A high efficiency liquid-liquid heat exchanger is made by imbedding 1 mill polyester plastic film in a mass of quartz pebbles. The quartz pebbles are 0.125 to 0.25 inches in diameter and are placed in 0.25 to 0.5 inches thick layers between the plastic sheets. The two liquids flow on alternate sides of the sheets and the flows of the liquids are given a 90.degree. angular spiral flow in relation to each other by strips of plastic cemented between the sheets. In this way a stream tube, or small division of the main flow of one of the liquids, is heated by short elements of a large number of stream tubes of the other liquid and the effects of uneven placement of the pebbles and the resulting channeling of the liquids are overcome. Heat transfer coefficients as high as several hundred BTU's per degree Fahrenheit per hour per square foot of plastic surface have been easily obtained with very low pressure drops. The plastic sheets and quartz pebbles are very cheap and the heat exchanger is easily assembled. The heat exchanger can be operated, if desired, at relatively high flow rates and pressure drops, and higher heat transfer rates obtained.
A heat exchanger unit comprising a particulate heat exchanging mass or pack consisting of relatively small, mechanically immobilized particles having a thermal diffusivity constant of at least 0.5 cm.sup.2 /sec at 20.degree. C., and compressively retained in an enclosure in heat transfer relationship to each other and to a fluid directed therethrough. Preferred materials for the particles are crystalline carbon, copper and aluminum. The pack may be in cylindrical form or planar form and may be contained within metal conduits or, for solar radiation, within a transparent or translucent enclosure. Interconnected units may be disposed in an array or bank to provide the desired quantitive degree of thermal transfer.
