Boiler Engleza

8/3/2019 Boiler Engleza

1/9

Boilers

Introduction

Boilers provide hot water or steam for industrial processes, for heating spaces and for hotwater. A wide range of types and sizes of boilers meet the varied needs of industrial and otherfacilities.

Most boilers have three main parts: a burner that converts the fuel to heat, a heat exchangerthat transfers the heat to steam or water, and a boiler vessel. A chimney stack draws off thecombustion by-products (flue gases), and the hot water or steam flows through a distributionsystem to its end uses. Figure 1 shows a schematic of the combustion process.

Natural gas and oil are the most common fuels used in boilers. Propane, electricity, coal andbiomass are also used. Electric boilers are generally found where combustion boiler firehazards pose safety risks and where it is important to reduce air pollution.

Figure 1: Boiler Configuration

Boiler life is approximately 25 years, so it is essential to consider both long-term fuel and

maintenance costs along with initial capital costs when buying or retrofitting. Fuel costs for anew high-efficiency model can be up to 40 percent lower than for a conventional one. Over25 years, this can be a great saving. In many cases, simply retrofitting an existing boiler canimprove efficiency by 20 percent or more.

Measuring Efficiency


2/9

Boilers with heat outputs of 300 000 Btu/hr to 2 500 000 Btu/hr are rated by ThermalEfficiency.

We are interested in the steady-state Thermal Efficiency i.e., after the flue gas temperaturehas warmed up and reached equilibrium. Many combustion systems do not operate in steady-state equilibrium: they cycle up and down, taking a significant time to reach equilibrium, if atall. Nearly all transient systems are significantly less efficient than ones that operate in thesteady state.

Thermal Efficiency is a steady-state measure only and does not include the effects of heatloss caused by on-off cycling or transient operation. This measure is different from the

Annual Fuel Utilization Efficiency (AFUE) rating, which measures the average efficiency ofa system over a year. The AFUE rating takes into account the cyclic on/off operation and

associated energy losses of the heating unit as it responds to changes in the load, which inturn is affected by changes in weather and occupant controls.

Standards and Regulation

In Canada, gas and oil boilers under 300 000 Btu/hr are regulated under the EnergyEfficiency Act. No similar Canadian standards exist for boilers over 300 000 Btu/hr. In theUnited States, boilers over 300 000 Btu/hr are subject to standards under the National Energy

Policy Act. Under this Act, large gas-fired boilers must have a steady-state ThermalEfficiency of at least 80 percent, and large oil-fired boilers must have a steady-state Thermal

Efficiency of at least 83 percent.

Heat Losses in Combustion

Losses result from:

y Dry flue gas loss: These heat losses are due to the temperature of the flue gases andare a function of excess air (which wastes energy by carrying heat up the stack) aswell as the flue and combustion air temperatures.

y Hydrogen loss: This is caused by the latent heat loss due to the boiling of watervapour by the combustion of hydrogen in the fuel.

y Moisture-in-fuel loss: This is latent heat loss due to boiling of water in the fuel.Significant levels of water are found naturally in biomass, garbage and lignite.

y Casing loss: This is the radiant heat loss from the furnace casing.y Incomplete combustion.y O

ther losses: moisture in the air, air or flue gas leaks, or heat in the ash.

How Can the Combustion System Be Improved?

Flue gases are the single most important cause of energy loss. As much as 18 to 22 percent ofavailable energy goes up the chimney. Heat radiation and convection from boiler walls raiseheat loss another 1 to 4 percent.

There are four main ways of reducing flue gas energy losses:


3/9

y by improving the efficiency of converting the fuel to heat (improved combustionsystem efficiency)

y by requiring less air for satisfactory combustiony by ensuring that the boiler casing is tight, so that there is no air/heat entering or

leaving the casing through leaksy by improving the efficiency of transferring the heat to the steam or hot water

(improved heat exchanger efficiency)

Operating practices such asblowdown cause other losses, as do inefficiencies in steam andhot water distribution systems.

Improved Combustion System Efficiency

New boilers generally incorporate several new technologies. These same technologies canalso be applied when retrofitting older boilers. The most important new technologies are asfollows.

y Fan-assisted combustion: Originally, boilers and furnaces relied on natural draft,i.e., the buoyancy of the hot air in the flue, to draw the air into the firebox and up the

flue. A draft hood limited condensation in the flue and ensured that the burner andflame were isolated from outside air pressure fluctuations by adding "dilution" air to

the flue. At the same time, the dilution air lowered the vapour pressure at which theflue gases would condense and cause damage to the flue. However, efficiency was

lost because of the loss of heated interior air up the chimney.

Newer-technology fan-assisted burners eliminate the draft hood and are better atmixing fuel and air. As a result, excess air is reduced. Fan-assisted burners alsodiminish losses by reducing the amount of hot air going up the chimney.

The fan also improves the heat transfer inside the boiler by improving combustion gasflow through the heat exchanger.

Two types of fan-assisted systems are available: a forced-draft system uses a fan toblow the fuel and air mixture into the boiler; an induced-draft system has the fanlocated at the outlet end of the heat exchanger passages.

y Motorized dampers: Motorized dampers stop heat from escaping up the chimney byautomatically closing the flue when the boiler is idle.

y Electric ignition: Older gas boilers have pilot flames that remain lit whether theboiler is firing or idle. Electric ignitions or other intermittent ignition deviceseliminate this waste of fuel. A control circuit energizes the ignitor and, if the burnerdoes not fire on the first try, the ignitor re-fires until the burner is lit.

y Sealed combustion: Sealed combustion controls the combustion process morecarefully by preventing boilers from inducing infiltration into the building. In a sealedcombustion boiler, air is drawn directly from outside through a sealed venting system,ensuring that heated indoor air is not mixed with the outside air during the combustion


4/9

process.

y Pulse combustion: Instead of a continuous flame, pulse systems create discrete, rapidcombustion pulses in a sealed chamber. This intensely turbulent process results in a

highly efficient heat transfer to the heat exchanger and allows for flue gascondensation in condensing boilers.

Condensing Boilers

High-efficiency condensing boilers feature additional advanced heat exchanger designs andmaterials that extract more heat from the flue gases before they are exhausted. Thetemperature of the flue gases is reduced to the point where the water vapour produced duringcombustion condenses back into liquid form, releasing the latent heat, which improves energyefficiency. With some 12 percent of the energy of a gas-fired boiler tied up as latent heat, thisrepresents a significant energy-savings potential. A side effect is that this condensate isusually acidic and has to be piped to a drain.

Modern condensing boilers have energy efficiencies of 90 to 96 percent. New conventionalnon-condensing models have energy efficiencies of only 70 to 85 percent. Many boilers over20 years old typically operate at only 60 to 70 percent efficiency, making them goodcandidates for upgrading or replacement. A number of natural-gas-fired condensing boilersare available, but very few oil-burning ones are on the market.

An important point is that for the water vapour in the flue gases to condense, the temperatureof the flue gas must be reduced to below the water dew point of the flue gas. For this to

occur, the return water temperature to the boiler proper must be below 60C. If there are noheat exchange surfaces at the back of the boiler below this dewpoint, condensing will not

occur, and this energy opportunity will be lost, even if the boiler is a condensing boiler.

In retrofit applications where you wish to retain your existing boiler, boiler efficiency can beimproved by adding an economizer, which is a heat exchanger that utilizes the waste heatfrom the flue gas to preheat the boiler feedwater. A condensing economizer improves theeffectiveness of reclaiming flue gas heat by cooling the flue gas below the dewpoint. Thecondensing economizer thus recovers both the sensible heat from the flue gas and the latentheat from the moisture that condenses. You do have to ensure, however, that the condensatedoes not enter the boiler, as the condensate is highly corrosive.

Oil condensing boilers are more expensive, and it is much harder for them to actually achievecondensing because:

y Sulphur in the oil turns the condensed water into sulphuric acid that must beneutralized. The heat exchanger must be of very high quality to prevent corrosion bythe acid.

y Oil has 50 percent less energy tied up in latent heat, compared with natural gas.y The dew point for oil is low 47C compared with 60C for natural gas, making the

water vapour in the flue gas very difficult to condense.


5/9

How Much Will I Save?

The cost of a high-efficiency boiler can be up to twice that of a conventional boiler. On theother hand, high-efficiency models often pay for themselves in less than five years when thehigh price of fuel and the long boiler life (25 years on average) are factored in.

Table 1 shows how much can be saved by buying a high-efficiency condensing boiler insteadof a non-condensing boiler.

Table 1. Simplified Payback Analysis of New Gas-Fired Hot Water Boilers: A Standard

Boiler Compared with a Condensing Boiler

Non-

Condensing

(Mid-efficiency)

Condensing

(High efficiency)

Capacity 2 100 000 Btu/hr 2 100 000 Btu/hr

Rated Thermal Efficiency 83 percent 95 percentOperating Efficiency*(see note below)

79 percent 91 percent

Initial Cost $25,000 $50,000

Incremental Cost $25,000

Fuel Price** $9.14/GJ $9.14/GJ

Annual Full-Load Heating Hours*** 2080 hrs 2080 hrs

Annual Cost of Fuel $53,283 $46,256

Annual Savings $7,027

Payback Period 3.6 years

*Thermal Efficiency has been lowered by 4 percent to account for reheating after being off.**The price is from 2001, Source: Natural Resources Canada,Energy Use Data Handbook,1990 and 1995 to 2001.***Assumed: a two-shift cycle, 16 hrs/day, 5 days/week, at 50 percent load factor.

Note: This assumes an installation where condensing can occur most of the time.

This is a highly simplified scenario. Actual savings will depend on incoming watertemperature, on-off duty cycle, maintenance, controls and other variables.

Table 2 shows the potential payback period for replacing an older, inefficient boiler with a

new, efficient one.

Table 2. Simplified Payback Analysis: An Old Non-Condensing Boiler Compared with a

New Condensing Boiler

Old Non-

Condensing

(Low-efficiency)

Condensing

(High efficiency)


6/9

Capacity 2 100 000 Btu/hr 2 100 000 Btu/hr

Rated Thermal Efficiency 74 percent 95 percent

Operating Efficiency* 70 percent 91 percent

Initial Cost NA $50,000

Incremental Cost $50,000

Fuel Price** $9.14/GJ $9.14/GJ

Annual Full-Load Heating Hours*** 2080 hrs 2080 hrs

Annual Cost of Fuel $60,133 $46,256

Annual Savings $13,877

Payback Period 3.6 years

*This considers cyclic, off-cyle and heat air losses.**The price is from 2001, Source: Natural Resources Canada,Energy Use Data Handbook,1990 and 1995 to 2001.***Assumed: a two-shift cycle, 16 hrs/day, 5 days/week, at 50 percent load factor.

Note: This assumes an installation where condensing can occur most of the time.

This is a highly simplified scenario. Actual savings will depend on incoming watertemperature, on-off duty cycle, maintenance, controls and other variables.

Purchasing Energy-Efficient Boilers

Information and databases to help in selecting smaller boilers (under 300 000 Btu/hr andgenerally designed for residential use) can be found on the ENERGY STAR

and the

American Council for an Energy Efficient Economy Web sites. No comparable databaseexists for larger boilers.

Financial and other consequences of buying a new boiler will be felt for more than 25 years the normal life of a boiler. Several factors should be considered for making a wise choice.

Timing

The best time to purchase a high-efficiency boiler is when new plant is being built, when new

capacity is planned, or when a boiler has failed.

For new systems and capacity changes, the type and size of the boiler are determined in theplanning. Failed boilers must be replaced or repaired quickly. This is why it is always best to

have a boiler management plan in place to ensure that a suitable model is available. A boilermanagement plan can be as simple as having written-out criteria boiler type, size, supplierand so on for both repair and replacement.

Following are some other important points to consider when choosing the right boiler.

Seriously consider condensing boilers.Condensing boilers extract heat from the watervapour as it condenses out of the flue gas. These boilers can be 95 percent energy efficient orhigher. New, non-condensing models are only 70 to 80 percent efficient.


7/9

Where low-temperature hot water is wanted for example, for space heating a condensingboiler is the most efficient. Where high temperatures are wanted and there is no means tocool the return below the flue gas dewpoint, condensing is unlikely, so consider a mid-efficiency boiler.

It is crucial to thoroughly discuss existing and potential needs with suppliers because of the

higher cost of a condensing boiler. Long-term fuel costs and other factors must also beconsidered.

Use a high-modulating boiler with excess-air control. The simplest method of controllingthe flame is on-off firing. The fuel flow is either fully on or fully off. In a modulating boiler,the burner firing rate is matched to the required load. This is similar to an automobileaccelerator controlling speed.

Modulated firing results in fewer heating cycles and lower standby losses than on-off firing.On-off firing can cause short-cycling (frequent firing) and is inefficient. Short cycling occurswhen a boiler quickly satisfies demand and shuts down its burner until more heat is needed.Modulating the firing rate results in fewer heating cycles and lower standby losses.

If the boiler is firing at a 25 percent rate, the fuel will be in the boiler four times as long as itwould at a 100 percent fire rate, so the same amount of fuel has four times as long to transferits heat, resulting in higher efficiency.

Excess-air control improves efficiency by reducing airflow to the burner when less air isneeded at lower firing rates.

Use a number of integrated smaller boilers instead of oversizing. Oversize boilers arecommonly installed to handle peak demand and anticipate possible expansion. This iswasteful since oversize boilers rarely operate at peak load, and their part-load efficiency can

be as much as 20 percent lower.

Several smaller units can be more efficient and economical than a single large one. They canbe staged (or sequenced) to operate at or near peak efficiency if piped and controlledappropriately. An added up-front benefit is lower installation costs since small boilers do notrequire a crane to be installed.

For retrofitting, a multiple boiler approach can improve the seasonal inefficiency of large, oldboilers. A small boiler can supply basic heating, and the large boiler fires only whennecessary to supplement the heating during periods of high heating load.

Consider cogeneration. The most energy-efficient system for supplying both heat and

electricity is a cogeneration system. These systems generate both reliable electricity and heatwhile reducing emissions and saving money. A cogeneration system consists of a gas turbineor reciprocating engine and a heat recovery steam generator, which is a type of boiler. If anold boiler requires major improvements, this may be the time to replace it with a cogenerationsystem. To learn more about cogeneration, visit the Canadian Advanced Buildings Web siteor the U.S. Department of Energy Web site.


8/9

Assess heat cascading. When steam heat is needed for one process and hot water for another,the heat exhausted from one process can be input to the next process. The heat finallyexhausted should be at the lowest temperature that can be economically achieved.

Assess the life-cycle costs, not just the first or up-front cost. Fuel costs are by far themost significant expenditure over the life of a typical boiler. For example, fuel costs would be

more than 50 times the capital cost of the conventional gas-fired boiler in the example givenin the How Much Will I Save? section. Considerable savings can be realized by taking intoaccount all costs over the boiler's entire life cycle.

Rated Thermal Efficiency is a good starting point, but it does not tell the whole story.Thermal Efficiency is a steady-state measurement that applies only to specific operatingconditions. Real-world conditions are rarely the same as test conditions and can have a greatimpact on efficiency. This is especially true for condensing boilers whose efficiency is afunction of the operating temperature.

The Annual Fuel Utilization Efficiency (AFUE) rating may apply. It measures the averageefficiency of a system over a year and takes into account the cyclic on/off operation and

associated energy losses of the heating unit as it responds to changes in the load. This in turnis affected by changes in weather and occupant controls.

Boiler suppliers can help conduct life-cycle cost analyses to meet specific needs.

Operation and Maintenance Tips

A systems approach is the best way to save energy. This means looking at the boiler, thesteam distribution system and the end uses together. Learn more about saving energy insteam and hot water distribution systems.

Following are some common energy-saving tactics.

Control excess combustion air. Controlling excess air is the most important tool foroptimizing boiler efficiency. Too little air results in incomplete combustion, while too muchair wastes energy, as the excess air is heated to the stack temperature.

Reducing the excess air nearly always yields a greater increase in efficiency. This resultsfrom a reduction in the flue gas temperature due to reduced mass flow and consequentimproved heat transfer through the system. Stack temperature and flue gas oxygen (or carbondioxide) concentrations are primary indicators of combustion efficiency.

A number of controls are available for monitoring and optimizing the air-fuel mixture. Theserange from simple, low-cost on-off control to more expensive automatic oxygen trim control.Burner size determines which is the right control. The burner should be adjusted only byqualified personnel, so work with a supplier to correct the air-fuel mixture.

Train personnel. Have only well-trained, qualified personnel run, adjust, inspect andmaintain boiler systems.


9/9

Keep the boiler clean. The fireside of the boiler tubes can accumulate deposits from burningfuel. This fouling can dramatically reduce heat transfer. Boilers that use solid fuels tend tofoul much more than liquid- and gas-fuelled boilers. No. 6 (resid, heavy) oil has a greaterfouling tendency than No. 2 oil. Natural gas boilers have a very low fouling tendency.

The waterside of the boiler tubes can become covered with a mineral deposit, or scale.

Scale causes the tube's temperature to rise, raising the flue gas temperature and reducing theefficiency. Scale buildup can be tested with an automatic sensor while the boiler is runningand can be treated chemically.

Boiler water should be tested daily in small low-pressure boilers and hourly in large high-pressure boilers. A gradual rise in flue gas temperature usually indicates that a deposit isaccumulating on either the fireside or the waterside. If flue gas temperatures are too high,clean the system and adjust the water chemistry and the air-fuel mixture.

Large boilers often have soot blowers to clean fireside tube surfaces while the boiler isoperating. Soot blowing can consume large amounts of energy, so it must be done carefully.Smaller boilers should be opened regularly for inspection and cleaning.

Minimize boiler short-cycling losses. When a boiler is too big, boiler short-cycling lossesmay occur. An oversized boiler will turn on and off more often than a boiler that has been

properly matched to the demand. Every time the boiler turns on, extra energy is required toheat it back up to steady-state. A number of staged (or sequenced) smaller boilers use anautomatic controller to lower costs by efficiently shifting the load between boilers. Piping andcontrolling the multiple boilers correctly is necessary for saving energy.

Minimize wasted blowdown water. Boiler water must be blown down periodically toprevent scale from forming on boiler tubes. This process can be wasteful if too muchblowdown water is used. Automatic blowdown controls measure and respond to boiler water

conductivity and acidity to ensure that only the right amount of blowdown water is used.

For further savings, a blowdown waste heat recovery system that preheats boiler feedwatercan improve the system's efficiency by approximately 1 percent.

Use outdoor reset. Outdoor reset is used for hot-water building heating. In older systems, hotwater arrives in the distribution system at the highest temperature the boiler provides. On-offcontrols regulate building temperature. Outdoor reset varies the temperature of the water inthe distribution system in response to outdoor temperatures. When it is cold outside, the watertemperature rises to match the heat loss from the building. When it is warm outside, there isless heat loss from the building, so the distributed water is cooler. Fuel consumption isreduced.

Conservative estimates put savings at 10 to 15 percent. This can also be an effective way ofincreasing latent heat recovery for high-efficiency condensing boilers.

Improve boiler insulation. A simple and cost-effective way of reducing heat loss throughradiation and convection is by adding insulation directly to the outer walls of the boiler.Removable insulation pads will reduce losses and will not interfere with maintenance.

Boiler Engleza

Documents

Transcript of Boiler Engleza