1. Field of the Invention
The present invention relates to a heat exchange matrix of the type that can be used as an adiabatic humidifier to introduce moisture to a flow of fluid. The invention further relates to methods of manufacturing such devices. A matrix of this type can operate to provide adiabatic cooling for domestic purposes and may also be used for humidification in combination with conventional air conditioning and heating systems. It may also be used to remove moisture from an airstream in combination with a suitable desiccant.
2. Description of the Related Art
Heat exchange devices of one form or another are present in virtually every device and process. The performance of an action invariably involves the release of energy in the form of heat. If not required, the heat will often be released to ambient via an appropriate heat conducting surface provided e.g. with cooling fins. If the quantity of heat is excessive or if it can be employed for useful purposes, a specific heat exchanger may be provided to transport the heat away e.g. to another system. Heat exchange may also take place between different media:—gas, liquid and solid media can be interfaced in all combinations according to the performance required.
Adiabatic systems are also known, which act by evaporation of a liquid, usually water, into an air stream. Such systems are not heat-exchangers in the strictest sense, since they do not primarily cause heat to enter or exit the system. In fact, they serve to merely change the temperature of the air stream while raising its enthalpy only by the sensible heat of the added water. As water evaporates into the air stream, the latent heat of evaporation of this water is provided by the cooling of the air stream. For relatively hot, dry air, this method of cooling can be very efficient.
Conventional adiabatic coolers are disclosed in U.S. Pat. No. 3,792,841 and U.S. Pat. No. 5,143,658. Such devices generally comprise a matrix formed by stacks of corrugated plates placed on top of each other such that the corrugations in adjacent layers are angled with respect to one another. The plates may be formed of various materials but resin reinforced organic or inorganic fibrous materials are the most common. The plates are supported by a housing or frame which may provide inlet and outlet connections to guide an air flow through the stack. A watering arrangement is provided for continuously or intermittently applying water or another evaporable liquid to the plates. In operation, air to be cooled is passed through the stack. Assuming that the air is not fully saturated, it will absorb water vapour from the plates. In doing so, the temperature of the air is lowered, toward the so-called wet bulb temperature, which is the theoretical minimum. For given operating conditions, the efficiency of such devices may be determined by the input energy required to drive the air stream through the stack. This is in fact the only significant external energy required and is largely determined by the overall flow resistance of the stack and the speed of the air stream. A major problem with prior art devices is that in order to optimise efficiency they tend to become relatively voluminous.
In addition to adiabatic cooling, devices of similar construction are also used to humidify airstreams for other purposes. In particular, in integrated heating, ventilation and air conditioning systems (HVAC) it is often desirable to increase the absolute humidity of the air for comfort purposes. In particular during the winter months, heating systems tend to cause the air to become relatively dry. This may lead to respiratory problems, static electricity and other discomforts. Addition of moisture in an efficient manner without increasing the volume of the HVAC installation would be desirable.
An important distinction of the above devices over conventional heat exchangers is that they may interact with only one media flow. This avoids complex inlet and outlet manifolds and there is generally little need to consider the thermal conductivity of the matrix. Conventional air to air heat exchangers operate in cross-flow or counter-flow with heat transfer taking place between primary channels and secondary channels, which must be sealed from one another e.g. by heat conducting walls. An important design consideration is the manner in which an adequate heat transfer coefficient is achieved for the conducting walls. Another consideration is how multiple interlaced channels connect to inlet and outlet manifolds.
Another class of heat exchangers is the heat recovery wheel. Such devices use sensible and latent heat exchange media in the form of a matrix that provides air passages through which an air stream can be directed using a fan or blower. The matrices support a desiccant material that can absorb moisture and can take a variety of forms, such as a fibrous mesh or honeycomb. One type of honeycomb matrix is formed of a plurality of spaced, substantially parallel layers of a sheet material, particularly alternating layers of a corrugated sheet material and a flat sheet material. In the latter case, the corrugations are generally parallel and provide a plurality of axially extending passageways extending along the depth of the wheel. Such devices are disclosed in U.S. Pat. No. 4,769,053 and U.S. Pat. No. 5,542,968. Different materials have been proposed for the matrix construction including aluminium and fibrous materials. Considerable attention has also been given to the optimised use of the desiccant coatings. Nevertheless, one particular disadvantage of such wheels is their overall bulk. This is generally related to the volume of material required for effective heat exchange and to the need for an effective flow through area that will not lead to significant pressure drop and an associated high power consumption by the fan.
Many other forms of heat exchange construction have been proposed in the past for various different purposes. U.S. Pat. No. 4,147,210 discloses a screen heat exchanger comprising alternating screens and spacers. The screens are made of a conductive material such as copper or aluminium in the form of a mesh.
It would therefore be desirable to improve on the existing designs, not least in terms of matrix volume for a given performance but also in terms of the energy efficiency of flow through the device.