Solid fuel boiler issue 1

[bluetobits]

Coming from a maintenance background I have little knowledge of boiler design and theory. I have been tasked with resolving a performance issue with a solid fuel boiler. Conflicting information has been given as to why the boiler under performs but the problem does seem to point to an undersized heat exchanger.

To give some detail, a boiler manufacturer makes a range of solid fuel MPHW boilers in a range of outputs from 250kw to 1MW. The boilers are constructed in a modular way (lower furnace section, upper heat exchanger) to allow for, let’s say, the 250kW to 600kW range to employ the same furnace section but use different heat exchanger sections. Posing a somewhat hypothetical question, if in assembling a 500kW boiler a 350kW heat exchanger was inadvertently fitted;

a) would that boiler consistently under perform?
b) could the error be compensated for by increasing the flow rate through the heat exchanger from say 5 litres per second to 10 litres per second?
c) could the error be compensated for by increasing fuel rate and furnace temperatures?

Many thanks in advance.

 

[EnergyMix]

a) possibly
b) hard to say without more information. Often increasing flow rate doesn't help and may actually hurt in terms of heat transfer.
c) again hard to say without more information. However, this might be more likely to be beneficial, if it is possible to do within limits.

If you're the end customer, I take it that it is not possible to replace your hypothetically undersized heat exchanger with a correctly sized one? It seems like this would be the best solution (that's quite a size difference.)

If you're the manufacturer, I would think the customer who got the incorrect unit would be justified in demanding a replacement.

 

[bluetobits]

I am writing as an end user. I posed the question hypothetically as the project involving the installation of two such boilers is still a live one and has still not achieved completion after 11 months.

Both boilers consistently underperformed from the point of commissioning onwards. The installers increased the fuelling rate and consequently the furnace temperature but without any great success. However, it did produce a number of negative effects – the excessively high furnace temperatures resulted in silica slag formations and, eventually, the collapsing of the furnace refractory lining. After rebuilding the refractory, the next change made by the installers (on the advice of the manufacturer/supplier) was the addition of flue gas recirculation and doubling of the boiler flow rate from 5 litres per second to 10 litres per second. Whilst the boiler output is now closer to the desired 500kW, output is not consistent, fuel usage is high and too much ash is being produced. Only this week did we discover that the heat exchanger is manufactured by a different company to the boiler supplier. The heat exchanger manufacturers have confirmed that they rate the heat exchanger at 350kW. They also said that increasing the furnace temperature to try to achieve 500kW would improve the output but lower the boiler efficiency to around 65%.

I think your comment that we would be justified in demanding replacements is exactly what we now propose to do.

Many thanks.

 

[macmet]

Can you tell us what your fuel is and what the moisture level is? Fuel quality is one of the biggest factors for solid fuel fired boilers. One combustor can not effectively use every kind of solid fuel. Some are more versatile than others, but all have some limitations.

 

[bluetobits]

Sorry macmet for not replying sooner - been away for a few days.

Fuel is wood pellet with a typical moisture level of 7.5%. The boiler controllers vary fuel and air depending on load. At start-up control panel indicates 'phase 2 - boost', at 5c below set point control panel will indicate 'phase 3', once set point is reached panel indicates 'phase 4' and so on to 'phase 6' when set point has been exceeded. In the summer months the boilers were mainly in phase 3 or 4, the boiler ash bins had to be emptied every 10 to 14 days, furnaces were mostly clear of ash and flame pattern was good. Now that we are into the heating season, boilers are mainly in phase 2, ash bins can take as little as 2 days to fill, furnaces have a build-up of ash and the flame pattern is more ragged.

In reply to the point made by EnergyMix above about increase of flow rate affecting heat transfer - the boilers were struggling to meet the flow set point of 120c. By changing the setting on the boiler shunt pump invertors from 100% to 75% the boiler flow temperature quickly increased by 2c and boilers moved from phase 2 to phase 3. However, the energy meters on both boilers showed a reduction of around 80kW but we have seen little change in main system flow and return temperatures.

 

[TomDOT]

There's only so much you can do with an undersized heat exchanger. I think you are justified in requiring the manufacturer to fit the proper heat exchanger.

 

[rmw]

The furnace will fit one heat exchanger and the rest will be over or undersized. Sorry to hear about yours. Energy conversion in any boiler, but especially a solid fuel fired boiler is a function of heat release volume and if this is a grate fired furnace, heat release area on the grates. So the furnace you have is sized to release "X" amount of heat and if you have the corresponding heat exchanger section, you have a match. If not, then you will have performance issues, either too much furnace or too little. Too much furnace is probably not as bad as too little, but I suspect that you have too little based on your comments.

rmw