples the inlet 6 of the air treatment section 4 to the indoor air intake, and the outlet valve 26 couples the outlet 8 to the indoor air expulsion port. In this mode, indoor air is circulated by the fan 14 through the filter 12, and the heater 10 is turned off. Moisture is absorbed by the salt in the filter which then effectively becomes a hygroscopic salt solution.

In the second mode of operation, the filter 12 is regenerated using outdoor air. For this purpose, the inlet valve 16 couples the outdoor air intake to the inlet 6, and the outlet valve 26 couples the outlet 8 to the outdoor air expulsion port. The fan 14 then circulates outdoor air through the air treatment section 4. The outdoor air is heated by the heater 10 so that the heated air causes evaporation of the water held by the hygroscopic salts within the filter 12. The regeneration efficiency may be improved by reducing the air flow through the filter, for example to half the air flow rate through the filter during dehumidification. This can be achieved by controlling the fan speed.

By having these two separate modes of operation, it is possible to avoid all exchange of indoor air with outdoor air, which may be desirable when the outdoor air is cold, polluted or itself has a very high relative humidity.

Feedback control is required to enable automatic system operation, and for this purpose a first measurement unit 40 is provided at the indoor air intake

20 for measuring the temperature and relative humidity of the indoor air. A second measurement unit 42 is provided at the outlet 8 of the air treatment section 4, for measuring the temperature and relative humidity of the air exitting the treatment section 4.

As described above, the dehumidifying (condensative heating) mode is initiated when the indoor relative humidity has reached an upper level of typically 60% to 70%. The dehumidification can continue until the relative humidity has been reduced to a lower acceptable level (preferably somewhere within the range 30 to 50%), or until the filter salts require regeneration, whichever occurs first. The device may also have idle periods, if the humidity rises from the lower level to the upper level more slowly than the time required to regenerate fully the filter.

Filter regeneration is necessary when the filter approaches saturation, namely when the amount of salt solution in the filter (comprising the dry salt and the absorbed moisture) approaches the critical volume at which the entire volume of the paper walls in the filter is filled with salt solution.

For the correct timing of filter regeneration, the initial amount of dry salt impregnated in the filter must be known. When the paper wall volume of the filter is also known, it is possible to calculate the salt concentration corresponding to saturation of the filter. Filter regeneration is required at or slightly before this saturation. As discussed above, a (near) equilibrium between the exitting air and the salt solution can be attained, and a unique relationship then exists between the salt solution concentration inside the filter and the relative humidity and temperature of the exitting air. At this equilibrium, the temperature and water activity of the salt solution in the paper walls near the filter exit equals the temperature of and the water activity in the air exitting the filter. The relative humidity of the air equals the water activity in the air if the water activity in the air at the point of air saturation with moisture is set (arbitrarily) at 100 (i. e. relative humidity = 100%).

Given the impregnated amount of (dry) salt in the filter and the total available wall volume that can be filled with salt solution, the point of filter saturation will be attained when a salt concentration in the filter walls exists that

equals the ratio of the total amount of dry salt to the total paper wall volume.

Provided that a (near) equilibrium exists between the air exitting the filter and the salt solution near the filter exit, the salt solution concentration inside the filter can be followed over time from a measurement of the temperature and the relative humidity during the dehumidifying mode at the second measurement unit 42, downstream of the filter, for example using look-up tables. In these tables, the salt solution concentration is tabulated as a function of the temperature and the relative humidity of air that has come to equilibrium with the salt solution.

Although a specific example of impregnated filter has been described, there are of course a large number of possible filter configurations. A detailed description of the heater and fan used in the embodiment above has not been given, since conventional components can be used. The heater power will be selected according to the desired air treatment rate and the air flow conditions through the filter.