A counter flow heat exchanger is to heat air entering at $${400^ \circ }C$$ with a flow rate of $$6$$ $$kg/s$$ by the exhaust gas entering at $${800^ \circ }C$$ with a flow rate of $$4$$ $$kg/s$$. The overall heat transfer coefficient is $${100^ \circ }C$$ $$W/\left( {{m^2}K} \right)$$ and the outlet temperature of the air is $${551.5^ \circ }C.$$ Specific heat at constant pressure for both air and the exhaust gas can be taken as $$1100$$ $$J/(kgK).$$ Calculate the heat transfer area needed and the number of transfer units.

If the demand for an item is doubled and the ordering cost halved, the economic order quantity

Consider the following data for a product:
Demand $$=1000$$ units/year
Order cost $$=$$ Rs.$$40$$/order
Holding cost $$ = 10\% $$ of the unit cost / unit-year
Unit cost $$=$$ Rs.$$500$$

(a) What is the economic order quantity?
(b) Under the $$EOQ$$ what is the number of annual orders?
(c) With a policy of ordering every month what would be the total annual cost as a percentage of the cost at $$EOQ$$?

In $$PERT,$$ the distribution of activity times is assumed to be