Heaters
Operating temperatures can be classified into three ranges: low (< 120C), medium (120-250C), and high (>250C). low temperature-range heaters generally tend to use condensate or steam, medium temperature-range heater tend to steam, and high temperature-range heaters tend to use fired heaters or hot oil loops (<400C)
Temperature and required heat load are important factors in selecting a heater.
Fired Heaters
Fired heaters are used for heating up to high temperatures. They are able to reach these high temperatures because they generate energy by combustion of natural gas, fuel oil, or process off-gas. Fox example, they are used to generate heat for hot oil loops or steam generators (boilers). Hot oil can be used up to 600F (35C), and fired heaters provide a way to reach those temperatures. High pressure steam, used for utilities, is approximately 480F (250C).
Because fired heaters carry out combustion, factors such as pollutant emissions and excess air feed need to be accounted for in addition to the required heat duty. Reducing emissions can incur substantial additional costs.
Common types of fired heaters include cabin heaters, U-tube heaters, and vertical cylindrical heaters.
Electric Heaters
In electric heaters, heat is generated by running electricity through wires of high resistance. Heat is exchanged with fluid passed over axially through MgO insulation. The maximum duty that is typically obtained from electric heaters is approximately 1 MW.
Advantage of electric heaters include: the ability to reach very high temperatures (up to 1200F), no cross-leakage (because only one fluid is being used), good control (because of electrical system), no site emissions (initial power generation occurring elsewhere), and applicability in cyclic operations (metal cycles are not constantly used and/or fouled). However, disadvantages of electric heaters include: both higher capital and operating costs (equipment purchase and paying for electricity), and the large voltages make them extremely hazardous if they are not properly installed, operated, and maintained.
Steam Generators
Steam is typically the primary source of heat in processes. Steam used for this purpose is generated in boilers. The heat used to generate steam comes from combustion of natural gas, liquefied petroleum gas, or heating oil (commonly #2 or #6).
Often, high-pressure steam is generated in boilers. As it moves through a plant, it can be expanded in turbines to recover energy. Most plants use more than one level of steam.
Boilers are typically sold as packaged units. The two main types are water-tube and fired-tube.
Coolers
If heat cannot be recovered directly through exchange with another process stream, high-temperature heat can still be recovered by generating steam or preheating the feed water to the boiler. This way at least some energy is recovered.
Water and air are used for most cooling applications because they are available in large quantities at minimal cost. A comparison between the two makes the tradeoffs easily apparent.
Air Coolers
Advantages of air coolers are that they do not require additional infrastructure, and air is free so that the only operating cost is the electricity for the fans. Additionally, it is easy to add extra capacity for a higher duty. However, air coolers have much lower heat transfer coefficients compared to water coolers, and as a result can take up a lot of space. They can also lead to over-cooling depending on external temperatures.
Air coolers can use forced or induced draft. In forced draft, air is pushed up through the tubes by fans, located below. In induced draft, air is pulled up through the tubes, by fans located above. Forced draft air coolers allow easier access for maintenance, and allows recirculation of air for winterizing. Induced draft air coolers have better air distribution and less air recirculation, as well as a better natural draft.
Water Coolers
Water coolers require extensive infrastructure (pipes, cooling towers, water treatment) in order to provide water for cooling. Depending on the location, cheap water is not always available, and it is expensive to add capacity to handle an increased duty. Also, fouling is a problem. However, they have much higher heat transfer coefficients than air coolers, and thus are more compact.
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