WO2002077536A1

WO2002077536A1 - Ventilation, dehumidification and heat recovery apparatus

Info

Publication number: WO2002077536A1
Application number: PCT/FI2002/000204
Authority: WO
Inventors: Kari Moilala
Original assignee: Mg Innovations Corp.
Priority date: 2001-03-19
Filing date: 2002-03-14
Publication date: 2002-10-03
Also published as: WO2002077536B1; JP2004530093A

Abstract

The invention relates to an apparatus for air ventilation, drying and heat recovery, consisting of a recuperative (1) and a regenerative (2) heat exchanger and supply and exhaust air flows between indoor and outdoor spaces. The heat of indoor air is transferred to outdoor air in a regenerative heat exchanger (2), and then outdoor air and moist air is led from the indoor space to the recuperative heat exchanger (1). The outdoor air conducted to the recuperative heat exchanger (1) has a temperature such that outdoor air generates condensation, i.e. drying of indoor air in the recuperative heat exchanger (1). After this, dry indoor air is exhausted through the regenerative heat exchanger (2) and warm outdoor air is supplied indoors through the recuperative heat exchanger (1). Owing to this arrangement, indoor air is replaced with outdoor air while indoor air is being dried with free outdoor air, thus providing substantially enhanced air-drying efficiency.

Description

Ventilation, dehumidification and heat recovery apparatus

This invention relates to an apparatus for air ventilation, drying and heat recovery, which consists of air ventilation and recuperative and regenerative heat exchangers, by means of which indoor air is dried with high efficiency using outdoor air.

Dehumidification of indoor air at temperatures below zero requires much energy. Both humans and buildings suffer from deficient dehumidification. Air dehumidifi- cation involves a problem at objects such as public swimming baths, spas, saunas and shower rooms, various objects of the processing industry, underground rooms, laundries, kitchens, bakeries and any other processes involving moisture in abundance. As a solution to this, it has even been suggested to recycle indoor air that has been dried once, however, then the lack of fresh air will cause problems, resulting in lower vigour, raised carbon dioxide levels, indisposition, moisture and mildew problems, allergic conditions and mildew-stricken buildings.

Air dehumidification may be performed by

- air ventilation, because outdoor air usually has lower absolute humidity than in- door air,

- dehydration of circulating air either on the adsorption principle,

- or by condensing with a cooling coil.

The following description of dehumidification of public swimming baths also ap- plies to the drying of other moist spaces, given the research data available with regard to public swimming baths. Thus, for instance, the work Aittomaki, A., Karkia- inen, S., Nehmaan-Kreula, M., "Comparaison of air drying systems in public swimming baths", University of Technology of Tampere, Energy and Process Technology, UDK 694, 97, 725, 74, report 131, Tampere 1997, ISBN 951-722-938- 0, ISSN 1238-4747, states on page 36:

- the larger the heat recovery surface area, the less a heat pump reduces heat consumption

- an increase in the heat recovery surface area reduces heat consumption more than an increase in the surface area of an evaporator - the target moisture in public swimming baths has a strong impact on heat consumption. By contrast, a limit temperature (-10 °C or 0 °C) of outdoor air has no significant impact - a heat pump may achieve reduced energy costs, yet the difference is too small for investments to be repaid.

Under cold conditions, the drying of outdoor air is a good means of dehumidifica- tion, however, it entails energy waste. On the other hand, a considerable portion of the heat content of used air can be recovered by means of heat exchangers. Heat exchange is the more effective, the higher the temperature difference between the exothermic and the endothermic air flows. Heat may be transferred from exhaust air directly to outdoor air through an air-flow isolating plate, which is a direct recupera- tive heat exchanger. A heat transfer substance that stores heat during alternating heating and cooling in an air flow is a regenerative, heat-accumulating heat exchanger. In any situation, supply air absorbs heat in an amount equal to that of heat released by exhaust air. If water vapour is condensed in the heat exchanger, exhaust air dries simultaneously. In that case, the temperature rise of supply air is higher than the temperature drop of exhaust air, even if the air flows were of equal size.

High (recuperative) efficiency of plate heat exchangers is achieved with a large heat exchange surface. In the optimal case, the efficiency is about 70%. If the exhaust air is moist, it will condense into water on the heat exchanger surface, and at tempera- tures below zero, freezing water may obstruct and break the heat exchanger. To prevent damage, air can be heated, and then the annual efficiency will decrease. A heat exchanger dries air only when its surface temperature is below the dew point.

Two types of regenerative heat exchangers can be distinguished: - a rotating heat exchanger, whose efficiency can be varied with the speed of rotation of the disc, and

- a flow-inversing heat exchanger, allowing efficiency to be simply varied by varying the length of hot and cool cycles of its heat storing mass.

In terms of temperature efficiency, regenerative heat exchangers are better than recuperative heat exchangers, but on the other hand, they return moisture from exhaust air to supply air. As a rule, the better the temperature efficiency, the more moisture is returned. Freezing can be decreased with a lower speed of rotation and shorter cycles.

The purpose of the invention is to eliminate the shortcomings of heat exchangers mentioned above and to provide a system in which indoor air can be dried using outdoor air with high energy efficiency in cold outdoor conditions without using a heat pump.

The warmer the air, the higher water vapour potential it has. However, air at a given temperature may contain a specific maximum amount of water vapour. This is referred to as air saturated with water vapour. If air moisture is added at this stage or the air temperature drops, any excess moisture will be condensed from the air as liquid droplets. The temperature at which water starts separating from air is called the dew point. Since energy cannot disappear from cooled air, it will pass to the condensate surface, resulting in the heating of this.

In the known Mollier Diagram, the condition of mixed air is on the joint line of the states of air flows to be mixed. In the air flow relation, the mixing point is inversely proportional, so that the mixing point is closer to the point of the larger mass flow. If the mixing point of warm and moist indoor air and cool outdoor air gets to the right of the saturation curve, water vapour will condense. In heating, the absolute humidity of air remains constant, but the relative humidity will drop. In this situation, we move upwards on the constant moisture line in the Mollier Diagram.

The temperature of the adsorption surface should be below the dew point for water to condense and air to cool and dry.

To achieve good indoor quality, the entire air quantity should be replaced from once to three times an hour. In moist spaces, the target temperature of pleasant indoor air is 30 °C, and the temperature difference between cool replacement air and indoor air should not exceed 12 °C. for reasons of comfort and moisture stresses exerted on constructions, about 60% should be considered the upper limit of relative humidity. In ordinary public swimming baths, a value of 0.1 m/s is suitable for the air rate, and in whirlpools, for instance, an air rate in the range 0.2-0.3 m/s is suitable with a water temperature of 35 °C.

The following example studies absolute moisture contents of air at different temperatures with 60% relative moisture content.

The difference between the amounts of maximum and minimum water vapour is up to 15- fold. Considering that even small swimming baths have a volume of hundreds of cubic metres, it is easy to grasp the magnitude of the water volumes dis- cussed here.

In terms of heating and air ventilation, a swimming bath differs from ordinary indoor spaces mainly by the fact that water in great amounts is continually evaporated from the pool and the wet surfaces into the air. Unless this evaporated water is re- moved, indoor air moisture will increase, and at a certain point, it will start condensing at the poorest isolated location, usually a window. To prevent condensation, it is usually desirable to maintain a slight under-pressure in the pool for moisture and chlorine not to be transferred to other parts of the building and to the wall structures. Swimmers are very lightly dressed, barefoot and wet even before swimming, so that intense evaporation binds heat, and the swimmers will feel cold unless the air temperature and the relative humidity are suitable. In addition, with a decrease in relative humidity, evaporation from the pool increases, resulting in a need for heating the pool water. According to various sources, air temperature should be maintained at a temperature 2...3 °C above the temperature of the pool water. Ventilation planning should also consider whether the object is an ordinary swimming bath or a spa comprising whirlpool baths and similar factors increasing the moisture load. If the swimming baths has an audience stand, separate air separation is usually called for so that moist and chlorine-containing pool air is prevented from passing to the audience. Planning should also consider varied conditions of use at different hours of the day, for instance. During hours of use of the pool, the ratio of wet surface to pool surface is 1.5:1, there is 1 person/4 m² of pool and in addition, there is 1 person with wet surface/10 m² of the bath premises. By air-flow dimensioning, one seeks to achieve maximum difference of enthalpy and moisture between indoor air and outdoor air. However, the risk of draught in the room caused by too cold supply air and the risk of freezing may set limits to the planning.

As stated above, regenerative heat transfer involves the paradox that, when high efficiency and non-freezing conditions are desired, the amount of moisture returned by the apparatus from exhaust air into supply air increases as the warm/cold cycles are shortened. In moist and hot outdoor conditions, this is a significant advantage, but at temperatures below zero, moist and warm indoor spaces cause a problem. Consequently, efficiency and moisture recovery are inverse properties. Because of these properties, regenerative systems are not efficient in moist indoor spaces. However, regenerative heat exchangers have a property that recuperative heat exchangers do not have: they do not freeze as easily as recuperative heat exchangers, not even at temperatures well below zero, when correctly used. In addition, the temperature efficiency of regenerative heat exchangers can easily be varied between 0 and maximum.

In the optimal case, regenerative heat exchangers with alternating flow may have good temperature efficiency of about 88% even at temperatures below zero. Thus, for instance, the invention of FI patent specification 100133 achieves the following benefits (on the basis of the tests VTT/4 October 1995; RTE10406/95 and SI TEF/1982-08-11/STF15 F 82029, 1982-0528/150164, among other tests): 1) extremely high temperature efficiency (tests VTT: 87.8%; SINTEF: 98%) 2) In hot (above 30 °C) and moist (relative humidity above 80%) outdoor conditions, the apparatus even cools indoor air by 3-5 °C without supplementary energy. 3) Correctly used, the heat recovery cell system does not freeze. 4) Low connection power. 5) 38-67% balancing of the relative humidity of indoor air.

The purpose of the invention is to cool indoor air with outdoor air in a regenerative heat exchanger. The regenerative heat exchanger is controlled in such a way that the cell system still does not freeze. Indoor air that has been cooled in this manner is led to a recuperative heat exchanger, the temperature being at least above zero to avoid freezing of the cell system. Moist and warm indoor air reaching the recuperative heat exchanger will be cooled below the dew point and will thus be dried. Since the recuperative heat exchanger does not return moisture indoors, the air drying occur- ring within it can be completely utilised, unlike regenerative heat exchangers. Heated outdoor air is conducted indoors and not dried indoor air is conducted outdoors.

It is extremely difficult to carry out dehumidification calculations, because load pa- rameters such as moist transfer coefficients are uncertain and may vary within a large range in the practice. Similarly, moisture transfer figures of regenerative heat exchangers are poorly known. With outdoor temperature of -10 °C and indoor temperature of +30 °C and 90% efficiency, the exhaust air from the regenerative heat exchanger has a temperature of - 6 °C, which does not normally cool a recuperative cell. If the temperature still drops, freezing is prevented by decreasing the efficiency of the regenerative heat exchanger. On the other hand, to achieve condensation, the air temperature must not exceed the dew point.

The results of a test situation are presented in the following: A drying system operating without heat recovery requires 29.8 kW for heating ventilation air to the tem- perature of the pool hall, while the thermal power consumed by evaporation is 23.8 kW, i.e. totally 53.6 kW. With the system of the invention - 85% regenerative heat recovery - heating of the pool space required 4.5 kW and the thermal power consumed by moist recovery was 7.9 kW, i.e. totally 12.4 kW. The gain of 41.2 kW, i.e. 77%, represented a pure net gain, both the system having the same current con- sumption, i.e. required the same input power. Compared with a conventional regenerative drier, the system of the invention yielded savings of 73.9%.

The apparatus for air ventilation, drying and heat recovery of the invention is characterised by the features defined in the characterising part of the claims.

One of the main achievements of the invention is combining a recuperative and a regenerative heat exchanger known per se into an assembly in which drying of indoor air takes place with free energy and high efficiency. EP patent specification A 0922483 (B01D 53/26) comprises both a recuperative and a regenerative heat ex- changer, but it has no ventilation, so that it circulates indoor air alone. This is precisely the problem this invention resolves. Similarly, patent specifications US A 5,709,736 (B01D 53/26) and WO A 8,806,261 (F24F 3/14) do not comprise ventilation, for instance; the apparatuses disclosed in all of these references are intended merely for air drying.

The system eliminates the return of moisture into indoor spaces by the regenerative heat exchanger, yet without detriment to good heat recovery. FI patent specification A 895354 (F24F 12/00) merely comprises temperature control of the air flows in a regenerative heat exchanger. There remains the problem of air drying at tempera- tures below zero, because the apparatus returns moisture indoors. This problem is solved by the invention with high energy efficiency by using a recuperative heat exchanger alongside other means for the actual drying after the heat of indoor air has been recirculated indoors along with fresh outdoor air. Although it comprises two heat exchangers, the apparatus of the invention is straightforward and inexpensive, because it has one single fan and one single control system, instead of two. The system is silent, lightweight, easy to service and re- liable, and it does not contain any hazardous substances.

Various solutions, which are methods known per se, have been developed for all these known purposes. This invention allows the problems mentioned above to be solved with one single apparatus, which has a short repayment period, owing to its low acquisition price.

Various embodiments of the invention are described in the dependent claims.

The invention is explained below by means of an example and with reference to the accompanying drawing, which shows schematic views of an air drying and heat recovery installation and the air flows provided in this.

In drawings 1, the apparatus for air ventilation, drying and heat recovery is formed of a recuperative 1 and a regenerative 2 heat exchanger, supply 3 and exhaust air 4 ducts leading to the indoor space, supply 5 and exhaust air 6 ducts leading outdoors, and an air duct 7 leading from the regenerative 2 to the recuperative 1 heat exchanger and an air duct 8 leading from the recuperative 1 to the regenerative 2 heat exchanger. The air flows are shown with arrows, with outdoor air indicated with a broken line and indoor air with an unbroken line. Cool outdoor air is conducted from the supply duct 5 first to the regenerative heat exchanger 2. Then outdoor air is conducted through the air duct 7 to the recuperative heat exchanger 1, from where outdoor air finally is conducted indoors via the supply duct 3. Indoor air to be dried is conducted through the exhaust duct 4 to the recuperative heat exchanger 1, and then through the air duct 8 to the regenerative heat exchanger 2. In the regenera- tive heat exchanger 2, outdoor air cools indoor air, and subsequently indoor air is exhausted through the exhaust duct 6. In other words, indoor air releases its heat to outdoor air. Outdoor air transferred from the regenerative heat exchanger 2 to the recuperative heat exchanger 1 has a temperature such that outdoor air results in condensation, i.e. drying of indoor air in the recuperative heat exchanger 1. The temperature of outdoor air may be lower or at the most equal to the dew point temperature of indoor air. With this arrangement, it is ensured that moist indoor air is condensed and consequently dried. However, the temperature of outdoor air in the recuperative heat exchanger 1 is not allowed to be so low that it causes freezing of the recuperative heat exchanger 1. If necessary, the correct temperature is achieved in the regenerative heat exchanger 2 by varying its efficiency, for instance, with changes of the rotation speed of the rotating device and the length of the warm and cold cycles of the heat accumulating mass in the flow-inversing device. With an outdoor temperature of - 20 °C and an indoor temperature of + 28 °C in the arrangement of figure 1, air transferred from the regenerative 2 to the recuperative 1 heat exchanger has a temperature of about + 18 °C with 80% efficiency, this temperature being high enough to dry the air, because air at + 28 °C and with 55 % relative humidity has a dew point of about 18 °C; thus the arrangement has almost optimal efficiency while ensuring supply of fresh indoor air. Should this air drying be insufficient, the system can be combined with conventional cooling, with an evaporator 11 acting as a drier mounted in the supply duct 3 and a condenser 10 in the exhaust duct 6. A condenser may have been mounted also for heating water, for instance. Water condensed in the heat exchangers is conducted in pipe 9 to the drain, a separate receiver or to a swimming pool, for instance. At swimming baths, there is much warm waste water from the showers, among other things. This warm waste water can be utilised in absorption cooling, which can replace a conventional refrigeration compressor. All the test have proved that the invention yields extremely high efficiency rates, allowing air to be dried merely by means of cool out- door air.

Claims

1. An apparatus for air ventilation, drying and heat recovery, consisting of a recuperative (1) and a regenerative (2) heat exchanger, supply (3) and exhaust air (4) ducts leading to an indoor space, supply (5) and exhaust air (6) ducts leading to an outdoor space, an air duct (7) leading from a regenerative (2) to a recuperative (1) heat exchanger and an air duct (8) leading from a recuperative (1) to a regenerative (2) heat exchanger, characterised in that the outdoor air flow is conducted from an supply duct (5) leading to outdoor air through the regenerative heat exchanger (2) in air duct (7) to the recuperative heat exchanger (1), from where outdoor air is led indoors from the supply duct (3), indoor air is led from an exhaust duct (4) leading to the indoor space through the recuperative heat exchanger (11) in air duct (8) to the regenerative heat exchanger (2), from where indoor air is exhausted through the exhaust duct (6).

2. An apparatus for air drying and heat recovery as defined in claim 1, characterised in that outdoor air is conducted from the regenerative (2) to the recuperative (1) heat exchanger at a temperature low enough for indoor air to condensate in the recuperative heat exchanger (1), yet at a temperature high enough for the recuperative heat exchanger (1) not to freeze.

3. An apparatus for air drying and heat recovery as defined in claim 1 or 2, characterised in that the efficiency rate of the rotating regenerative heat exchanger (2) is varied by varying the speed of rotation of the disc, and in that the efficiency rate of the flow-inversing regenerative heat exchanger (2) is varied by varying the length of the warm and cold cycles of a solid heat-accumulating mass.

4. An apparatus for air drying and heat recovery as defined in any of the preceding claims, characterised in that the efficiency of the regenerative heat exchanger (2) is controlled with a thermostat or a hygrometer.

5. An apparatus for air drying and heat recovery as defined in any of the preceding claims, characterised in that indoor air is cooled with outdoor air in the regenerative heat exchanger (2), indoor air being led to the recuperative heat exchanger (1), where air is dried.

6. An apparatus for air drying and heat recovery as defined in any of the preceding claims, characterised in that the evaporator (11) of a cooler leading to the indoor space is mounted in the supply air duct (3) and a condenser (10) is mounted in the exhaust duct (6) leading to outdoor air, or the condenser has been mounted so as to heat water, for instance.

7. An apparatus for air drying and heat recovery as defined in claim 1, characterised in that the boiler of the absorption-cooling apparatus is heated with shower water or any other waste and free heat.

8. An apparatus for air drying and heat recovery as defined in claim 1, character- ised in that the water condensed in the recuperative (1) and regenerative (2) heat exchanger is conducted in a pipe (9) to the drain, a separate receiver or a swimming pool, for instance.

Fig.l