Water tanks are often heated with a steam sparger to provide hot water for production, cleaning, and a variety of other industrial uses. While steam spargers have a low initial cost, they are prone to noise, vibration, relatively short operating life, and limited heating rates. What’s more, high steam flow rates can result in incomplete condensation, energy loss from steam escaping to the atmosphere, and damage to tank linings from steam hammer or condensation shocks in the holding tank.
Another common method of tank heating is to utilize a shell and tube heat exchanger and a pump to circulate water from the tank through the heater. Tube bundles, however, are prone to fouling and require periodic cleaning to maintain capacity. A system to return condensate must be maintained as well.
An alternative solution is to use a direct steam injection water heater. Steam is injected into a recirculating loop. This steam not only supplies the thermal energy to heat the water, but also the pumping power to circulate the water in the loop. Because the steam condenses in the heater, no condensate return is required.
In addition, condensation occurs rapidly inside the heater body in the turbulence created by the steam jet. Noise and vibration levels are substantially lower with a direct injection heater than with a steam sparger and the heating rates can be much higher.
Figure I shows the required steam flow rate for a range of tank sizes and heating rates. To use the chart, select the tank size, the desired heating rate, and read the required steam flow. For example, if the tank volume is 8,000 gallons and the desired heating rate is 70 °F/hour, then the required steam flow will be 4,000 lb/hr.
In general, the heater size required will increase with the volume of water to be heated. A rough estimate of the nominal heater size for a given volume is shown in Figure II.
Figure III shows a piping schematic for a typical installation of a direct steam injection tank heater. A PID controller automatically modulates the opening of the steam nozzle in the direct steam injection heater to control the water temperature in the discharge line from the tank.
A large turndown up to 10:1 is possible with the direct steam injection heater. This adjustability allows for large steam flows for cold startup and much lower flow rates for maintaining the required temperature during operation.
The momentum from the steam jet propels warm water out of the unit and draws cold water from the tank to replace it. Because the heater is better at “pushing” than “pulling” water, the suction line should be located near the bottom of the tank and be designed to have low pressure loss while a higher frictional loss can be allowed with the discharge line.