Best practice no 22

Number 22

October 2023

BEST

Manufacturing Engineering PRACTICE

EFFICIENCY OF A BOILER OR STEAM GENERATOR

EFFICIENCY OF A BOILER OR STEAM GENERATOR

Many factors have influenced the development of modern Steam Generators. Operating loads, size, environmental restrictions, the use of low-quality fuels and the ability to adapt these changes over the useful life of the units are some examples. A specification that is usually on the list when selecting a steam generator is: "EFFICIENCY". Package boilers continue to be one of the most economical ways to generate steam and hot water. Selecting the right equipment, however, requires evaluation according to different criteria:

WHAT DOES BOILER EFFICIENCY MEAN?

PACKAGE BOILERS

TUBE BOILERS

• Selection of the type of boiler or steam generato r

• Comparison of features and benefit s • Determination of maintenance cost s

WATER TUBE BOILER

• Determination of fuel characteristics and cost s

• Evaluation of real boiler efficienc y

MONOTUBULAR BOILER

A typical boiler consumes four to seven times the initial investment cost in fuel in a single year, so the selection of a high-efficiency boiler represents the recovery of investment in a very short time due to savings in fuel consumption.

WHAT DOES BOILER EFFICIENCY MEAN?

How can we evaluate efficiency, in such a way that it reflects true fuel consumption? It is really difficult to verify the efficiency of a boiler after it has been installed, without an expensive measurement procedure. However, there are techniques available that provide the efficiency of a boiler before making an investment, by reviewing its basic design criteria and the data used to calculate its efficiency. The initial investment cost is the smallest part of the total investment of a boiler. Understanding how much the operating costs of a boiler are is the key to knowing the true annual investment cost when purchasing it. The following example compares four alternatives and their efficiencies. ne boiler has a thermal efficiency of 85%, another has a combustion efficiency of 87%, another has a boiler efficiency of 80%, and another has a fuel-steam efficiency of 83%. Which boiler consumes the least amount of fuel for the same application?

Based on this information, there is no way to know.

FOR-IM-024

REVISION DEL FORMATO: C

FECHA DE EFECTIVIDAD DEL FORMATO:10/03/2022

Typically, thermal efficiency reflects how well the boiler vessel transfers heat. This concept usually excludes radiation and convection losses. Combustion efficiency implies only the burner's ability to burn the fuel without the presence of carbon monoxide or unburned hydrocarbons. Boiler efficiency may not mean anything, although if we relate this efficiency to the energy supplied to the boiler in relation to the energy it delivers converted into steam, we can consider this fuel to steam ratio as a true efficiency ratio.

The efficiency of a boiler has been defined as the ratio of heat delivered through steam generation, divided by the heat supplied by the combustion process.

The total heat generated in a combustion process is not completely transferred to the generation of steam due to the natural losses that the boilers or steam generators have in the design, these losses are due to:

• Losses due to radiation and convectio n

• Losses due to waste of hot gases in the chimne y

• Losses due to high temperature condensate drain s

• Losses due to humidity in the steam generate d

These losses have a very significant impact on total efficiency, so it is worth doing a detailed analysis of each of them.

Losses are generally due to the construction design of the boiler or steam generator, so a general description of the design features will be given.

PACKAGE BOILERS

The concept of package boilers has been around the world for around 70 years. In them, all components are designed and installed on a single platform, in such a way that the only connections required for their operation are the installations of: water, electrical energy. , fuel, steam and condensate and chimney for the discharge of combustion gases.

The package boiler is compact, usually requires little space and is assembled complete with burner, fan, insulation, refractories, controls, auxiliary pump and preheater electrically and mechanically interconnected. This gives the owner the simplicity of a single source of responsibility for warranties, repairs and maintenance.

During the Second World War, a new type of package boiler was introduced to the marine industry (Scotch Boiler), which is now widely known in the industry as the Drum Boiler.

TUBE BOILER:

These boilers are built by a cylindrical vessel, with a central burner called "Morrison Tube", the combustion gases pass inside the tubes, called smoke tubes, the water and steam are found on the outside of the tubes. and the burner, contained by the main body. Essentially a flue tube boiler is constructed similarly to a shell and tube heat exchanger.

One of the inherent characteristics of this boiler is the large volume of water it contains; this allows it to respond to load variations with minimal changes in steam pressure.

The ideal design for this type of exchangers is the spherical shape, however, it is not practical, so a cylindrical shape is used.

In a cylindrical body, transverse forces are easily absorbed by the structure, however, longitudinal forces limit the construction of said boilers.

Longitudinal stresses tend to cause lateral failure. These forces are proportional to the pressure and diameter of the container, the thickness of the plate becomes increasingly larger as they try to absorb these efforts, so the pressure limit that these equipment can reach is a maximum of 300 psig. . (21 kg/cm²g).

FOR-IM-024

REVISION DEL FORMATO: C

FECHA DE EFECTIVIDAD DEL FORMATO:10/03/2022

Combustion occurs in the Morrison tube and the combustion gases travel through the tubes until they reach the chimney. These boilers are built with 2,3 and 4 steps. The gases pass through the tubes, giving up heat and decreasing their temperature and specific volume, so the passages decrease in total area in order to maintain adequate speeds for the best heat exchange with water and steam. Technologies such as Gas Recirculation (FGR) are currently available for smoke tube boilers. This design reduces NOx emission by recirculating a portion of relatively cold combustion gases within the combustion chamber, which reduces the combustion gas temperature. An efficient burner design must be provided in order to ensure complete combustion of fuels. The walls of the burner and the tubes can suffer from embrittlement due to high temperature if there is not a rapid transfer of heat between the steel and the water, so possible incrustation due to the salts present in the water should be avoided as much as possible, with a adequate treatment of the feed water, either by softening or demineralization as the case warrants. Tube boilers are non-condensing type, this means that there is a low limit on the temperature of the combustion gases and therefore a decrease in efficiency. If the temperature of the gases is allowed to drop too low, condensation occurs, this condensation would cause serious corrosion problems and failure of the tubes due to rupture. As a rule of thumb, the temperature of the chimney gases should be maintained around 100°F (55°C) with respect to the saturation temperature of the steam. Usually in standard designs 5 ft² of heating surface is required for each boiler horsepower (BHP). This type of boiler is designed up to a maximum of 1000 BHP. Efficiency increases with the number of steps, however, there is a limit; The addition of an extra step improves the average efficiency by 2 to 3%, however, this causes greater pressure losses, which must be compensated with an increase in fan power. Disadvantages of tube boilers: 1) High temperature differential between the chimney gases and the water/steam mixture in the upper part of the boiler. 2) Due to the large dimensions of the body, a large investment in insulation is required to avoid losses due to radiation and convection; in some places, freezing problems may occur when the boiler is stopped. 3) The large volume of water contained implies very long start-up times before steam can be generated. 4) Due to the large volume of water contained, a large amount of blowdown is required to eliminate sludge at the bottom. The transfer coefficient due to the combustion gases is a function of the speed with which they pass inside the tube, so maximum efficiency is obtained at high load ratings, dropping considerably at low loads. You must balance the relationship between the number of steps and the increase in power, avoiding lowering the temperature of the chimney gases too much.

WATER TUBE BOILER

Unlike smoke tube boilers, water tube boilers are designed with a minimum of two steel domes, one upper and one lower, joined by curved tubes, within which water is stored, in the dome. upper part the steam is separated. The combustion process takes place in the home where a part of the tubes form a wall to absorb the radiation coming from the combustion process. The combustion gases are taken to the chimney, causing them to be deflected touching the other tubes until maximum heat transfer is achieved.

Due to their design, they withstand high steam pressures and can even superheat the steam, which is why their use is widespread in power applications for the generation of electrical energy.

FOR-IM-024

REVISION DEL FORMATO: C

FECHA DE EFECTIVIDAD DEL FORMATO:10/03/2022

MONOTUBULAR GENERATOR

This steam generator, also called monotubular, uses a continuous coil, fed by a positive displacement pump. The space between the coil tubes is graduated to optimize the characteristics of combustion gas velocity, mixing and heat transfer.

The water flow through the coil in the opposite direction to the combustion gases, behaving as a cross-flow heat exchanger. The outlet of the heating unit is connected to an external container where the steam is separated. With this type of separator, you can obtain up to a steam quality of 99.5%.

Compared to a tube boiler, this steam generator is more compact, lighter in weight and therefore less susceptible to losses due to radiation and convection. Although the tubes in the steam generator are relatively smaller, the velocity of water and steam is higher. The high flow speed allows the suspended solids to be dragged to the vapor separator, where they are separated by gravity, with a high concentration of salts. Because of this only a small amount needs to be purged to keep suspended solids under control. Because the heat transfer process is carried out through a forced convection system, the amount of transfer surface required by Boiler Horse Power is 1.25 ft2. Steam separation is done in an independent container, so the entire surface of the coil is used to transfer heat. For all this, the volume and weight of a steam generator is reduced by up to 75% compared to a smoke tube boiler.

During startup, shutdown, or change in steam demand, response time is affected by thermal mass. Because the thermal mass of a smoke tube boiler is greater, its response is less.

The energy contained in the thermal mass is lost during a shutdown and can be recovered when the unit starts again, and any amount of steam can be called upon. This loss can be significant if the unit is not booted continuously. Due to the spiral design of the heating unit and the small amount of water stored, it is possible to quickly absorb the thermal stresses caused by the heating of the tubes, generating steam in just 5 minutes after starting completely cold.

LOSSES BY RADIATION AND CONVECTION:

A hot boiler, due to its high temperature, radiates heat to the outside because it is at a lower temperature. This heat loss increases when we have air circulating around it at high speed. This loss is a direct function of the outside temperature and the exposed surface. The best way to reduce this loss is by insulating the outside and avoiding having the boiler outdoors. The way to measure this loss is by taking the outside temperature of the boiler body and measuring the exposed surface. These losses are constant and do not depend on the steam generation capacity.

LOSSES DUE TO WASTE OF HOT GASES IN THE CHIMNEY:

Fuels require oxygen to achieve the combustion process, this oxygen is obtained from ambient air, unfortunately air only contains 21% oxygen, the remaining 79% is nitrogen. Nitrogen is a gas that does not intervene in the combustion process, so it is only heated and discharged into the atmosphere at high temperature through the chimney. Depending on the fuel, we use 13 to 20 kg. of air per kg. of fuel, which implies that 75% of the gases escaping through the chimney is hot nitrogen. The combustion process is not a natural process, so to guarantee that all the fuel is burned, it is necessary to handle excess air, this depends on the efficiency of the burner. The way to calculate this loss is by measuring the temperature of the stack gases and analyzing the amount of oxygen and carbon monoxide in the stack gases. These parameters are a direct relationship of the losses due to chimney gases.

FOR-IM-024

REVISION DEL FORMATO: C

FECHA DE EFECTIVIDAD DEL FORMATO:10/03/2022

LOSSES DUE TO HIGH TEMPERATURE CONDENSATE PURGE

The water used to feed the boilers contains dissolved salts that are concentrated because the steam generation process only evaporates water, due to this we must throw away a certain amount of water concentrated with salts at high pressure and temperature, therefore that we lose heat invested in bringing the water to those conditions, this loss is a direct relationship of the amount of water that must be thrown away.

MOISTURE LOSSES IN THE GENERATED STEAM

The presence of humidity in the steam is due to the dragging of liquid in the generation of steam, this water does not intervene in the heating processes, so it is discarded as condensate, losing the latent heat it has.

The acquisition of a boiler is normally done by calculating the maximum demand of our steam-consuming equipment and increasing it by 30 to 50% more due to future expansions, so that finally the boiler operates at 30 to 60% of its design capacity. Conventional boilers at low operating loads decrease their design efficiency.

The capacity of a boiler is normally given with feed water at 100°C and one atmosphere of pressure, but the water we feed to our boilers is normally at room temperature, so we need to use part of the steam generated to preheat the boiler water. feeding from 20°C to 100°C, lowering the real efficiency of the boiler.

CALCULATION OF CAPACITY LOSS DUE TO PREHEATING

Assuming steam is generated at 10 kg/cm² DATA: T = 185°C

Enthalpy of the saturated liquid........................hf = 185.6 kcal/kg. Enthalpy of vaporization ..................................hfg = 478.4 kcal/kg. Enthalpy of saturated vapor.............................hg = 663 kcal/kg.

HEAT REQUIRED TO CARRY A BHP FROM 20°C TO 100°C

Q = 15.6 kg/h x 1 kcal/kg°C x 80°C = 1,248 kcal/h

HEAT REQUIRED BY BHP FROM 100°C TO 185°C AS STEAM

Q = 15.6 kg/h x 1 kcal/kg°C x 85°C + 15.6 kg/h X 477 kcal/kg

Q = 8,767 kcal/h

CAPACITY LOSS = 1,248 Kcal/h / 8,767 Kcal/h = 14.2 %

The monotubular forced convection Steam Generators by design recover 25% in humidity of the amount of steam that is being generated in the separator, this condensate is recovered to the condensate tank acting as a preheating system.

RECOVERED HEAT

QR= 15.6 Kg/h x 0.25 x 186 kcal/kg = 725 kcal/h

HEAT NECESSARY BY BHP TO TAKE FROM 20°C TO 100°C

Q = 1,248 Kcal/h - 725 kcal/h = 523 kcal/h

CAPACITY LOSS = 523 Kcal/h / 8,767 Kcal/h = 6.0 %

FOR-IM-024

REVISION DEL FORMATO: C

FECHA DE EFECTIVIDAD DEL FORMATO:10/03/2022