Hi.. I'm kinda confused with this one... i hope someone helps me here !A dryer operates for 60 hours/week, 50 weeks/year. It exhausts 200 kW/hr of heat and the value of energy is 6p/kWh. A heat exchanger with a 50% efficiency is put into the exhaust stream to recover a proportion of this heat. The installed cost of the heat exchanger is ?22,500. The heat exchanger pressure drop needs a 2 kW fan to overcome it, and the cost of electricity is 10p/kWh. (I have the final answer - but no luck getting it !! - 10 points to the one who gets it right with good enough explanation for me to understand :)Thanks !
Energy Loss/Yr.: 6p x 200kwh = 1200 x 60 x 50 = 360,000 p/Yr. Exchanger saves: 50% = 180,000p/yr, less fan power costof 10p x ( 50 x 60 x 2) = 60,000 P So 'Payback' is 120,000p/Yr. (less Maint.) How many p/ Pound now?
The dryer exhausts heat at the rate of 200 kW (joules/sec) (not kW/hr). Operated 60 hrs/wk 50 wks/yr it exhausts 200 kW X 60 hr/wk X 50 wk/yr = 600,000 kWh/yr. Value of energy lost = .06?/kWh X 600,000 kWhr/yr = 36,000 ?/yr. With 50% recovery from heat exchanger, value of energy saved = 18,000 ?/yr. If the heat exchanger uses 2 kW, that is 2 X 50 X 60 = 6000 kWh/yr with an annual cost of 6000 kWh X .1?/kWh = 600 ?. Net savings = 18,000 ?/yr - 600 ?/yr = 17,400 ?/yr. Simple payback = 22,500 ? / 17,500 ?/yr = 1.29 years.
