Cooling boxes are large water-filled structures containing a pipe coil. They usually are made of concrete or steel. Typically, the process pipes inside are 3 in. diameter or larger.
Cooling boxes often handle one of two roles. The first is as an inexpensive substitute for a tempered water system. The second is as an emergency shutdown cooler on standby in hot processes.
Conventional tempered-water-system heat exchangers recycle their discharge water to heat their inlet water. Process duty requirements control the recycle rate.
The main objective of a tempered water system is to remove heat from the process via the hottest possible cooling utility — because this enables more effective recovery of the heat for other duties. Heat integration schemes often rely on tempered water.
Other uses for tempered water systems include situations where the process fluid may solidify if it gets too cold. The tempered water keeps film temperatures as high as possible while still removing the required duty. The higher temperature helps avoid solidification on a cold tube surface.
Tempered water systems require a recirculation pump. In contrast, a cooling box provides tempering without a pump.
Emergency cooling backup is crucial for plants where emergency shutdowns may happen at the same time cooling water is lost. A cooling box holds an inventory of water. This inventory provides a heat sink for a shutdown when cooling water isn’t available.
Three points about cooling boxes merit further discussion: overall design and evaluation, fouling, and water makeup.
Cooling boxes aren’t included in conventional design tools or software. A usable shortcut is to consider them either as a kettle reboiler or a tank bayonet heater. For a cooling box operating at boiling conditions, kettle-reboiler calculation methods will allow for a reasonable performance approximation. For cooling boxes operating below boiling conditions, tank stab-in-heater methods often will give better results. The tank immersion heater approach usually under-predicts heat transfer because tanks have lower circulating velocities than cooling boxes. Nevertheless, this remains the best shortcut method for a cooling box without vaporization.
Because cooling boxes are open to the atmosphere, they lose or gain water from evaporation or precipitation. Most cooling boxes vaporize most of the water entering them. These boxes are not intended to overflow; so, a level controller manipulates the makeup water rate. In addition, they usually use a purge to attempt to control the content of dissolved material in the water.
Less commonly, cooling boxes are intended to overflow. Such boxes typically have a fixed water rate high enough that the box will never dry out in normal operation. Alternatively, a temperature controller on the overflow water will set the water rate.
The dominant factor in cooling box performance usually is outside fouling. Temperature is high. Any carbonate or sulfate content will precipitate at the tube surface to cause scale. The open top of the box also allows dirt, dust, pollen and other organics into the water. Oxygen from the air, relatively high temperature and long residence time conspire to foster formation of fouling deposits.
Exterior fouling factors for cooling boxes routinely reach 0.1 hr-ft2-°F/BTU or higher. This is orders of magnitude higher than those of a conventional exchanger. It’s best to derive a fouling factor from experience for the location, box configuration and water used, if possible. Otherwise, for design purposes, a value of 0.1 hr-ft2-°F/BTU is a reasonable starting point for boxes in continuous services. The fouling allowance will directly determine the interval between cleanings. The higher its value, the more expensive the cooling box becomes but the longer the period between cleanings. For boxes used as emergency shutdown coolers that may sit at low temperatures while on standby, lower fouling factors — 0.005–0.010 hr-ft2-°F/BTU — are reasonable for most applications, assuming some effort is made to keep the sitting water in the box “reasonably” clean.
ANDREW SLOLEY is a Chemical Processing Contributing Editor. He recently won recognition for his Plant InSites column from the ASBPE. Chemical Processing is proud to have him on board. You can email him at ASloley@putman.net