Recently, the trend among process steam users, particularly food processors, has been to find alternatives to producing steam with conventional steam boilers. One approach is the use of thermal fluid or hot oil heaters with unfired steam generators.
Reasons for replacing a steam boiler with a thermal fluid system include the elimination of water treatment and reduction in maintenance functions. Additional thermal fluid system advantages include the elimination of steam traps and related components; limitation of the process to a single (liquid) fluid phase; and the benefits of a totally closed-loop system. However, these may not be applicable if steam is retained in the process. Environmental considerations also may favor the use of hot oil over fired boiler-produced steam.
Steam Boilers. Boilers generally are classified as either firetube or watertube. Regardless of boiler type or whether it is fossil-fuel fired or electric, its purpose is to convert heat energy into steam. To heat water and generate steam, a continuous supply of boiler feedwater is required. In addition to mechanical and chemical water treatment, a means of pumping feedwater is necessary.
The principle components of any steam boiler system are the boiler, a feedwater storage tank and a pump. Because heated water evaporates and chemicals build up in concentration, boilers require that a certain portion of the feedwater be drained to remove dissolved solids. Known as blowdown, the removed water must be compensated for with feedwater makeup.
Liquid-Phase Heaters. Utilizing thermal fluid as the heating medium, liquid-phase heaters are closed-loop systems. Although they require a reservoir for the fluid and a pump to circulate fluid through the system, blowdown is not required. Therefore, in the absence of leaks, fluid volumes remain essentially constant during operation.
The fluids generally do not require mechanical or chemical treatment but are subject to degradation over time. If the fluid is overheated or contaminated, degradation may be accelerated and result in premature fluid failure. Thus, fluid selection and quality are as critical to system performance as heater design.
In a forced circulation coil-type thermal fluid heater, a fluid under relatively low pressure circulates through a set of nested, parallel, connected coils while forced-draft combustion gases travel across the outer coil surface. The hot gases envelope the entire coil surface; this maximizes the use of both radiant and convective heat to achieve high heat transfer rates. Velocities on both sides of the coil wall are high enough to reduce film resistance to heat transfer.
For thermal fluid, flow through the coils must be turbulent enough to ensure constant mixing of the film layer with the bulk of the fluid but not so high as to cause excessive pressure drop that could result in higher pump motor horsepower. Therefore, the size of the system circulating pump should be selected to provide proper flow rate and head for the application while optimizing motor horsepower and maintaining low film temperatures.
Figure 1 diagrams the flow in a forced circulation, coil-type thermal fluid heater. As shown, temperature controls monitor the output temperature of the thermal fluid and regulate fuel and air to the burner. This results in efficient combustion over the modulating range of the heater. This design requires a small volume of fluid -- generally 10% of the fluid volume required for a helical coil-type heater of the same thermal capacity -- which can result in faster response to load changes.
Water TreatmentWater condition is critical to any type of steam boiler operation. All steam boilers are subject to failure if feedwater is not treated properly for hardness and corrosion control. Feedwater treatment generally has two components:
- Removal of scale-producing elements such as calcium.
- Removal of dissolved oxygen and carbon dioxide that can cause corrosion in the boiler, piping and other components.
Typical feedwater treatment systems consist of both mechanical and chemical processes.
Mechanical Feedwater Treatment. Eliminating oxygen and carbon dioxide in feedwater prevents oxygen and acidic corrosion, a common cause of tube failure. Deaerator systems provide a method for eliminating dissolved oxygen and carbon dioxide in feedwater. Deaerator systems are feedwater tanks that both heat and mechanically work the water by moving it over trays and through spray nozzles, thus placing the gas molecules close to the surface to facilitate their escape. Although the solubility of oxygen in water decreases with increasing temperature and theoretically oxygen cannot stay in solution at saturation conditions, some oxygen can remain and must be treated with chemical oxygen scavengers (figure 2).
Chemical Feedwater Treatment. Boiler feedwater treatment generally begins with sodium zeolite, a resin bed water softener. It removes hardness, which is the principal cause of scale in boiler systems. Scale acts as a heat insulator and reduces heat transfer efficiency across the boiler tube wall, and it eventually leads to burnout. In addition to reducing hardness, most softeners slightly increase the alkalinity of the feedwater, helping reduce acidic corrosion. If demineralizers are used, they often can decrease the feedwater's pH. Adding caustic soda or soda ash will increase pH to the desired range.
MaintenanceWater treatment can reduce the likelihood of oxygen and acidic corrosion in steam boilers; however, boiler installations still can exhibit a high potential for corrosion problems. This phenomena is exacerbated by the abrasiveness and natural lack of lubricity of high pressure steam. Tube replacement in steam boilers is considered normal maintenance; the ancillary or auxiliary components such as condensate pumps, steam traps and water treatment equipment require ongoing, regular maintenance.
Thermal fluid heaters also require periodic maintenance. However, the intervals for sampling and performing preventive maintenance tasks and the number and complexity of those tasks can be reduced in a hot oil heater. For example, feedwater quality in a steam boiler system often is monitored on a daily or weekly basis. Some blowdown systems use conductivity cells to automatically control the rate of blowdown based on continuous water analysis. Thermal fluid samples typically are analyzed only once per year. The water softeners used in feedwater treatment may, like a residential water softener, require the frequent addition of salt to a brine tank.
Efficiency. Most fired thermal fluid heaters will operate in the same combustion efficiency range as steam boilers. However, if deaerator, blowdown and flash losses are considered, the thermal fluid heater will show higher overall efficiency. The efficiency of a steam boiler can be reduced by tube fouling due to poor water treatment. Thermal fluid heater performance also is impaired by tube or coil fouling caused by fluid breakdown, but it occurs less frequently than scale buildup in boiler tubes.
Process steam and thermal fluid generally are used to provide heat for a particular application. As anyone who has taken a basic course in physics knows, water at 14.7 psia boils at 212oF (100oC); as pressure increases, a higher temperature is required to achieve boiling. For every saturation pressure, there is a corresponding saturation temperature. The relationships between saturation pressures and temperatures are published as tables of thermodynamic properties. From these steam tables, the pressure required to achieve a certain temperature of dry saturated steam can be discovered.
For example, for a temperature of 600oF (316oC), saturated steam must be produced at approximately 1,543 psia. As table 1 shows, many thermal fluids have vapor pressures less than 1% of that pressure at 600oF (316oC).
Obviously, not every thermal fluid system benefit applies to every situation. If strictly hot oil is being used, most benefits will apply. If steam is still being produced, even in a separate heat exchanger loop, some of these advantages may be only partially pertinent.