ABOUT INDIRECT HEAT EXCHANGE: FAQS

The basis of the indirect cooling process is the dry transfer of outside air through a plate heat exchanger. In this process, outside air moves through the horizontal passages of the heat exchanger and comes into contact with the dry plate and transfer spines in those sealed passages (see drawing above). Heat is then transferred to the vertical passages where it is exhausted from the top of the unit by an exhaust fan.

Because the system separates the exhaust air from the outside (or supply) air, cross-contamination does not occur. This allows the use of building exhaust air to be used in the indirect heat exchange process and thereby recover energy both in the summer and winter.

The efficiency of the indirect cooling heat exchanger is generally a function of the material used in its fabrication and the velocity of the outside air traveling through the heat exchanger's passages. Spec-Air uses an engineered polymer, which has several special characteristics that make it ideal for indirect cooling. The material has a UL Flame and Smoke Spread rating of less than 5 which makes it safe for public buildings, schools, hospitals, churches, etc.; the material permits the thermal forming of spines or detents which function as heat transfer points and mix the incoming air to promote better contact with the heat exchanger structure. The heat exchanger is specifically selected based on the volume of incoming supply air divided by the most cost effective velocity of air, i.e., approximately 450 feet per minute. This selection is critical since the efficiency of the heat transfer process is also a function of the duration of time in which outside air comes into contact with the heat exchanger plates and indirect cooling occurs. Typically, the cooling efficiency of the Spec-Air heat exchanger is 75%+ (based on factory selection).

Indirect cooling is not a new process, many other methods have been attempted, however, only recently has materials technology improved to the extent that a polymer material became available to support this high efficiency with an equally high fire safety rating (critical in institutional settings). The polymer used by Spec-Air is thermally formed in sheets and then hand assembled using special epoxy with a rubberized edge coating. This joining process serves the specific function of ensuring that there is virtually no cross-impingement of supply air and exhaust air. The heat exchangers are then subjected to four quality inspections, which include pressurized testing.

Although used alone in many applications, the indirect cooling heat exchanger is often applied with other types of cooling, e.g., direct evaporative cooling, chilled water coils and conventional direct expansion systems (DX systems are used in standard packaged rooftop air handlers). In these applications, cooling is staged to provide the most energy efficient air conditioning. During periods of moderate outside air temperatures the supply fan is operated alone and only outside air is provided to the space. As the temperature increases the indirect cooling heat exchanger is activated to provide additional cooling of the outside air, and, finally, when the space temperature exceeds a predetermined set point, the mechanical cooling system is activated. Thus, the most expensive cooling process, mechanical cooling, is only operated during the hottest part of the day. As temperatures decrease, the cooling stages are turned off sequentially until, once again, only outside air is provided.

Spec-Air is the largest manufacturer of indirect cooling modules in the United States and has completed numerous projects worldwide. The success of the Spec-Air system has been the close coordination with specifying mechanical engineers in using our indirect cooling heat exchanger on projects which require large volumes of outside air for ventilation, cooling and heating. With indirect cooling and winter pre-heating through the heat exchanger, a significant reduction in the size (capacity) of supplemental mechanical cooling systems and heater sizes is gained. In operation, staged cooling and heating dramatically reduces the operating time (i.e., energy costs) of such systems.