Date post: | 18-Dec-2015 |
Category: |
Documents |
View: | 214 times |
Download: | 0 times |
Exercise #12 Engines
V1 = 6.25X10-4 m3, P1 = 12X106 Pa, n = 3 moles
P1V1 = P2V2
P2 = P1V1
/V2 = 385 MPa
= 1 – T1/T2 = 0.75
W = QH, QH = ncVT23
P3 = 500X106 Pa, V3 = 7.8X10-5 m3
QH = (3)(3/2)(8.31)(1564-1205) = 13424 J
Limits on Engines
Engines convert heat into work and waste heat
Second Law of Thermodynamics
An engine cannot have 100% efficiency
1st and 2nd Laws
Converting heat completely into work does not violate the 1st law
The second law is an independent statement
Refrigerators
A refrigerator is a device that uses work to move heat from low to high temperature
A heat pump does this to heat a room (want large QH)
How a Refrigerator Works
Fluid flows through the cold chamber and evaporates, adding heat QL to the fluid from the chamber
The fluid is pumped into the hot chamber and compressed, adding work W
The fluid condenses releasing heat QH
Refrigerator Cycle
Liquid
Gas
Compressor (work =W)
Expansion Valve
Heatremovedfrom fridgeby evaporation
Heat added to room bycondensation
HighPressure
Low Pressure
QL QH
Refrigerator Performance
The equivalent of efficiency for a refrigerator is the coefficient of performance K
K = QL/(QH-QL)
Unlike efficiency, K can be greater than 1
Heat Pump
The heat removed from the inside of a refrigerator is ejected into the kitchen
A refrigerator that has the cold chamber as the outdoors and the hot chamber as the house is called a heat pump
Many heat pumps can be reversed in summer to function as air conditioners
Refrigerators and the Second Law
You cannot move heat from low to high temperature without the addition of work
Equivalence
One implies the other For example: A 100% efficient engine connected to a high T
reservoir powering a refrigerator cooling a low T reservoir to the same high T reservoir
The refrigerator by itself is “legal” but the net effect to is move QL from low to high T with no other effect