EVAPCO ATWB Induced Draft Closed Circuit Cooler

A closed circuit cooler that keeps the process fluid out of the air

The EVAPCO ATWB is an induced draft, counterflow closed circuit cooler: the process fluid circulates inside the tubes of a coil and never touches the outside air, while spray water and air are used outside the coil to take the heat away. That is the difference from an open cooling tower, whose water is exposed to dust and debris and often has to be separated from the process by a plate heat exchanger that costs approach temperature. EVAPCO describes the ATWB as the original Advanced Technology closed circuit cooler and lists a very wide range, from 91 to 37,765 MBH, in many box sizes.

The process fluid enters the Thermal-Pak II coil at the top connection, and water from the sump is pumped over the tubes while an axial fan draws air up through the unit against the falling water. A small part of the spray water evaporates and carries heat away, and the cooled fluid leaves through the bottom connection. Because the coil sits in the air stream, the unit can also run dry when ambient conditions allow, stopping the spray pump and saving water and pump energy, and EVAPCO states inlet fluid temperatures up to 170 F are possible where an open tower is limited by its fill to 130 F.

Technical specifications

Model range and engineering data

Models are written as, for example, ATWB 9-3I14: the first number appears to be the cabinet width series, the middle number and letter the coil and fan selection, and the final number the cabinet length in feet (derived reading from the drawings, confirm on certified prints). The printed tables give shipping weight, heaviest section weight, operating weight, fan hp, air flow, spray pump hp and gpm, coil volume, and the remote sump data. Selected rows:

Notes in the tables state that the number of coil connections doubles when the flow rate exceeds 900 gpm on the 9 x 14, 9 x 18 and 9 x 21 models, and 1,800 gpm on the 17 x 12 models, as a required high-flow coil configuration. Series flow piping, stainless steel coils, low sound fans and non-certified options are marked by suffix letters on the model number.

Construction, access and water treatment

The basin is accessible from all four sides by lifting out the lightweight louver, the bottom of the fill section stands at least 4 ft above the basin floor, and the pan slopes completely to a depressed section so that dirt can be flushed through the drain. Optional equipment printed in the bulletin includes a sloped ladder, a working platform with davit, a basin level platform, a hinged louver access door, an all stainless steel option excluding fans and coils, and a vibration cutout switch.

EVAPCO offers factory mounted water treatment on the unit: the Smart Shield solid chemical system or the Pulse-Pure chemical-free system, plus the Pass-Protect passivation process for galvanized steel. A remote sump arrangement omits the spray pump and strainer and supplies an oversized outlet, and a bleed line is required to control concentration of impurities in the recirculating water.

Using the ATWB in Oman and the GCC

Closed circuit coolers are used in the region where the process fluid must stay clean, such as cooling for compressors, furnaces, induction heating, data centres, power generation equipment and water-cooled heat pumps. Because the fluid stays inside the coil, fouling of the process side is avoided, which is valuable where dust and sand would contaminate an open tower system. Selection must be made at the project’s design wet-bulb temperature, which can be high in coastal summer conditions.

The spray water outside the coil still needs a water treatment programme, blowdown control and a Legionella control plan, because scale on the coil reduces capacity quickly. Plan regular cleaning of the basin and strainers.

Information to send with an enquiry

• Process fluid, flow rate and temperatures
• Design wet-bulb temperature
• Make-up water quality
• Sound limits and quantity

Documents and quotation

The attached engineering data bulletin includes the full ATWB model tables, drawings, steel support layouts and the guide specification text. Seven Star LLC is an independent supplier in Muscat; to quote a closed circuit cooler we need the process fluid and flow, entering and leaving temperature, design wet bulb and dry bulb, allowable pressure drop and footprint limits. Related items are under cooling equipment.