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Industrial Water Tube Boiler Design
Formulas in Practice
Mehran Mousapoor
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eBook - ePub
Industrial Water Tube Boiler Design
Formulas in Practice
Mehran Mousapoor
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About This Book
This book consists of steps for the thermal & mechanical design of d-type water tube boilers. It provides the reader with guidance for burner, fan size, and capacity selection, furnace thermal and dimensional design, super heater primary thermal and mechanical design, and boiler bank and economizer design. Calculations of the methods and efficiency are also described.
Mr Mehran Mousapoor, [email protected]
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1 Introduction
This guideline gives direction as to how to design a water tube D-type boiler. This design guideline can help engineers understand the basic design of a boiler with an appropriate dimension, materials and heat of combustion. Good execution of a boiler is affected by the greatest heat absorbed and least heat misfortune. The design of the boiler may be impacted by variables, counting process requirements, financial issues and safety. All vital parameters and equations used within this guideline are clarified, which help the readers understand the meaning of parameters or the term utilized. The theory area clarifies how to calculate sizing and determination of a boiler. This guideline makes a difference by the readers to get almost the heat balance concept. The application of the boilerâs theory with the illustrations will make the engineers understand boilerâs basics and perform the actual design of boilers [5].
1.1 Boiler basics
A boiler is a closed vessel in which liquid (for the most part, water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in different processes or heating applications [1]. Water tube boiler is a shape of the boiler in which steam is produced by circulating water through tubes exposed to the source of heat.
Boilers utilize a heat source, ordinarily combustion of a fossil fuel, to heat water to generate hot water or steam. Boilers nowadays burn fuel gas and oil as well as solid fuels and proceed to play a bigger role in industries.
Boilers are separated by their arrangement, dimension and the quality of the steam or hot water delivered. Boilerâs dimension is most regularly measured by the fuel input in million Btu per hour (MMBtu/h). Size may be measured in pounds of steam per hour (pph). Output may be measured in horsepower. One boiler horsepower = 33,475 Btu/h evaporation capacity or about 34.5 pph. Since huge boilers are regularly used to create power, it may moreover be valuable to relate boiler size to power output in electric producing applications. Utilizing ordinary boiler and producing efficiencies, 100 MMBtu/h heat input is increased to almost 10 MW electric output. Hot water boilers, for the most part, heat water to 250 °F or less at pressures of 250 pounds per square inch (psig) or less. Huge numbers of small low-pressure steam boilers (<10 MMBtu/h) have been utilized at small factories or operate in support of larger manufacturing processes. Low-pressure steam boilers for the most part deliver saturated steam at temperatures of 350â400 °F at pressures between 125 and 250 psig. The bigger steam usage is for industry, power generation and area heating. The main steam consuming industries are refineries, petrochemical, power plants, paper, food industries and metal factories. Large boilers generate high-pressure steam and may be rated at 250â10,000 MMBtu/h. High-pressure boilers can generate steam temperatures above 700 °F and pressures till 3,000 psig [2].
1.2 Boiler types
Boilers can be characterized by the arrangement of heat transfer surfaces â either fire tube or water tube â and by the combustion system. The appropriate arrangement is determined by suitable fuel, steam conditions and capacity. Figure 1.1 shows the capacity ranges between fire tube and water tube boilers on a heat input basis [3].
Fig. 1.1: Water tube versus fire tube boiler [3].
1.3 Boiler arrangement
There are two types of boilers. In a fire tube boiler, the water is in the main part of the boiler, and the combustion gases pass through metal tubes. Heat is transferred to the water by conduction from the fire tube(s) to the surrounding water. Increasing the number of âpassesâ that combustion gases make through the boiler upgrades heat extraction. The simplicity and low cost of fire tube boilers are their point of interest, nearly all fire tube boilers burn oil, natural gas or both. The mixing of water in a huge chamber makes a fire tube boiler well suited to generate hot water or low-pressure steam. For high-pressure (>200 psig) or high-capacity (>10 MMBtu) applications, fire tube boilers (Fig. 1.2) are not desirable because of pressure vessel failure, which in a water tube boiler is just can be failure of a single tube [3].
Fig. 1.2: Fire tube boiler, HKB Boiler Solution.
Fig. 1.3: Water tube boiler, HKB Boiler Solution.
In water tube boilers (Fig. 1.3), the fuel burns in a furnace and the exhaust gases flow surround metal tubes. Heat transfer to the water tubes is achieved by radiation, conduction and convection. The water tubes are welded together to form the shape of the combustion chamber in a âwaterwall.â Water tube boilers can generate steam at high temperatures and pressures, and these boilers are more complex and expensive than fire tube boilers [3].
1.4 Boiler designing sequence
Sequences for designing a water tube boiler in this book will be as follows:
- Calculating heat duty of a boiler is based on subtracting outlet steam enthalpy from inlet steam enthalpy and multiplying outlet steam flow rate.
- Calculate the required fuel and combustion air for burning mixture of air and fuel.
- Calculate the heat input rate by generating burners and then selecting burner quantity and size from burner manufacturer catalog.
- Calculate the width and height of a furnace, which is approximate of flame diameter and height.
- Calculate the flame temperature, density and flue gas volumetric flow rate to find the required volume of furnace. For this matter, velocity of flue gas will be taken from boiler manufacturer handbook. Finally, we can calculate the height of furnace. Please note that this height, width and length are preliminary and need to recalculate the end of our calculations.
- At this point, some items such as furnace flue gas draft pressure drop, furnace flue gas exit temperature, heat flux and tube metal temperature need to be controlled.
- Calculate the boiler design pressure.
- Designing of a superheater package includes the design of superheater tube size, thickness, length, tube rows number and tube rows deep number. Please note that the mentioned item will be calculating preliminary and by calculating heat duty and optimization, corrected amount will be obtained.
- Calculate the outlet steam and flue gas temperature from superheater package.
- Calculate the steam and mud drum diameter and thickness.
- Calculate the height between drums center, and then bank tubes average length.
- Designing of a bank tube package by predicting its heat duty, and then predicting tube rows number, tube rows deep number, heat flux, steam drum outlet steam temperature and bank tube flue gas draft pressure drops. By iteration and checking limits, our prediction comes to reality, and final data will be obtained.
- Designing of an economizer package by predicting heat duty to obtain final size and specification, which is same as that of superheater and boiler bank packages.
- In all mentioned sequence draft, pressure drops, steam pressure drops, velocities and dimension of each package should be checked and even total package dimensioned should be controlled.
- Till now we assumed the circulation ratio which is related to the dimension and pressure of boiler and we continued our calculation. But at this moment we should calculate and control the circulation ratio.
- At this point, stack and safety valves will be sized.
- We are calculating our boilerâs efficiency at the next step. This item can show us our design was good or need to be revised. If it will be okay, then we can go through drum hold-up time and number of tubes and each part and packageâs weight.
- All reports can be seen at the end of this book.