A parent pushes a small child on a swing so that the child moves rapidly away while the parent remains at rest. How does the magnitude of the force that the child exerts on the parent compare to the magnitude of the force that the parent exerts on the child?
A. The force on the child is larger in magnitude.
B. The force on the parent is larger in magnitude.
C. These forces have equal magnitudes.
D.The child exerts zero force on the parent.
E.There is no way to tell.
C. These forces have equal magnitudes.
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It is proposed to preheat the water for a boiler with flue gases from the boiler stack. The flue gases are available at the rate of 0.25 kg/s at 150°C, with a specific heat of 1000 J/(kg K). The water entering the exchanger at 15°C at the rate of 0.05 kg/s is to be heated at 90°C. The heat exchanger is to be of the reversed current type with one shell pass and 4 tube passes. The water flows inside the tubes which are made of copper (2.5 cm-ID, 3.0 cm-OD). The heat transfer coefficient at the gas side is 115 W/(m2 K), while the heat transfer coefficient on the water side is 1150 W/(m2 K). A scale on the water side offers an additional thermal resistance of 0.002 (m2 K)/W. (a) Determine the overall heat transfer coefficient based on the outer tube diameter. (b) Determine the appropriate
mean temperature difference for the heat exchanger. (c) Estimate the required tube length. (d) What would be the outlet temperature and the effectiveness if the water flow rate is doubled, giving a heat transfer coefficient of 1820 W/(m2 K)?
GIVEN
• Reverse current heat exchanger - 1 shell pass , 4 tube passes
• Water in tubes, flue gases in shell
• Copper tubes
? Inside diameter (Di) = 2.5 cm = 0.025 m
? Outside diameter (Do) = 3.0 cm = 0.03 m
• Specific heat of gases (cpg) = 1000 J/(kg K)
• Gas inlet temperature
? Tg,in = 150°C
• Water temperatures
? Tw,in = 15°C
? Tw,out = 90°C
• Gas flow rate ( m g) = 0.25 kg/s
• Water flow rate ( m w)= 0.05 kg/s
• Tubes are copper
• Gas side heat transfer coefficient ( )ch = 115 W/(m2 K)
• Water side heat transfer coefficient ( )ih = 1150 W/(m2 K)
• Scaling resistance on the water side (Ri) = 0.002 (m2 K)/W
FIND
(a) The overall heat transfer coefficient (Uo) based on the outside tube diameter
(b) The appropriate mean temperature difference (?Tmean)
(c) The required tube length (L)
(d) The outlet temperature and effectiveness if the water flow rate were doubled, making ih = 1820 W/(m2 K)
SKETCH
PROPERTIES AND CONSTANTS
the specific heat of water at the average temperature of 52.5°C (cpw) =4179 J/kg K
the thermal conductivity of copper (k) = 392 W/(m K) at 127°C
A chicken is running in a circular path with an angular speed of 1.52 rad/s. How long does it take the chicken to complete one revolution?
A) 4.13 s B) 2.07 s C) 118 s D) 4.77 s E) 8.26 s
What type of particle is a proton? (There may be more than one correct choice.)
A) meson B) baryon C) nucleon D) lepton E) hadron
Warm air has a higher capacity for water vapor, because in warm air
A) fast-moving water-vapor molecules are too energetic to stick together. B) molecules expand. C) water-vapor molecules condense. D) there are more water vapor molecules than in cool air.