Thermal design of heat exchangers pdf

Cast iron tubes are easily equipped with fins, but also steel tubes can be equipped with fins. Finned tubes are more difficult to clean than unfinned tubes, thus economizers with unfinned steel tubes are used in boilers burning fuels with a high ash content. Figures provide some examples on finned steel tubes.

Spiral finned tubes are often used in Figure 1: Spiral finned tubes [Aircoil]. By bending fins heat transfer properties can also be improved. Steel tube with aluminium fins endures better in corrosive conditions.

Compound composition conists of a cast iron tube equipped with fins and steel tube inside. A compound composistion endures higher pressure. In air preheaters finned steel tubes are not used, since the heat transfer properties are practically the same on both air and flue gas sides. When cast iron tubes are used, heat transfer surfaces are usually finned on both sides to improve the heat transfer. Superheaters and evaporators use unfinned Figure 2: Finned tubes [Vulcan finned tubes].

Sizing of heat transfer surfaces When sizing the heat transfer surface of a heat exchanger the heat power to be transferred and stream temperatures of inlets and outlets have to be known.

The heat surface area can be calculated from equation 15, when temperatures and the heat transfer coefficient have been determined, which is the capability of the heat exchanger to transfer heat between two fluids. Figure 4: Heat exchanger stream descriptions for a cross-flow heat exchanger , used in equation The heat transfer surface area of furnace consists of sides, base and beak, which is an "L"-formed bending of the evaporator tubes that protect the superheaters from radiation.

Most of utility and industrial boiler furnaces have a rectangular shape. A large number of package boilers have a cylindrical furnace. Furnace bottom for typical PCF boiler is double inclined or v-form, as shown in figure 5. Flat bottom is more typical for grate or bubbling fluidized bed boilers. The ratio of height and width varies for boilers with two-pass layout. The larger the boiler is, the larger is also the ratio. The largest boilers have a width of 20 m and a height of m.

The fuel and vaporization efficiency determines the size of the furnace. To be able to dimension furnaces the overall mass balance, h A heat balance and heat transfer must be specified.

The furnace heat balance can be specified Figure 5: Furnace dimensions. The painted similarly: areas are the total effective furnace heat transfer area. Furnace strain level The furnace is preliminarily dimensioned with a net fur suitable strain level. Tube wall design When the size of the furnace has been dimensioned, the tube size and material can be chosen and the wall thickness can be calculated according to the SFS standard.

Then input velocity of water to furnace is chosen and number of necessary tubes is calculated. Load characteristics When designing of a steam-generating unit it is necessary to determine the following load characteristics: 1. Minimum, normal and maximum load 2.

Time duration of each load rate 3. Load factor 4. Nature of the load constant or fluctuating The load factor is the actual energy produced by a power plant during a given period, given as a percentage share of the maximum energy that could have been produced at full capacity during the same period. The design will determine the boiler's ability to carry a normal load at a high efficiency as well as to meet maximum demand and rapid load changes. It will also determine the standby losses and the rapidity with which the unit can be brought up to full steaming capacity.

In smaller boiler sizes it is possible to select a standardized unit that will meet the requirements; larger units are almost always custom designed. Fuel type effect on furnace size The most important item to consider when designing a utility or large industrial steam generator is the fuel the unit will burn.

The furnace size, the equipment to prepare and burn the fuel, the amount of heating surface and its placement, the type and size of heat recovery equipment, and the flue gas treatment devices are all fuel dependent. The major differences among boilers that burn coal or oil or natural gas result from the ash in the products of combustion. Natural gas produces no ash. For the same power output, due to the high ash content of coal, coal-burning boilers must have larger furnaces and velocities of the combustion gases in the convection-based heat exchangers must be lower.

Figure 9 presents an example of the relative sizes of furnaces using three different fuels: natural gas, oil and coal. The power of the boiler is the same in all three cases. Typical furnace outlet temperatures Furnace outlet temperature is the flue gas temperature after the radiation-based heat transfer surfaces before entering the convection-based heat transfer surfaces.

The outlet temperature depends on the characteristics of the combusted fuel. If the temperature is too high, ash layers build up on the surface of the superheater tubes. This leads to poorer heat transfer, increased corrosion and it can even block flow paths.

It is necessary to provide air in excess of this quantity to assure complete combustion. The amount of this excess air is determined by the following factors: 1. Composition, properties, and condition of fuel when fired 2. Method of burning the combustible 3. Arrangement and proportions of the grate or furnace 4. Allowable furnace temperature 5. Turbulence and thoroughness of the mixing of combustion air and volatile gases Excess air reduces efficiency by lowering the furnace temperature and by absorbing heat that would otherwise be available for steam production.

NOx can be reduced decreasing temperature, decreasing air excess, or using low-nox-burners. In using low-nox-burner air will be fed into flame in two or three phases. CFB furnace design When dimensioning a circulating fluidized bed CFB furnace the high content of sand has to be taken into consideration. This means that the temperature profile and thus the heat transfer near to the furnace wall differs from other types of furnaces.

Download Download PDF. Translate PDF. A characteristic of heat exchanger 1. U tube heat exchanger design is the procedure of specifying a design, then 2.

Straight tube 1- Pass or 2- Passes. Process fluid assignments to shell side or tube side. Then 2. Selection of stream temperature specifications. Setting shell side and tube side pressure drop has been performed by applying several thermal design limits. Viscous fluids go 4. Setting shell and tube side velocity limits. Selection of heat transfer models and fouling rate of heat transfer. The pressure values from the coefficients for shell side and tube side.

Selection of heat exchanger TEMA layout and number of passes. Specification of tube parameters —size, layout, pitch and material. Heat exchangers are devices used to enhance or 3. Setting upper and lower design limits on tube facilitate the flow of heat.

Every living thing is length. Specification of shell side parameters — materials, exchangers. They are commonly used as oil coolers, baffles cut, baffles spacing and clearances. Setting upper and lower design limits on shell in both fossil fuel and nuclear-based energy diameter, baffles cut and baffle spacing. To develop calculations there are several design andrating packages available. Shell and tube heat exchangers still take a noted place in many industrial processes.

They are widely used because of their robut and flexible design. However, conventional heat exchangers with segmental baffles in shell side have some shortcomings resulting in the relatively low conversion of pressure drop into a useful heat transfer.

The exchanger, the Bell-Delaware method is usually used equation given below has been shown to be accurate in the shell and tube heat exchanger design. In this for any arrangement having 2-tube passes per shell method, the shell side heat transfer coefficient is pass. They are comparing with given heat load. Revised and updated with new problem sets and examples, Heat Exchangers: Selection, Rating, and Thermal Design, Third Edition presents a systematic treatment of the various types of heat exchangers, focusing on selection, thermal-hydraulic design, and rating.

Each chapter contains examples illustrating thermal design methods and procedures and relevant nomenclature. End-of-chapter problems enable students to test their assimilation of the material. Step-1 : Read the Book Name and author Name thoroughly. Step-4 : Click the Download link provided below to save your material in your local drive.