Boiler investment: Selection matters
13 March 2017
If you’re currently, or will be in the future, looking to invest in an industrial boiler then these tips from Stephen Fleming, sales project engineer at Byworth Boilers, may help your decision-making process
Not many purchasing decisions for your factory have larger or longer-term energy consequences than that of purchasing a new or replacement boiler. However, purchasing a boiler is frequently undertaken without considering all of the options and alternatives.
What is the operating efficiency and how well does the new system fit my application?
If your boiler is maintained correctly, its life span can range over decades. Therefore, significant considerations would be the operating efficiency over the whole load cycle and how well the end product fits your specific application –
Tip: Don’t concentrate on the initial cost of the unit but consider the entire ROI
Will my new boiler comply with the MCPD?
Directive 2015/2193 'on the limitation of emissions of certain pollutants into the air from medium combustion plants' was published in November 2015 and will pass into UK Law on the 19th December 2017. Referred to as The Medium Combustion Plant Directive or MCPD, it plugs the gap between the Industrial Emissions Directive 2010/75/EU (the IED) and the Eco Design' Directive 2009/125/EC. It will apply to all operators of combustion plant rated between 1 and 50MW thermal input.
The responsibility for compliance lies with the user of the plant, not the supplier and compliance can sometimes only be achieved by control of the fuel or by abatement of the flue gases exiting the plant.
Tip: Involve potential suppliers early in the project to ensure that the plant does indeed meet the ELV’s applicable to you.
Start with a detailed evaluation
Evaluate your steam pressure and volume requirements remembering lower pressure boiler are slightly more efficient while high pressure is better for distribution efficiency. Analyse your daily, weekly and annual cycle of boiler loads particularly at low load as boilers that switch on and off frequently are considerably less efficient
Tip: Ensure the steam quality you will be getting will give you the utmost efficiency. Dry, saturated steam will contain no water droplets whereas 99% saturated steam will give you 1% water droplets. The water will transfer no heat and instead acts to reduce the amount of steam the pipe can carry, reducing the overall plant efficiency.
Have requirements changed?
It is not uncommon to find that over time you experience a change in the steam usage trends. You may have less demand for the end product, therefore, less steam demand or you may find your existing boiler operates at the wrong temperature and pressure for your heating process requirements.
Tip: It can often be more valuable to have multiple boiler installations instead of a single larger unit because it would allow redundancy, thus greater plant reliability.
Have you considered a waste heat or composite boiler?
With certain applications, it makes sense to install a waste heat boiler. Low-temperature applications, for example, could benefit from recovering some of the wasted heat from the exhaust and ‘reusing’ it elsewhere.
Tip: Consider investing a bit more money and buying a composite boiler which blends waste and fired-boilers into a single unit capable of satisfying the entire site demand.
What about economisers?
Economisers recover heat from the flue gases after they have left the boiler. This is frequently used to pre-heat the boiler feedwater and offers fuel savings of between 4 and 6%.
Tip: If you have a large demand for heated process water, then an economiser could be used to supplement the heating of that instead, offering savings of up to 15%.
Selecting the right burner with a new boiler is critical, especially with applications that require a very low NOx compliance and high turndown ratio to cater for low load conditions. If your minimum site load is 25% of the maximum, you only need a 4:1 turn down on the burner and anything higher will offer no further benefit.
Tip: Oxygen trim controls can be useful where you are burning a fuel of varying quality but the cost may outweigh the benefit for someone burning natural gas or LPG – specifying digital controls should give you a good enough level of repeatability on their own.
How many boilers?
When evaluating your boiler options, consider whether backup equipment would benefit your business:
• Having a spare boiler is desirable where the steam is mission critical
• Seasonal changes could mean it is more efficient to have several boilers so you can turn one off during periods of lower demand
Fuel, emissions, efficiency and technology are important considerations. A general rule of thumb with regards to fuel costs is that they typically account for 10% of an organisations total operating expenditure. So it makes sense to place fuel costs as a ‘top of the list’ consideration.
Natural gas burns the cleanest, leaving the least residue out of all the fossil fuels. This means that less maintenance is required.
LPG should be preferred over oil where natural gas is not available, due to its price stability and better environmental credentials.
Wood or heavy fuel can be significantly cheaper to run than any other fuel; HFO and wood-fired boilers need careful monitoring and regular cleaning in order to avoid costly downtime caused by losses in efficiency from soot build up.
Dual fuel burners are advantageous when the primary fuel supply is shut down due to maintenance and service or where the supply is inconsistent
or it is necessary to start a plant on one fuel before changing to another (as is often the case with biogas for instance).
Combustion efficiency refers to how effective the burner is at burning the fuel completely, while thermal efficiency refers to the boiler’s ability to transfer heat.
Fuel to steam efficiency refers to the actual true boiler efficiency. Fuel to steam efficiency can be calculated by considering the heat balance, i.e. the stack temperature and losses, radiation, excess air level and convection losses. However, when making these calculations, it does not consider variables such as the cycling and purge losses and operating levels, among others. The figures represent a moment in time and not a prolonged period of operation such as a production cycle, or a day of operation.
Stack losses refers to the temperature of the combustion gases that leave the boiler and the CO2 level in those gases (sometimes the O2 level). Generally speaking, the lower the stack temperature and the higher the CO2 (lower O2), the more effective the heat transfer was and less heat is being lost up the stack.
Radiation and convection losses are a function of the surface area of the boiler, the quality of insulation, and the working temperature. They are usually expressed by the boiler maker as a percentage of the maximum firing rate and because they are fairly fixed will be a higher percentage at lower firing rates
Generally speaking, boilers of the same basic design that incorporate a larger amount of heating surface are likely to be more efficient than those with a lower amount of heating surface. Alternative designs can seek to increase the amount of turbulence in the tubes (such as through the use of retarders or Byworth’s own XID tubing) to get increased heat transfer from the same amount of heating surface.
The number of passes in a boiler is NOT particularly important to heat transfer, the use of 3 or even 4 passes is about making an efficient use of space (making a shorter fatter boiler rather than a really long skinny one) and NOT specifically about efficiency. It is quite possible to buy reverse flame boilers that are more efficient than some 3-pass boilers.
The performance of the boiler is based on the capability of the boiler, burner and the control systems to match the steam load accurately so that they all work in unison. The emissions, efficiency, turndown, excess air and capacity all need to be reviewed together when identifying the performance. Whether the burner is able to operate at optimum excess air levels, will in part, affect the efficiency of the boiler. Burners that are designed insufficiently tend to produce soot or CO at these excess air levels; they can also foul the boiler and considerably reduce efficiency. Likewise, the control systems on your boiler can enhance efficiency and reduce a company’s running costs.
Research your decision thoroughly and don’t be afraid to ask the experts any questions that will help broaden your understanding.