Principles of Heat and Fluids class notes on Chapter 2

Energy

Energy is the capacity or the ability to do work. It can be categorized into two forms:

Transient Energy (Mechanical Work, Heat)

Stored Energy (Kinetic, Potential)

 

Mechanical Work

Work\quad =\quad Force\quad \times \quad Distance

Imperial SI
W = Work ft-lbf N-m or Joule (J)
F = Force Applied lbf Newton (N)
d = Distance travelled ft m

 

Ex 1. Find the Work done when a weight of 1000 lb is moved through a distance of 124 inches.

W\quad =\quad F\quad x\quad d
W\quad =\quad 1000\quad \times \quad (124\quad in\quad \times \quad (1ft/12in))
W\quad =\quad 1000\quad \times \quad 10.33
W\quad =\quad 10330\quad ft-lbf
$latex $

Ex. 2 A ventilation fan having a mass of 165 Kg is hoisted 96m from the ground to the roof of a building. Neglecting friction and other losses, compute the work done.

F\quad =\quad mg
W\quad =\quad (165kg)\quad \times \quad (9.8\quad m/{ s }^{ 2 })\quad \times \quad (96m)
W\quad =\quad 155,343\quad J
$latex $

 

Power

Power is the time rate at which work is done or work per unit time.

Power\quad =\quad Work\quad /\quad Time

Imperial SI
P = Power ft-lbf /sec Watt (J-sec)
W = Work ft-lbf N-m or Joule (J)
Time sec sec

*1\quad Horsepower\quad =\quad 550\quad ft-lbf\quad /sec\quad =\quad 746\quad watts

Ex 1. Determine the power required in kilowatts and horsepower to move an elevator weighting 2000 lbf vertically through 40 ft in 10 sec.

P = W/T = (F x d) / T = (2000 x 40) / 10 = 8000 ft-lbf /sec

P = 8000 ft-lbf /sec

P = 14.5 hp

 

Heat

Heat is an energy in transition between two objects or substances due to their temperature difference. Heat always flows from a substance with a higher temperature to a substance with a lower temperature, regardless of sizes.

Heat Measurement:

The British thermal unit (BTU): The amount of heat required to raise the temperature of 1 lb of water by 1 Fahrenheit degrees

The Calorie (Cal): The amount of heat required to raise the temperature of 1 gram of water by 1 Celsius degrees.

1 BTU = 252 CALORIES

Types of Heat

  • Specific heat: is the amount of heat required to raise the temperature of 1 lb. of a material by 1F
Specific Heat Specific Heat
Substance BTU/lb F KJ/kg C Substance BTU/lb F KJ/kg C
Ice 0.487 2.00 Mercury 0.033 0.134
Water 1.000 4.19 Aluminum 0.214 0.902
Steam 0.445 1.186 Brass 0.091 0.382
Wood 0.327 1.37 Copper 0.092 0.385
Alcohol 0.615 2.395 Glass 0.18 0.750
Machine Oil 0.400 1.675 Lead 0.031 0.129
Fuel Oil 0.564 2.362 Cast iron 0.120 0.420
Mild steel 0.116 0.485 Air 0.24 1.007
  • Sensible heat: causes a change in the temperature of a material without causing a change of state.

Q\quad =\quad mass\quad \times \quad specific\quad heat\quad \times \quad temperature\quad change

{ Q }_{ s }\quad =\quad m\quad \times \quad c\quad \times \quad \triangle T

  • Latent heat: not felt or sensed. It will not cause a thermometer to register a temperature change. However, it will cause a change of state of a substance. It is the amount of heat required to change liquid to gas or solid to liquid or vice versa.
Change of State Terminology Btu/lb KJ/kg
Ice to water Melting 144 Btu/lb 335 KJ/kg
Water to ice Fusion -144 Btu/lb -335 KJ/kg
Water to steam Vaporization 970 Btu/lb 2257 KJ/kg
Steam to water Condensation -970 Btu/lb -2257 KJ/kg

Latent\quad heat\quad =\quad Mass\quad \times \quad Latent\quad heat\quad per\quad unit\quad mass

{ Q }_{ 1 }\quad =\quad m\quad \times \quad l.h.

 

  • Total heat: is the sum of the sensible heat and the latent heat.

Total\quad Heat\quad =\quad Sensible\quad Heat\quad +\quad Latent\quad Heat

{ Q }_{ T }\quad =\quad { Q }_{ S }\quad +\quad { Q }_{ I }

 

 

About Hvac Master 32 Articles
All about Heating Ventilation Air Conditioning, property management, and etc