Common Terms, Definitions and Conversion Factors

Size: px

Start display at page:

Download "Common Terms, Definitions and Conversion Factors"

Loraine Morgan
6 years ago
Views:

1 1 Common Terms, Definitions and Conversion Factors 1. Force: A force is a push or pull upon an object resulting from the object s interaction with another object. It is defined as Where F = m a F = Force in Newton (N) in S.I. units m = Mass in kg a = Acceleration in m/sec 2. In M.K.S. units, force is defined as Kgf 1 Kgf = 9.81 N. In F.P.S. units, force is defined as lbf 1 lbf = kgf. 2. Pressure: Force exerted per unit area is called pressure. Pressure is defined as: P = F A Where P = Pressure in Pascals (Pa) Pascal = 1N m 1 2 F = Force in N A = Area in m 2 1 Bar = 10 5 Pa = 1.02 kgf/cm 2 = 14.5 lbf/sq. in. At sea level, atmosphere exerts a force of N on one square meter area.

2 2 REFRIGERATION AND AIR CONDITIONING HIGH SIDE DESIGN 1 Atm. pressure = N/m 2 = Bar = kg f/m 2 = lbf/sq. in. 3. Work or Energy: Force applied to a body to move it to a distance is called work. Work = Force Distance Unit of work or energy in S.I. units is Nm or Joule (J). i.e. 1 Nm = 1 J If a force of 1N moves a body by 1m, it is called work of 1J or 1 Nm is done. In M.K.S. Work/energy = kgf m In case of heat energy in MKS, it is called k cal. Conversion of heat energy into work is by mech equivalent of heat (J). 1 k cal = 427 Kgf m In F.P.S., unit of heat is British Thermal Unit (BTU)) 1 k cal = 4.19 kj = BTU 4. Power: Rate of doing work is called power. Unit of power in S.I. is Watts (W). 1 Watt = 1 J/s Electrical unit of work is also Watt i.e. 1 Watt = 1 amp 1 volt = 1J/sec. In F.P.S. System, unit of power is Horse Power (H.P. imperial). 1 H.P. (Imperial) = 550 Ft lb/s = 746 J/s = 746 W In M.K.S. System, unit of power is H.P. (metric) 1 H.P. (metric) = 75 Kgf m/s = 736 J/s = 736 W. 5. Unit of energy or work can also be derived from power W = J/sec J = W sec Now kwh is the unit of work 1 kwh = 3600 kj. Common Conversion Factors 1 k cal = BTU = kj 1 kj = k cal 1 kj = BTU

3 COMMON TERMS, DEFINITIONS AND CONVERSION FACTORS 3 1 BTU = k cal 1 BTU = kj 1 k cal/h = W 1 kw/h = BTU/h 1 H.P. (imperial) = 642 k cal/h 1 H.P. = 746 W. (H.P. normally refers to H.P. imperial. Unless specified as H.P. metric). 1 H.P. = BTU/h Heat Flux 1 k cal/hm 2 = W/m 2 = BTU/h ft 2. Density r 1 lb/ft 3 = kg/m 3 Heat Transfer Coefficient 1 k cal/h m 2 C = W/m 2 K = BTU/h ft 2 F 1W/m 2 K = BTU/h ft 2 F 1 BTU/h ft 2 F = 5.68 W/m 2 K 1 k cal/hmk = W/mK 1 W/mK = BTU/h ft 2 F 1 BTU/h ft 2 F = W/mK = k cal/h m C ( F/BTU)/h ft 2 = m 2 K/W. DEFINITIONS OF THERMODYNAMICS Thermodynamics is a science which deals with the interaction of energy and material systems. 1. System: System is a region of space or a finite quantity of matter under study. A system is surrounded by an envelope which is referred as the boundary of the system. There are three types of systems: (a) Closed System: It is that system in which there is transfer of energy across the walls of a system but there is no transfer of mass as shown in Fig Fig. 1.1

4 4 REFRIGERATION AND AIR CONDITIONING HIGH SIDE DESIGN (b) Open System: It is that system, in which there is a transfer of energy as well as mass. A common example of this system is a compressor in a refrigeration system. (c) Isolated System: In this system, there is neither the transfer of energy nor mass. Most common example of this system is universe. 2. Thermodynamic State: It is the condition of the system, as defined by thermodynamic properties like pressure, temperature etc. 3. Process: It is transformation of a system from the initial state to another state. 4. Path: A series of states through which the system undergoes during its transformation from initial state to another state as shown in Fig Fig Cycle: A system undergoes a cycle. When its initial and final states are the same as shown in Fig Fig. 1.3 (a) Thermodynamic cycle: Where the chemical properties of the substance (working fluid) do not undergo any change, it is called a thermodynamic cycle e.g. refrigeration cycle. (b) Mechanical cycle: Where chemical properties of the working fluid do undergo a change, it is called a mechanical cycle e.g. Internal Combustion Engines (where mixture of air and fuel after combustion sends out exhaust gases). 6. Heat: It is a form of energy which is transferred from one body to another due to temperature difference. 7. Cold: It is a relative term which means absence of heat. 8. Temperature: It is the measure of the thermal level of the substance. It is measured in centigrade, Fahrenheit, Rankins and Kelvin. C 5 = F 32 9 Kelvin is absolute temperature on centigrate scale. If the temperature of a body is t C.

5 COMMON TERMS, DEFINITIONS AND CONVERSION FACTORS 5 K = t Hence absolute zero temperature is 273 C. Rankin is absolute temperature on Fahrenheit scale R = t F Absolute zero on Rankin scale is 460 F. 9. Specific Heat: Specific heat of a substance is the quantity of heat added to a mass of 1 kg to raise its temperature by 1 C or 1K. It is written as C p or C v. Units are kj/kg K C p Specific heat at constant pressure C v Specific heat at constant volume. Quantity of heat transfer at constant pressure is defined as Q = m C p T 10. Types of Heat (a) Sensible Heat: It is that amount of heat added/subtracted which causes change in temperature. (b) Latent Heat: It is the heat which causes change of state without change in temperature. (i) Latent Heat of Fusion: It is the heat added/subtracted to change the state of a substance from solid to liquid and vice versa, without the change in temperature. Latent heat of fusion of ice is 335 kj/kg = 80 Cal/gm. (ii) Latent Heat of Vaporization: It is the heat added or subtracted to change the state of a substance from liquid to gas or vice versa, without change in temperature. Latent heat of vaporization of water is 2257 kj/kg = 546 Cal/gm. 11. Saturation Temperature: It is that temperature of the liquid at which a fluid will change its phase. Saturation temperature depends upon the pressure. As the pressure increases, the saturation temperature goes up and vice versa. 12. Saturation Pressure: Saturation Pressure is that pressure which changes saturation temperature. Saturation temperature of water at 1 atmosphere is 100 C and for ammonia it is 33 C. 13. Sub Cooled Liquid: Any liquid at a temperature below its saturation temperature is called a sub cooled liquid. 14. Super Heated Vapour: Any vapour at a temperature higher than its saturation temperature is called a super heated vapour. 15. Thermodynamic Relationships (a) Pressure, Volume and Temperature Relationships (i) Boyle s Law: At constant temperature, pressure is inversely proportional to its volume. P α 1 v (at constant temperature) (ii) Charle s Law: At constant pressure, volume is directly proportional to its temperature v α T (at constant pressure) (iii) Pressure Temperature Relationship: From Boyle s and Charle s law, at constant volume, pressure is directly proportional to its temperature.

6 6 REFRIGERATION AND AIR CONDITIONING HIGH SIDE DESIGN P α T (at constant volume) (iv) General Gas Equation: By combining Boyle s and Charle s Law or v α T P Pv α T or Pv = RT (where R is gas constant) v-specific volume of gas i.e. volume occupied by 1 kg of gas. Multiply both sides by m. since Where and P is in Pascal Pa V is in m 3 m is in kg T is in K R is in kj/kg K mpv = mrt m.v = V PV = mrt Value of R is 287 kj/kgk. (b) External Work Done: Whenever a substance changes volume, work is done. When volume increases, work is done by the substance i.e. either energy can be transferred or work can also be done by utilising the internal kinetic energy (K.E.) of the gas. Kinetic Energy is reflected in the temperature. When gas is compressed Work is done on the gas. The mechanical energy of the piston is converted into internal kinetic energy and thus the temperature is increased. (c) Energy Equation of a Gas: All the energy transferred to a gas is accounted for in one of the following ways: (i) Increase in internal kinetic energy. (ii) Increase in internal potential energy. (iii) External work done. (d) Internal Kinetic Energy: It is because of molecular motion. (e) Internal Potential Energy: It is due to molecular separation Q = K + P + W Heat transfer K.E. P.E. Work done (f) Work Done: In case of solids and liquids, when energy is transferred, all the energy is utilised to increase internal K.E. DP = O, DW = O, so DQ = DK In case of change of phase, the heat transferred

7 COMMON TERMS, DEFINITIONS AND CONVERSION FACTORS 7 DQ = DP + DW (DK = 0) (g) Ideal or Perfect Gas: It is a gas, in which molecules are free and independent of each other s force of attraction. All energy transferred will not affect the internal P.E. e.g. Oil at a low temperature will not flow due to high internal friction, but once heated, the friction will reduce, it will start flowing. In vapour state, molecular friction is still less. Vapour is a gas which is near its saturation temperature and for refrigeration, we will consider it as a perfect gas. So energy equation Q = K + W (h) Constant Volume: No work is done DQ = DK (i) Constant Pressure F = P A Work Done = Force distance = P A s Work Done = P (V 2 V 1 ) Fig. 1.4 Fig. 1.5 Area under PV diagram gives the work done in the process. C p > C v C p Specific heat at constant pressure. It increases K.E. and work is also done. C v Specific heat at constant volume. It increases only K.E. (j) Isothermal process (Temperature is constant): All energy transferred is utilised in doing work. v2 W.D. = pv 11log e v Minimum W.D. is during isothermal process Pv n = C (in isothermal, n = 1) In case of compressors Isothermal work Isothermal efficiency = Actual work done. Fig

8 8 REFRIGERATION AND AIR CONDITIONING HIGH SIDE DESIGN (k) Adiabatic Process: There is no transfer of heat from system to surrounding and vice versa, or no transfer of energy from the system to the surrounding or from the surrounding to the system W.D. = PV 2 2 PV 1 1. (g = 1.4) γ 1 In refrigeration, we consider compression as adiabatic process. This is also known as isentropic process as the entropy remains constant. Fig. 1.7 (l) Polytrophic Process: It is a process, in which part of the energy to do the work comes from an external sources and part of the energy comes from its own internal energy. Work done = PV 2 2 PV 1 1. n 1 It is between isothermal and adiabatic processes. Value of n is between 1 and 1.4. (m) Enthalpy: It is a term loosely used to indicate the total heat of a substance. Enthalpy is a calculated property of a matter which is loosely defined as the total heat. Enthalpy of a substance at a certain condition is the total energy transferred to it to bring it to that condition from a chosen datum. Enthalpy is equal to the sum of the internal energy and the product of pressure and volume. h = V + pv Where, h = Specific enthalpy or enthalpy/kg H = Total enthalpy. (n) Entropy: It is the measure of the unavailability of energy. You can measure only change in entropy. Entropy, like, enthalpy is a mathematical function. Entropy (s or φ) of a unit mass of a material at a given condition is an expression of the total energy transferred to the material per degree of absolute temperature to bring that material to that condition from a certain datum. Mathematically, entropy between two states 1 and 2 is given by the expression: 2 dq ds =. 1 Fig. 1.8 T Area underneath a T S (temperature entropy) diagram represents energy transferred. (o) Refrigeration: It is the branch of science which deals with the process of reducing and maintaining the temperature of a space or material below that of its surroundings. Unit of refrigeration Tons of refrigeration (TR) 1 ton of refrigeration is the quantity of heat required to be removed from 1 US tonne of water at 0 C to convert it into ice at 0 C in 24 hours. 1 US Tonne = 2000 lbs Latent heat of water = 144 BTU/lb TR = = 200 BTU/min

9 COMMON TERMS, DEFINITIONS AND CONVERSION FACTORS 9 or 1 k cal = 4.19 kj = BTU 1 TR = 211 kj/min = 50 k cal/min 1 TR = 3.5 kw (approx). REFRIGERATION AND PROCESSES Refrigeration is a process where the energy is transferred from a system to environment or a different substance. Air conditioning is an application of refrigeration. Some of the common processes that involve refrigeration are as follows: 1. Comfort Air Conditioning: This is a process that aims at improving the thermal parameters for human comfort. 2. Industrial Air Conditioning: It is a form of air conditioning which helps an industrial process. Some common examples are: (i) Textile Industry where humidity is controlled more precisely. (ii) Precision Manufacturing Plants where both temperature and humidity are controlled precisely. 3. Domestic Refrigeration: Common household refrigerators and deep freezers fall under this category. These are fractional tonnage plants. 4. Commercial Refrigeration: Design, installation and maintenance of refrigeration fixtures in hotels display cases, perishable food storage etc. come under this category. 5. Industrial Refrigeration: The manufacturing plants, food packing industry, cold storages etc. come under this category. 6. Marine and Transport Refrigeration: Marine ships carrying perishable foods, refrigerated trucks, vans and rail cars are examples of this types of refrigeration. Various Methods of Refrigeration 7. Principles of Refrigeration (a) Total energy transferred to a closed system is equal to the work done + charge in internal energy Q = W + du. (b) Claussius Statement: Heat will flow from a low temperature reservoir to a high temperature reservoir, when work is done on the system. 8. Ice Refrigeration: In this system, ice is kept in a container which needs to be refrigerated. Heat transfer takes place by convection. Some disadvantages of this system are: (a) Very low temperature is not possible. It is normally used where temperature required to be attained is 5 to 10 C. (b) Control of rate of cooling is not possible. (c) Ice needs to be replenished. Fig. 1.9

10 10 REFRIGERATION AND AIR CONDITIONING HIGH SIDE DESIGN 9. Evaporative Cooling: Evaporation causes dry bulb temperature drop closer to wet bulb temperature without change in wet bulb temperature. Common examples of this type of cooling are pitchers, air coolers sold as desert air coolers. 10. Air Cycle Refrigeration or Expansion of Air: We have seen in the principles of refrigeration Q = W + du In this process, gas is made to expand adiabatically so Q = O or W = du The adiabatic expansion causes cooling, as explained above. PROBLEM: Assume atmospheric air at 27 C is compressed isentropically to a pressure ratio of 5. Fig It is then cooled in the heat exchanger, before it is made to expand isentropically in the expansion cylinder. What is the final temperature of the air? SOLUTIONS: T T 2 1 = P 2 P 1 γ 1 γ T 2 ( ) It is then cooled to 300 K, so = ( 5) T 2 = 475 K T T 3 4 = P 3 P 4 γ 1 γ = ( 5) 14. T 4 T 4 = K

Introduction CHAPTER Prime Movers. 1.2 Sources of Energy

Introduction CHAPTER Prime Movers. 1.2 Sources of Energy Introduction CHAPTER 1 1.1 Prime Movers Prime mover is a device which converts natural source of energy into mechanical work to drive machines for various applications. In olden days, man had to depend