HEAT EXCHANGER

Heat exchanger

• Heat exchanger are thermal devices that transfer or exchange heat from one fluid stream to one or more others.
• Heat exchanger transfer heat by one of three ways

1. By recuperation, or recovery, of heat from hot stream to a cold stream
2. By regeneration, as the hot and cold stream alternatively flow through a matrix
3. By direct contact of one fluid stream with another

Classification of heat exchangers

• Heat exchanger are classified either by flow arrangement, by construction or by their degree of compactness

Classification by flow arrangement

• Heat exchanger can be classified according to flow arrangement as follows

Parallel flow heat exchanger

• In a parallel flow (also referred to as cocurrent or cocurrent parallel stream) exchanger, the fluid streams enter together at one end, flow parallel to each other in the same direction, and leave together at the other end

• In a parallel flow exchanger, a large temperature difference between inlet temperatures of hot and cold fluids exists at the inlet side, which may induce high thermal stresses in the exchanger wall at the inlet.

Counter flow heat exchanger

• In a counterflow or countercurrent exchanger, the two fluids flow parallel to each other but in opposite directions within the core.

• The counterflow arrangement is thermodynamically superior to any other flow arrangement.
• It is the most efficient flow arrangement, producing the highest temperature change in each fluid compared to any other two-fluid flow arrangements for a given overall thermal conductance (UA), fluid flow rates (actually, fluid heat capacity rates), and fluid inlet temperatures.

• The maximum temperature difference across the exchanger wall thickness (between the wall surfaces exposed on the hot and cold fluid sides) either at the hot- or cold-fluid end is the lowest, and produce minimum thermal stresses in the wall for an equivalent performance compared to any other flow arrangements.

Cross flow heat exchanger

• In this type of exchanger, the two fluids flow in directions normal to each other.
• In a crossflow arrangement, mixing of either fluid stream may or may not occur, depending on the design.

CLASSIFICATION OF HEAT EXCHANGER BY CONSTRUCTION

TYPES OF PUMP

Pump

• A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action.
• The basic purpose of pump is to transfer fluid or liquid or gases or slurries from a lower level to higher level.
• The pumps increase the mechanical energy of the liquid, increasing its velocity, pressure or elevation or all three
• Pump are widely used in variety of application
• Pump exist in a variety of shapes and sizes, depending on their intended function.
• When the flowing fluid is a gas, the pump is typically referred to as a compressor.
• There are two major classes of pump

1. Positive displacement pump
2. Centrifugal pump

Positive displacement pump

• It is the first major class of pump. 
• In this pump, a definite volume of liquid is trapped in chamber, which is alternately filled from the inlet and emptied at a higher pressure through the discharge.
• Positive displacement units apply pressure difference directly to the liquid by a reciprocating piston, or by rotating members which form chambers alternately filled by and emptied of the liquid. 
• Positive displacement pumps are a category of pumps designed to move fluid at a steady rate through a system. 
• These pumps are able to handle viscous fluids, which flow at lower speeds and create more resistance, more efficiently than kinetic (dynamic) pumps. 
• There are two sub classes of positive displacement pumps.

1. Reciprocating pumps
2. Rotary pumps

Reciprocating pumps

• In reciprocating pumps, the chamber is a stationary cylinder that contain a piston or plunger or diaphragm.
• They utilize a piston, plunger or diaphragm which draws fluid in (upstroke) and pushes it out (downstroke), using check valves to regulate and direct flow through the system.
• In a reciprocating pump, a volume of liquid is drawn into the cylinder through the suction valve on the intake stroke and is discharged under positive pressure through the outlet valves on the discharge stroke. 
• The discharge from a reciprocating pump is pulsating and changes only when the speed of the pump is changed. This is because the intake is always a constant volume. 
• Often an air chamber is connected on the discharge side of the pump to provide a more even flow by evening out the pressure surges. 
• Reciprocating pumps are often used for sludge and slurry.
• There are three example of reciprocating pumps

1. Piston pumps
2. Plunger pumps
3. Diaphragm pumps

Piston pumps

• In piston pump, liquid is drawn through an inlet check valve into the cylinder by the withdrawal of a piston and then is forced out through a discharge check valve on return stroke.
• Most piston pumps are double acting with liquid admitted alternately on each side of the piston so that one part of the cylinder is being filled while the other is being emptied.
• The piston may be motor driven through reducing gears , or a steam cylinder may be used to drive the piston rod directly
• The maximum discharge pressure for commercial piston pumps is about 50 atm. 

Plunger pumps

• For high pressure plunger pumps are used 
• A heavy walled cylinder of small diameter contains a close fitting reciprocating plunger, which is merely an extension of the piston rod
• At the time of stroke plunger fills nearly all space in the cylinder.
• Plunger pumps are single acting and usually are motor driven
• They can discharge against a pressure of 1500 atm or more.

Diagram pumps

• In a diagram pump, the reciprocating member is a flexible diaphragm of metal, plastic or rubber
• This eliminates need for packing or seals exposed to the liquid being pumped, a great advantage when handling toxic or corrosive liquid
• Diagram pumps handle small to moderate amount amounts of liquid, up to 100gal/min, and can develop pressure in excess of 100 atm 

Rotary pumps

• A wide variety of rotary positive displacement pumps are available.
• They bear such names as gear pumps, lobe pumps, screw pumps, cam pumps, and vane pumps.
• Rotary pumps do  not contain check valve.
• Rotary pumps operated best on clean, moderately viscous fluid, such as light lubricating oil.
• In rotary pumps discharge pressures up to 200 atm or more can be attained.

CLASSIFICATION OF CHEMICAL REACTION

Chemical reaction is classified according to number of reaction react to form specific product as follows

1. Single reaction
2. Multiple reaction
3. Combine reaction

Single reaction :- 

• As name suggest only one reaction takes part to form specific product.
• When a single stoichiometric equation and single rate equation are chosen to represent the progress of the reaction, we can called reaction as single reaction

Multiple reaction :-

• When more than one stoichiometric equation is chosen to represent the observed changes then more than one kinetic expression is needed to follow the changing composition of all the reaction components, then we can say its multiple reactions.
• Multiple reactions may be classified as:

• Series reactions

• Parallel reactions:-  which are of two types
• Competitive reaction

• Side by side reaction

Combine reaction

• In combine reaction, reaction proceeds in parallel with respect to B, but in series with respect to A, R, and S.

Chemical reaction classified according to relation between rate of reaction and stoichiometric equation of that reaction

1. Elementary Reactions 
2. Nonelementary Reactions

Elementary Reactions:- 

• The reactions in which the rate equation corresponds to a stoichiometric equation are called elementary reactions
• An elementary reaction is a chemical reaction in which one or more chemical species react directly to form products in a single reaction step and with a single transition state.
• Elementary reactions are single step reactions.
• Elementary reactions are simple in nature.
• For an elementary reaction, the order of reaction must be an integer.
• In a unimolecular elementary reaction, a molecule A dissociates or isomerises to form the products(s)

the rate of disappearance of A is

• In a bimolecular elementary reaction, two atoms, molecules, ions or radicals, A and B, react together to form the product(s)

the rate of disappearance of A is

Nonelementary Reactions:- 

• When there is no direct correspondence between stoichiometry and rate equation, then we can called as nonelementary reactions
• Non elementary reactions are multi-step reactions , i.e , such reactions occur through a series of reaction steps. 
• The non elementary reactions are complex in nature.
• For a non elementary reaction, the order of reaction may be an integer or have a fractional value.
• example 

FIRST LAW OF THERMODYNAMICS

Statement

" Although energy assumes many forms, the total quantity of energy is constant, and when energy disappears in one form it appears simultaneously in other form"

• First law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes.
• The first law makes use of the key concepts of internal energy, heat, and system work.
• For any process, the first law can be written as 

Energy balance for the closed system

• In closed system no streams enter or leave a system, and no energy associated with the matter is transported across the boundary.
• All energy exchange between a closed system and its surroundings then appears as heat and work.
• And hence total energy change of surroundings equals the net energy transferred to or from it as heat and work

                                                                  where Q= heat of system
                                                                          W= work done

Sign convention of heat and work

• The work done by the system on the surroundings is treated as positive quantities similarly the energy transferred as heat to the system from surroundings is also treated as positive quantity.

• Hence with reference to surroundings, heat transferred from the system to the surrounding and work done by the surroundings taken as follows
• Energy of the surroundings becomes 

• From equation of first law we can say that

• From these equation we can say that all energy exchange between a closed system and its surroundings then appears as heat and work, and the total energy change of surroundings equal the net energy transferred to or from it as heat and work.
• Closed system often undergo process during which only internal energy of the system changes. hence equation becomes

• Where Ut is the total internal energy of the system . for differential changes

Hence first law of thermodynamics can be stated as the change in the internal energy of a system is equal to the sum of the heat gained or lost by the system and the work done by or on the system.

ZEROTH LAW OF THERMODYNAMICS

Statement 

"The zeroth law of thermodynamics state that if each of the two given systems are in thermal equilibrium with third system then the two given systems are in thermal equilibrium with each other."

• Zeroth law of thermodynamics gives us the concept of temperature and thermometer.
• A thermometer is a portable device whose thermal state is related linearly to some simple property, for example its density or pressure. 
• Once a suitable temperature scale is defined for the device, one can use it to measure the temperature of a variety of disparate systems in thermal equilibrium. Temperature thus characterizes thermal equilibrium.
• Thermal equilibrium means, the object will approach the same temperature and in the absence of loss to the other objects, they will maintain a single constant temperature. 
• Zeroth law of thermodynamics was discovered way after the first second and third law of thermodynamics were discovered. But zeroth law is the most basic law. That's why this law is named as zeroth law of thermodynamics.
• For example: 

1. If A and C are in thermal equilibrium with B, then A is in thermal equilibrium with B. If an object with a higher temperature comes in contact with a lower temperature object, it will transfer heat to the lower temperature object. That means all three bodies are at the same temperature.
2. Consider two separate cups of boiling water. If we place a thermometer into the first cup, it gets warmed up by the water until it reads 100°C. We now say that the thermometer is in thermal equilibrium with the first cup of water. Next, we move the thermometer into the second cup of boiling water, and it continues to read 100°C. The thermometer is therefore also in thermal equilibrium with the second cup of water. Using the logic of the zeroth law, we can conclude that the two separate cups of boiling water are in thermal equilibrium with each other. The zeroth law therefore enables us to use thermometers to compare the temperatures of any objects we like.

MOLECULAR DIFFUSION

Molecular diffusion

• Mass transfer process can take place in a gas or vapour or in a liquid, and it can result from the random velocities of the molecules (molecular diffusion) or from the circulating or eddy currents present in a turbulent fluid (eddy diffusion).
• Molecular diffusion occurs due to movement of individual molecule through a substance by virtue of their thermal energy
• The phenomenon of molecular diffusion ultimately leads to completely uniform concentration of substances throughout solution which may initially have been nonuniform.
• For example:-

1. If a drop of blue copper sulphate solution is placed in a beaker of water, the copper sulphate eventually permeate the entire liquid. the blue color in times everywhere uniform, and no subsequent changes occurs.
2. In a two phase system not at equilibrium, such as layer of ammonia and air as a gas solution in contact with a layer of liquid water, spontaneous alteration through molecular diffusion also occurs, ultimately bringing the entire system to a state of equilibrium, after that diffusion stops.

• Hence in molecular diffusion ,if solution is not uniform everywhere in concentration of its constituents, the solution is spontaneously brought to uniformity by diffusion.
• It means that molecular diffusion is the mechanism of mass transfer in stagnant fluids or in fluids which are moving in laminar flow and the mass transfer takes place from a place of high concentration to low concentration.
• The rate at which a solute moves at any point in any direction must therefore depend upon concentration gradient at that point and in that direction.
• rate = molar flux [mol/ (area)(time)]
• The area is measured in the direction normal to the diffusion

HOW PROCESS CONTROL

Consider the tank heater system shown in Figure 

Assumption

Fi, Ti = flow rate (ft3/min) and temperature(°F) of entering liquid into the tank

F = flow rate of steam which heated liquid (lb/mm).

F, T = the flow rate and temperature of the stream leaving the tank. 

The tank is considered to be well stirred, which implies that the temperature of the effluent is equal to the temperature of the liquid in the tank. 

Objectives

• To keep the effluent temperature T at a desired value Ts
• To keep the volume of the liquid in the tank at a desired value Vs 

To maintain the temperature of effluent ‘T’ at desired temperature ‘Ts’

• The operation of the heater is disturbed by external factors such as changes in the feed flow rate and temperature (Fiand Ti).
• If nothing changed, then after attaining T = Ts and V = Vs, we could leave the system alone without any supervision and control.
• Consequently, some form of control action is needed to alleviate the impact of the changing disturbances and keep T and V at the desired values.
• In Figure we see such a control action to keep T = Ts when Ti or Fi, changes.
• A thermocouple measures the temperature T of the liquid in the tank.
• Then T is compared with the desired value Ts, yielding a deviation ε = Ts - T.
• The value of the deviation ε is sent to a control mechanism which decides what must be done in order for the temperature T to return back to the desired value T. 
• If ε > 0, which implies that T < Ts, the controller opens the steam valve so that more heat can be supplied.
• On the contrary, the controller closes the steam valve when ε < 0 or T> Ts.
• It is clear that when T = Ts (i.e., ε = 0), the controller does nothing.
• This control system, which measures the variable of direct importance (T in this case) after a disturbance had its effect on it, is called the feedback control system. 
• The desired value Ts is called the set point and is supplied externally by the person in charge of production.

To maintain the temperature of effluent ‘T’ at desired temperature ‘Ts’

• Returning to the tank heater example, we realize that we can use a different control arrangement to maintain T= Ts when Ti, changes.
• Measure the temperature of the inlet stream T, and open or close the steam valve to provide more or less steam.
• Such a control configuration is called feedforward control.
• The feedforward control does not wait until the effect of the disturbances has been felt by the system, but acts appropriately before the external disturbance affects the system, anticipating what its effect will be. 

To maintain the height of liquid ‘h’ in the tank at 

desired level ‘hs’

• In Figure we see a control action to keep h = hs when Ti or Fi, changes. So that tank will not overflow or go dry
• A level measuring device measures the height h of the liquid in the tank.
• Then h is compared with the desired value hs, yielding a deviation ε = h - hs.
• The value of the deviation ε is sent to a control mechanism which decides what must be done in order for the height h to return back to the desired value hs.
• It may open or close the valve that affects the effluent flow rate F
• It is also feedback control systems act post facto (after the fact), that is, after the effect of the disturbances has been felt by the process.
• If ε > 0, which implies that h < hs, the controller opens the steam valve so that more heat can be supplied.
• On the contrary, the controller closes the steam valve when ε < 0 or T>Ts.
• It is clear that when T = Ts (i.e., ε = 0), the controller does nothing.
• This control system, which measures the variable of direct importance (T in this case) after a disturbance had its effect on it, is called the feedback control system. 
• The desired value Ts is called the set point and is supplied externally by the person in charge of production.