Vari-Cool Evaporative (Hastings Industries Inc.)

The uses of the cooled dry-side air

The major use of the cooled, dry- side air is to pre- cool entering air to reduce the load on air conditioning system coils. Cooled, dry-side air may also be used as an exclusive cooling source in applications where design conditions permit. Two-stage cooling applies dry-cooled air to a second stage direct evaporative cooler. The two-stage I/D" (Indirect/Direct) unit is used as the total cooling source in many systems applications.

Wet-side air source

Normally outside ambient air is used as the secondary wet-side air source. In cases where sufficient exhaust air is available to handle the secondary air requirements (nominally 800 CFM per core), the pre-conditioned air will provide a lower wet-bulb temperature, thus increasing the capacity of the VARI-COOL unit for cooling; also, this application will allow wintertime heat reclaim operation. Never apply exhaust air at temperatures above 125°F.

The use of the cooled wet-side air

The wet-side(secondary) air is cooled as it passes over the tubes. The air may be directed, and used as the entering air, to air conditioning system air-cooled condensers. This will create further energy reduction (attributable to the VARI-COOL equipment) and increased capacity of the air conditioning system. The dry bulb temperature reduction of the secondary air can be expressed as approach to wet-bulb temperature. The VARICOOL unit, based on nominal8OO CFM per core of secondary air will achieve an approximate 1O°F W.B. approach.

Heat Reclaim

When the VARI-COOL unit is applied to pre-cool outdoor air in the cooling season and building exhaust air is provided as the wet side air source, an additional energy-saving by-product is obtained in the heating season: the outdoor air is pre-heated by reclaiming the heat of the exhausted building air. Heat reclaim is accomplished when the cooling thermostat de energizes the circulating pump and the knitted tube sleeves dry out, creating a sensible heat exchange. Automatic control can be provided to energize and de-energize the water pump based on the entering air temperature to the dry-side air passage.

Heat reclaim, based on equal air quantities (primary air through the tubes and secondary air across the tubes), is approximately 40% of the temperature difference between entering primary air and entering secondary air.

                                                                                                   Example: Entering Exhaust Air = 80°
                                                                                                                    Entering Outdoor Air = 35°
                                                                                                               Temperature Difference = 45°

At 40% efficiency, the temperature rise to the entering outdoor air is 45°x4O%= 18° rise or a leaving temperature of 53°F.

Do not apply exhaust air temperatures above 125°F. To avoid frost accumulation, do not provide outdoor air temperatures to the unit below 3O°F. Tempered (pre-heated or bypass) air may be provided at temperatures below 30°F. Do not apply exhaust air above 1000 CFM per core to avoid water carry over.

How Does The I/D^™ System Works?

On a call for cooling, the control dampers are positioned to introduce outside air fan, to the VARI-COOL™ indirect evaporative cooling units, which functions as first stage of cooling. The air is sensibly cooled (no moisture added) as it passes through the tubes (dry-side)of the indirect unit. This cooling process is achieved by water distributed which wets the outside surface of the heat exchange tubes.Secondary (wet-side)fan(s) move a separate air stream across the exterior of the wetted tube surface forcing evaporation and cooling the tubes adiabatically. Heat is transferred from the tube to the secondary air stream and rejected to the outside.

The leaving dry-side air, from the first stage indirect unit, passes through a highly efficient direct evaporative cooling unit which, as stage two, further cools the air. The process is adiabatic whereby the dry-bulb temperature is reduced while the wet-bulb temperature remains constant.

The leaving air from the direct evaporative unit is the cooled supply air is used to perform the cooling work in the space.

The temperature of the supply air will be a function of the ambient temperature (DB & WB) of the outdoor entering air, and the efficiency of the indirect and direct units. At the recommended design temperatures for many geographical areas, the leaving air temperature will be low enough to produce the total required cooling by two stage evaporation alone. No additional (third stage) refrigerated booster cooling will be required. The I/D two stage system will maintain satisfactory room conditions. Note that the supply air is relatively saturated (usually 90% plus) similar to the condition of the air leaving a conventional cooling coil. Following the psychrometric process, room sensible heat ratio and supply air quantity will determine the heat removal as well as resultant room temperature and relative humidity.

When a lower supply air temperature is required a booster third stage conventional cooling coil may be incorporated in the system to produce the required added temperature drop. Optionally, this coil may be located in the air leaving the indirect unit or leaving the direct unit.

During the non-cooling or heating cycle the damper positions are reversed whereby the return air damper is fully open and the outside air damper is set at a minimum position for ventilation. The system is on a return air cycle and the indirect unit is bypassed. The indirect and direct unit pumps and indirect unit wet-side fans are de-energized. The heating apparatus is energized if required. If required, a bypass humidistat can be incorporated to energize the direct evaporative unit pump during the heating cycle to increase humidity.

I/D units may be applied to provide cooling only. In this case no recirculated air would be required as part of the system mode. A short connecting duct section is provided to connect the indirect and direct units and no return air connection or return and outdoor air dampers are provided.

Vari-Cool Evaporative - Page 2