Boiler Burner Calculation

200psi = 13.8 bar.

From steam tables, h_g = 2790kJ/kg.

Assume inlet temperature = 15degC. h_f = 62.9kJ/kg.

So ΔH = 2790 - 62.9 = 2727.1kJ/kg.

Throughput = 5500kg/h = 1.53kg/sec

So, assuming no losses for the moment, the heat required = 1.53 x 2727.1 = 4166kW.

Divide by, say, 45% efficiency,

So the burner needs to be rated somewhere near 9¹/₄MW, as a first approximation.

Thankyou so much.

You have taken the feed water temperature to be 15 degree C its around 90-95 C secondly is it reasonable to assume 45 percent efficiency 45 percent seems pretty bad. Why do boiler manufacturers on natural gas claim effciency of 80 percent plus?

<...taken the feed water temperature to be 15 degree C its around 90-95 C ...>

If the feed water is 90-95degC, then it isn't coming from the mains. Which means that the heat needed to get it from 15degC to 95degC needs to come from a heat source within the works, i.e., the boiler.

h_f @ 95degC = 398.0kJ/kg (from steam tables)

So, the heat required to do this is (398.0 - 62.9) x 5000 / 3600 = 465kW, or about ¹/₉th of the total capacity of the boiler.

<...is it reasonable to assume 45 percent efficiency; 45 percent seems pretty bad...>

For a first approximation for this sort of thermodynamic cycle to get an order-of-magnitude, well, yes.

<...Why do boiler manufacturers on natural gas claim effciency of 80 percent plus...>

Good question. The flue gases have to be leaving the boiler tubes at or above 200degC, for the boiler to generate steam at 200degC.

CH₄(g) + 2O₂ (g) + 10N₂(g) (all at say 25degC) → CO₂(g) + 2H₂O(g) + 10N₂(g) (somewhere north of 250degC, say).

So there's quite a lot of hot gas coming out, which could go through energy recovery, perhaps, to pre-heat the incoming gases to the burner, thereby raising the combustion temperature and reducing the theoretical size of the boiler for any given duty. Energy recovery is missing from the original post. Reducing the temperature of the flue gases by pre-heating the feed gases will increase the nominal efficiency of the cycle, and thereby give a second approximation for the sizing of the burner.

Thermodynamics textbooks would suggest an overall efficiency of aroung 55% for a well-integrated energy recovery scheme on a major high-pressure boiler and power generation facility. Does this particular boiler fit this category? Do the manufacturers have something extra to offer on efficiency that would sell the boiler/burner combination in question?

In any case, the manufacturer will need to size the boiler and the burner to meet the Client's steam quality and throughput requirements. The easiest way to avoid doing the sizing oneself is to require a demonstration of performance of the package as part of the purchase contract.

Incidentally, it is always worth looking at maximising hot condensate recovery from steam users, and its re-use so as to maximise efficiency. ¹/₉ of the boiler's performance (≈ a megawatt) is not to be sneezed at! By re-using condensate one is reducing the water treatment plant throughput and the treatment costs of the feedwater per unit of steam delivered, so economies of operation could be made to the facilities upstream. Further, one is reducing the purchase volumetric costs of the incoming water, as well as the volumetric disposal costs of the condensate on its passing through the effluent treatment plant and out the door.

What about re-using any rejected condensate as a feed to the works' cooling towers, so as to save further water purchase and disposal costs (the treatment regime will require adjustment)?

Your approach is correct.........but.......two thoughts:1) you better not try to introduce the "feed" water at 15 degC. Feed water needs to be close to steam temperature. So, you should de-aerate and pre-heat the feed water before pumping into the boiler . 2) Manufacturer's average boiler efficiency as stated in another post is 80%; all of this assuming 200 psig saturated steam............

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