Comparative calculation of consumption of diesel vs LPG in a steam gen

[pc1968]

i want to know the per hour consumption of a) Diesel, b) LPG(Butane 90% + Propane 10%)in liquid state
in a steam generating bolier with following specs
1. 4000 kg water evaporation/hr
2. Boiler efficiency 88%

 

[TBP]

I think you're in the wrong forum. Try "Boilers & Pressure Vessels".

Anyway, you need to determine your feedwater temperature. This will allow you to look-up the heat value per unit, then subtract that from the heat value of the steam leaving the boiler. This will give you net heat added by the boiler. Then multiply by the mass flow, then divide this by the percent boiler efficiency. This will tell you how much heat input the burner must deliver. I seriously doubt that you'll hit 88% boiler efficiency. A safe value is 80%.

I work in Imperial, so I've converted the units.

Most steam boilers will add about 1,000 BTU/lb of water to make steam.

4,000 kg X 2.2 = 8,800 lbs/hr.

8,800 X 1,000 = 8,800,000 BTU/hr output.

8,800,000 / 0.8 = 11,000,000 BTU/hr fuel input.

So, at 138,5000 BTU/US gallon for #2 oil, you'll require just under 80 gallons per hour.

I don't have the heat value for LPG in front of me, but it should be easy to look-up.

 

[TBP]

Oops - got an extra "zero" in the heating value for #2 oil. Should read 138,500.

 

[25362]

The gross heating values (GHV) at 298 K and 1 ata, would be:

1. butane, 49.5 MJ/kg gas
2. propane, 50.3 MJ/kg gas
3. #2 light oil, 38.2 MJ/L

As TBP says it would be difficult to achieve a level of 88% efficiency. For that to happen, and assuming no heat losses, the flue gases exit temperatures would have to be below, say, 450 K.

Since you didn't state the cold water and saturated steam temperatures, and assuming the heat needed to heat and boil water at about 2500 kJ/kg, 4000 kg/h steam would need 10,000 MJ/h, which divided by the efficiency would give the heat to be liberated in combustion and the quantities of fuel needed.

 

[quark]

GHV and GCV are applicable to condensing type boilers, otherwise go with NCV(LHV).

 

[25362]

The high (or gross) heating value [HHV] of the indicated fuels is about 6.7% larger than their low heating values [LHV].

As I was given to understand, HHV usage is preferred in the USA for all types of boilers, not so in Europe.

As quark says, LHV's when used on "condensing boilers" may result in efficiencies surpassing 100%, since the LHV's are based on the flue combustion gases leaving at 150^oC, while the HHV's are estimated by bringing the combustion gases to a pre-selected reference state, say 25^oC and 1 ata, thereby "exploiting" the latent heat released by condensation of the water vapor present in the combustion gases.