Chapter 5, Problem 5.6DOEP

Using the information in Problem 1.22, estimate the ambient air temperature that could cause frostbite on a calm day on the ski slopes. 1.22 In order to prevent frostbite to skiers on chair lifts, the weather report at most ski areas gives both an air temperature and the wind-chill temperature. The air temperature is measured with a thermometer that is not affected by the wind. However, the rate of heat loss from the skier increases with wind velocity, and the wind-chill temperature is the temperature that would result in the same rate of heat loss in still air as occurs at the measured air temperature with the existing wind. Suppose that the inner temperature of a 3-mm-thick layer of skin with a thermal conductivity of 0.35W/mKis35C and the air temperature is 20C. Under calm ambient conditions the heat transfer coefficient at the outer skin surface is about 20W/m2K (see Table 1.4), but in a 40-mph wind it increases to 75W/m2K. If frostbite occurs when the skin temperature drops to about 10C, do you advise the skier to wear a face mask? What is the skin temperature drop due to the wind?

Task 01 An industrial freezer is designed to operate with an internal air temperature of -20 0C  when the external air temperature is 25 0C  and the internal and external heat transfer coefficients are 12 W/m2K  and 8 W/m2K  , respectively. The walls of the freezer are composite construction, comprising of an inner layer of plastic ( k=1 W/m.K , and the thickness of 3 mm) , and an outer layer of  stainless steel  (k=16 W/m.K  , and thickness of 1mm). Sandwiched between these two layers is a layer of insulation material with k=0.07 W/m.K and the thickness of 195mm. Find the overall heat transfer coefficient and determine the heat transfer through the industrial freezer.

You are charged with specifying a cooling system for a house. The solar and internal gains have been calculated already, but you need to increase its capacity to account for ventilation. The ventilation system will constantly exhaust 200 cubic feet per minute (CFM) of stale air and replace it with 200 21 CFM of fresh air. On a design day, the outdoor air is 90°F dry bulb temperature and 73°F wet bulb temperature. Indoor air is expected to resolve to "standard 21 air" at 75°F and 50% relative humidity (%RH). What is the additional amount of coolir acity required to account for the ventilation system? Your answer should be in Btu/h. Answer:

Solve using the methodology : Known, Find, Schematic Diagram, Assumptions, Properties, Analysis and Comments.

Residual oil, with a heating value of 18,480 BTU/lbm, is used as a fuel in a power plant which produces 947,870 BTU/s of electricity. The overall efficiency of the power plant is 52%. Determine the: a. Heat input, in BTU/s b. Daily fuel consumption, in lbm c. Dimensions of the cylindrical storage (assuming length is equal to the diameter), in ft. d. Heat rejected to the environment, in BTU/s. The density of the oil is 62 lbm/ft3

The wall of an industrial kiln is built in refractory brick with 0.18 m thick, whose thermal conductivity is 1.7 W / (m.K). In steady state, the oven has a temperature of 1400 K on the inner wall and 1000 K on the outer wall. What rate of heat loss through the wall that is 1.6 m² in area? Draw a wall with a graph of temperature variation in its thickness.

Metal spheres, 10 mm in diameter, are to be annealed by heating them to 827°C then allowing them to cool slowly in air at 27°C to the point where they are in thermal equilibrium with the air. 2000 balls are annealed in one hour’s time. What is the total rate of their heat transfer if they are made of steel, lead, or copper? a.Sketch the problem. b.Draw lines identifying the control volume, or control mass. c.Identify the states with numbers, letters, or descriptions such as “in” and “out”. d.Write down the knowns and unknowns. e.Identify what is being asked for. f.State all assumptions.

3. The U.S. auto industry produces approximately 6 million vehicles a year, each containing roughly 1.5 metric tons of fabricated metal. Consider a future wind-turbine industry having half of the auto industry's mass throughout of wind turbine units. Each unit has a specific weight of 75 kg/kW rated (nacelle components plus tubular tower only). a. At a wind-turbine capacity factor of 0.3, how long would it take to replicate the 2008 U.S.-installed average electric power rating of 1035 GW? b. Discuss (5-10 sentences) the relevant factors that would determine whether a wind machine industry of this size could be successfully operated sustainably over periods on the order of several centuries.

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