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Answer:
The net energy transfer from the student's body during the 20-min ride to school is 139.164 BTU.
Explanation:
From Heat Transfer we determine that heat transfer rate due to electromagnetic radiation (), measured in BTU per hour, is represented by this formula:
(1)
Where:
- Emissivity, dimensionless.
- Surface area of the student, measured in square feet.
- Stefan-Boltzmann constant, measured in BTU per hour-square feet-quartic Rankine.
- Temperature of the student, measured in Rankine.
- Temperature of the bus, measured in Rankine.
If we know that ,
,
,
and
, then the heat transfer rate due to electromagnetic radiation is:
Under the consideration of steady heat transfer we find that the net energy transfer from the student's body during the 20 min-ride to school is:
(2)
Where is the heat transfer time, measured in hours.
If we know that and
, then the net energy transfer is:
The net energy transfer from the student's body during the 20-min ride to school is 139.164 BTU.
Answer:
7.15
Explanation:
Firstly, the COP of such heat pump must be measured that is,
Therefore, the temperature relationship,
Then, we should apply the values in the COP.
The number of heat rejected by the heat pump must then be calculated.
We must then calculate the refrigerant mass flow rate.
The value is 145.27 and therefore the hot reservoir temperature is 64° C.
The pressure at 64 ° C is thus 1849.36 kPa by interpolation.
And, the lowest reservoir temperature must be calculated.
the lowest reservoir temperature = 258.703 kpa
So, the pressure ratio should be = 7.15