Question

10. A boiler system is being designed for a building expected to require heating for approximately 3,500 hours per year. The LDC is indicated below (left). The total annual heating load is about 280,000 kWh. The load will be met by three identical boilers, with each boiler having the following characteristics: Heating Fuel: natural gas (heat content may be estimated to be = 10.33 kWh/m?). • Burner Type: on/off • Rated Output = 100 kW • Rated Efficiency: NA= 82% The boiler operating sequence will be as follows: Boiler-1 fires first and operates to meet whatever load it can, up to its capacity. Boiler 2 fires next and operates to meet whatever load it can (above that met by Boiler-1), up to its capacity. Boiler-3 is then added sequence in a similar way to that of Boiler-2. The average heat output of each boiler will be adjusted by cycling its burner on and off. The hourly part-load performance of each boiler can be estimated using the adjustment curve below (right). Load Duration Curve Boiler Performance Adjustment Curve 300 250 100% 200 90% Heating Load [kW] PL 80% 150 NFL 100 - 70% 50 60% 0 50% 20% 40% 0% 60% 80% 0 100% 3000 1000 2000 Hour Number PLR Based on the information provided above, make reasonable estimates of the following: annual heating load met by Boiler-1 ii) average efficiency of the three boilers when operating on the coldest day of the year. i)

Answer 1

Soln.

Information from question:

Heating requirement is for 3500 hrs per year.

Total annual heating load = 280000 kWh

Rated output of each boiler = 100 kW

Rated efficiency of boiler at full load = 82%

i) annual heating load met by boiler-1:

Considering 82% efficiency, each boiler can cater 82 kW.

Based on load distribution curve, boiler-1 will be on for whole period of requirement of 3500 hours. Except for 500 hours, boiler-1 will will operate at full load for rest 3000 hours. For 500 hours average load will be 50%. Hence, annual heating load met by boiler-1 will be = 3000*82 + 500*41 = 266500 kWh.

ii) average efficiency of the 3 boilers on the coldest day:

As per load distribution curve, heating requirement on coldest day is 275 kW. All the 3 boilers put together can cater heating requirement of 100*0.82*3 = 246 kW..

Hence, all the 3 boilers have to run at full load on coldest day. So, they will operate at full load efficiency of 82%. So average efficiency of 3 boilers will be 82% on coldest day.