There is some possibility that humanity could ruin large parts of the environment through concentration on individual gain at societal cost. This is known as
a. the tragedy of the commons.
b. freedom.
c. the free lunch.
d. the outcome of economic progress.
e. sustainable development.
A
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Water is heated by hot air in a heat exchanger. The flow rate of the water is 12 kg/s and that of the air is 2 kg/s. The water enters at 40°C and the air enters at 460°C. The overall heat transfer coefficient of the heat exchanger is 275 W/(m2 K) based on a surface area of 14 m2. Determine the effectiveness of the heat exchanger if it is (a) parallel-flow type or(b) a cross-flow type (both fluids unmixed). Then calculate the heat transfer rate for the two types of heat exchangers described and the outlet temperatures of the hot and cold fluids for the conditions given.
GIVEN
• Water heated by air in a heat exchanger
• Water flow rate m w= 12 kg/s
• Air flow rate m a= 2 kg/s
• Inlet temperatures
? Water (Tw,in) = 40°C
? Air (Ta,in) = 460°C
• Overall heat transfer coefficient (U) = 275 W/(m2 K)
• Transfer area (A) = 14 m2
FIND
(a) The effectiveness (e)
(b) The heat transfer rate (q)
(c) The outlet temperature (Tw,out, Ta,out) for: 1. parallel-flow 2. cross-flow
ASSUMPTIONS
• Steady state
SKETCH
PROPERTIES AND CONSTANTS
the specific heat of dry air (cpa) = 1059 J/(kg K) at 400°C
the specific heat of water (cpw) = 4178 J/(kg K) at 50°C
Resistance and Resistivity: The length of a certain wire is doubled while its radius is kept constant. What is the new resistance of this wire?
A. It stays the same. B. It is 2 times as large. C. It is 3 times as large. D. It is 4 times as large. E. It is ½ as large.
In a RLC circuit, a second capacitor is connected in parallel with the capacitor previously in the circuit. What is the effect of this change on the impedance of the circuit?
A) It increases. B) It decreases. C) It does not change. D) It increases for frequencies below resonance and decreases for frequencies above resonance. E) It decreases for frequencies below resonance and increases for frequencies above resonance.
The commutator in a direct-current motor:
a. starts the loop rotating. b. holds the loop steady. c. allows the motor to run on alternating current from a standard wall socket. d. keeps the current flowing in the same direction through the coil. e. reverses the direction of electric current through the coil.