CENTRIFUGAL PUMP

Centrifugal pump

• A centrifugal pump is one of the simplest pieces of equipment in any process plant.
• Centrifugal pumps are the most common type of dynamic pump or kinetic pump and are used most often in applications with the moderate-to-high flow and low head.
• These pumps all rely on the centrifugal force as the fundamental principle by which they operate.

Centrifugal pump increase the mechanical energy of the liquid by centrifugal action

Its purpose is to convert the energy of a prime mover (an electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped. 

 

The energy changes occur by virtue of two main parts of the pump, the impeller, and the volute or diffuser. 

• The impeller is the rotating part that converts driver energy into the kinetic energy.
• The volute or diffuser is the stationary part that converts the kinetic energy into pressure energy.

Working of centrifugal pump

The process liquid enters the suction nozzle and then into the eye (center) of a revolving device known as an impeller. 

• When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and provides centrifugal acceleration.
• As liquid leaves, the eye of the impeller a low-pressure area is created causing more liquid to flow toward the inlet. 
• Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. 
• The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.

Components of Centrifugal Pumps

• A centrifugal pump has two main components: 

A rotating component comprised of an impeller and a shaft.

A stationary component comprised of a casing, casing cover, and bearings. 

Stationary Components 

1. Casing

• Casings are generally of two types: volute and circular. 
• The impellers are fitted inside the casings. 
• Volute casings build a higher head.
• Circular casings are used for low head and high capacity. 
• A volute is a curved funnel increasing in area to the discharge port. As the area of the cross-section increases, the volute reduces the speed of the liquid and increases the pressure of the liquid. 
• One of the main purposes of a volute casing is to help balance the hydraulic pressure on the shaft of the pump. However, this occurs best at the manufacturers recommended capacity. 
• The circular casing has stationary diffusion vanes surrounding the impeller periphery that converts velocity energy to pressure energy. 
• Conventionally, the diffusers are applied to multi-stage pumps.
• The casings can be designed either as solid casings or split casings. 
• Solid casing implies a design in which the entire casing including the discharge nozzle is all contained in one casting or fabricated piece. 
• A split casing implies two or more parts are fastened together. 
• When the casing parts are divided by horizontal plane, the casing is described as horizontally split or axially split casing. When the split is in a vertical plane perpendicular to the rotation axis, the casing is described as vertically split or radially split casing. Casing Wear rings act as the seal between the casing and the impeller.

2. Suction and Discharge Nozzle

• The suction and discharge nozzles are part of the casings itself. 

3. Seal Chamber and Stuffing Box

• Seal chamber and Stuffing box both refer to a chamber, either integral with or separate from the pump case housing that forms the region between the shaft and casing where sealing media are installed. 
• When the sealing is achieved by means of a mechanical seal, the chamber is commonly referred to as a Seal Chamber. 
• When the sealing is achieved by means of packing, the chamber is referred to as a Stuffing Box. 
• Both the seal chamber and the stuffing box have the primary function of protecting the pump against leakage at the point where the shaft passes out through the pump pressure casing. 
• When the pressure at the bottom of the chamber is below atmospheric, it prevents air leakage into the pump. 
• When the pressure is above atmospheric, the chambers prevent liquid leakage out of the pump. 
• The seal chambers and stuffing boxes are also provided with cooling or heating arrangement for proper temperature control. 

3. Bearing housing

• The bearing housing encloses the bearings mounted on the shaft. 
• The bearings keep the shaft or rotor in correct alignment with the stationary parts under the action of radial and transverse loads. 
• The bearing house also includes an oil reservoir for lubrication, constant level oiler, jacket for cooling by circulating cooling water.

Rotating Components

1. Impeller

The impeller is the main rotating part that provides the centrifugal acceleration to the fluid. 

• The number of impellers determines the number of stages of the pump. 
• A single-stage pump has one impeller only and is best for low head service. 
• A two-stage pump has two impellers in series for medium head service. 
• A multi-stage pump has three or more impellers in series for high head service. 

2. Shaft

• The basic purpose of a centrifugal pump shaft is to transmit the torques encountered when starting and during operation while supporting the impeller and other rotating parts. 
• It must do this job with a deflection less than the minimum clearance between the rotating and stationary parts.

• PERFORMANCE PARAMETER OF CENTRIFUGAL PUMP

1. CAPACITY
2. HEAD
3. BHP(Brake horsepower)
4. BEP (Best efficiency point)
5. SPECIFIC SPEED

• NET POSITIVE SUCTION HEAD

FICKS LAW OF DIFFUSION

Fick's law of diffusion

• Mass transfer is the natural tendency to transfer a given component (species) in a mixture from a region of high concentration to a region of low concentration to bring about a uniform or equilibrium condition. 
• The mass transfer has three requirements: 

1. that transfer occurs only in a mixture,
2. that at least one substance within the mixture moves from region of high concentration to a region of low concentration,
3. that the rate of mass transfer—i.e., the “flux” of a given substance—be proportional to the concentration gradient of that substance. 

Fick's  law

• In 1855 FICK proposed a relation between the flux of the diffusing substance and concentration gradient as the first law of diffusion which named as Fick's law of diffusion.

Fick’s law states that the flux of a diffusing component A in the z-direction in a binary mixture of A and B is proportional to the molar concentration gradient.

• Hence Fick's law of diffusion for component A in a binary mixture of A and B for steady-state diffusion is

           Where

• The negative sign in the equation indicates that diffusion occurs in the direction of decrease in concentration. Hence the term dCA/dz is -ve and flux become positive
• JA = molar flux of A in the z-direction [kmol/(m2.s)]
• CA = molar concentration of A [ kmol/ m3]
• dCA/dz = concentration gradient in the z-direction
• DAB = proportionality constant, diffusion coefficient for component A diffusing through B [m2/s]
• Z = distance in the direction of diffusion

COMPARISION OF SINGLE AND MULTIPLE EFFECT EVAPORATOR

Comparison of single  and multiple effect evaporator

Single-effect evaporators 

• Single-effect evaporators are used 

1. when the throughput is low, 
2. when a cheap supply of steam is available, 
3. when expensive materials of construction must be used as is the case with corrosive feedstocks 
4. when the vapour is so contaminated so that it cannot be reused. 

• The feed and saturated steam with temperature at TF and TS respectively enter the heat- exchange section 
• Condensed steam leaves as condensate or drips.
• The solution in the evaporator is assumed to be completely mixed.
• Hence, the concentrated product and the solution in the evaporator have the same composition.
• Temperature T1 is the boiling point of the solution.
• The temperature of the vapor is also T1, since it is in equilibrium with the boiling solution.
• The pressure is P1, which is the vapor pressure of the solution at T1
• If the solution to be evaporated is assumed to be dilute and like water, then 1 kg of steam condensing will evaporate approximately 1 kg of vapor (if the feed entering has TF near the boiling point) 
• Single-effect evaporators are often used when the required capacity of operation is relatively small and/or the cost of steam is relatively cheap compared to the evaporator cost.
• However, for large-capacity operation, using more than one effect will markedly reduce steam costs
• The heat requirements of single-effect continuous evaporators may be obtained from mass and energy balances. 

Multiple - effect evaporator

• The single effect evaporator uses rather more than 1 kg of steam to evaporate 1 kg of water. 
• The latent heat of the vapor leaving in single effect evaporator is not used but is discarded.
• Much of this latent heat, however, can be recovered and reused by employing a multiple - effect evaporator, that is, vapor from one effect serves as the heating medium for the next.
• The economy of the system, measured by the kilograms of water vaporized per kilogram of steam condensed, increases with the number of effects.
• in multiple effect evaporator, the pressure in each effect is lower than that of the effect to which it receives steam and higher than that of the effect to which it supplies vapors
• Each effect, in itself, act as a single effect evaporator, and each has a temperature drop across its heating surface corresponding to the pressure drop in that effect.

Forward-feed multiple - effect evaporator 

• A simplified diagram of a forward-feed triple- effect evaporation system is shown in Fig. 

• If the feed to the first effect is near the boiling point at the pressure in the first effect, 1 kg of steam will evaporate almost 1 kg of water.
• The first effect operates at a temperature that is high enough that the evaporated water serves as the heating medium to the second effect.
• Here, again, almost another kg of water is evaporated, which can then be used as the heating medium to the third effect.
• As a very rough approximation, almost 3 kg of water will be evaporated for 1 kg of steam in a three-effect evaporator.
• Hence, the steam economy, which is kg vapor evaporated/kg steam used, is increased.
• This also holds approximately more than three effects.
• However, the increased steam economy of a multiple-effect evaporator is gained at the expense of the original first cost of these evaporators
• In forward-feed operation as shown in Fig. fresh feed is added to the first effect and flows to the next in the same direction as the vapor flow.
• This method of operation is used when the feed is hot or when the final concentrated product might be damaged at high temperatures.
• The boiling temperatures decrease from effect to effect. This means that if the first effect is at P1 = 1 atm abs pressure, the last effect will be under vacuum at a pressure P3.
• The concentration of the liquid increases from the first effect to the last effects
• This pattern of liquid flow is the simplest
• It requires a pump for feeding dilute solution to the first effect, since this effect is often at about atmospheric pressure, and a pump to remove thick liquor from the last effect.
• The transfer from effect to effect, however, can be done without pumps, since the flow in the direction of decreasing pressure, and control valves in the transfer line all that is required.

Backward-feed multiple - effect evaporator 

• In the backward-feed operation shown in Fig. for a triple-effect evaporator, the fresh feed enters the last and coldest effect and continues on until the concentrated product leaves the first effect. 

• This method of reverse feed is advantageous when the fresh feed is cold since a smaller amount of liquid must be heated to the higher temperatures in the second and first effects.
• However, liquid pumps must be used in each effect, since the flow is from low to high pressure.
• This reverse-feed method is also used when the concentrated product is highly viscous.
• The high temperatures in the early effects reduce the viscosity and give reasonable heat-transfer coefficients.
• Backward feed often gives a higher capacity than forward feed when the thick liquor is viscous, but it may give a lower economy than forward feed when the feed liquor is cold.

Parallel-feed multiple-effect evaporators

• Parallel-feed in multiple-effect evaporators involves the adding of fresh feed and withdrawal of the concentrated products from each effect.
• The vapor from each effect is still used to heat the next effect.
• This method of operation is mainly used when the feed is almost saturated and solid crystals are the product, as in the evaporation of brine to make salt

MEASURES OF EVAPORATOR PERFORMANCE

There are three main measures of evaporator performance:

• Capacity (kg vaporized / time)
• Economy (kg vaporized / kg steam input)
• Steam Consumption (kg / hr)

The performance of a evaporator is evaluated by the capacity and the economy.

Economy

• Economy is the number of kg of water vaporized per kg of steam fed to the unit.
• The rate of heat transfer q through the heating surface of an evaporator, by the definition of overall heat transfer coefficient, is product of three factors

1. The area of heat transfer surface A
2. The overall heat transfer coefficient U
3. The overall temperature drop ΔT

Q = U * A * ΔT

• Economy calculations are determined using enthalpy balances.
• The key factor in determining the economy of an evaporator is the number of effects.
• The economy of a single effect evaporator is always less than 1.0. 
• Multiple effect evaporators have higher economy but lower capacity than single effect.
• The thermal condition of the evaporator feed has an important impact on economy and performance. 
• If the feed is not already at its boiling point, heat effects must be considered. 
• If the feed is cold (below boiling) some of the heat going into the evaporator must be used to raise the feed to boiling before evaporation can begin; this reduces the capacity.
• If the feed is above the boiling point, some flash evaporation occurs on entry.

Capacity

• Capacity is defined as the no of kilograms of water vaporized per hour.
• If the feed to the evaporator is at the boiling temperature corresponding to the absolute pressure in the vapor space, all the heat transferred through the heating surface is available for evaporation and the capacity is proportional to q.
• If the feed is cold, the heat required to heat it to its boiling point may be quite large and the capacity for a given value of q is reduced accordingly, as heat used to heat the feed is not available for evaporation.
• if the feed is at a temperature above the boiling point in the vapor space, a portion of the feed evaporates spontaneously by adiabatic equilibration with the vapor-space pressure and the capacity is greater than that corresponding to q. This process is called flash evaporation.

Steam consumption

• Steam consumption is very important to know, and can be estimated by the ratio of capacity divided by the economy. 
• That is the steam consumption (in kg/h) is

                                          Consumption = Capacity/Economy.

Heat transfer in evaporators

• The rate equation for heat transfer takes the form:

Q = U * A * ΔT

where:

1.  Q is the heat transferred per unit time
2. U is the overall coefficient of heat transfer
3. A is the heat transfer surface
4. T is the temperature difference between the two streams. 

• In applying this equation to evaporators, there may be some difficulty in deciding the correct value for the temperature difference because of what is known as the boiling point rise (BPR) or boiling point elevation (BPE)
• If water is boiled in an evaporator under a given pressure, then the temperature of the liquor may be determined from steam tables and the temperature difference is readily calculated. 
• At the same pressure, a solution has a boiling point greater than that of water, and the difference between its boiling point and that of water is the BPR or BPE. 
• For example, at atmospheric pressure (101.3 kN/m2 ), a 25 per cent solution of sodium chloride boils at 381 K and shows a BPR of 8 deg K. If steam at 389 K were used to concentrate the salt solution, the overall temperature difference would not be (389 − 373) = 16 deg K, but (389 − 381) = 8 deg K. Such solutions usually require more heat to vaporise unit mass of water, so that the reduction in capacity of a unit may be considerable. 
• The value of the BPR cannot be calculated from physical data of the liquor, though Duhring’s rule is often used to find the change in BPR with pressure. 
• Duhring’s rule states that the boiling point of given solution is a linear function of the boiling point of pure water at the same pressure.
• Thus, if the boiling point of the solution is plotted against that of water at the same pressure, then a straight line is obtained.
• Thus, if the pressure is fixed, the boiling point of water is found from steam tables, and the boiling point of the solution from Duhring’s plot.
• Different lines are obtained for different concentrations.
• The boiling point rise is much greater with strong electrolytes, such as salt and caustic soda.

TYPES OF EVAPORATOR

Types of Evaporator

Single effect evaporator

When a single evaporator is used, the vapor from the boiling liquid is condensed and discarded. This method is called single effect evaporation, and although it is simple, it utilizes steam ineffectively.

• The solution to be concentrated flows inside the tubes. 
• The heating medium is steam condensing on metal tubes. 
• Usually boiling liquid is enter under a moderate vacuum.
• This increases the temperature difference between the steam and boiling liquid.

Multiple effect evaporator

Increasing the evaporation per kilogram of steam by using a series of evaporators between the steam supply and the condenser is called multiple effect evaporator

• If vapor from one evaporator is fed into the steam chest of the second evaporator and the vapor from second is then sent to a condenser, the operation becomes the double effect. 
• The heat in the original steam is reused in the second effect, and the evaporation is achieved by a unit mass of steam in the same manner.

Once through evaporator

• In once-through evaporation, the feed liquor passes through the tube only once, releases the vapor, and leaves the unit as thick liquor. 
• All the evaporation is accomplished in a single pass. 
• The ratio of evaporation to feed is limited in single pass units: the evaporators are well adopted to multiple effect operation, where the total amount of operation can be spread over several effects. 
• Agitated film evaporators are always operated once- through  falling film and climbing film evaporators can also be operated in this way. 
• Once through evaporators are specially useful for heat-sensitive materials. 

Circulation evaporators

• In circulation evaporators a pool of liquid is held within the equipment. 
• Incoming feed mixes with the liquid from the pool, and the mixture passes through the tubes. 
• Unevaporated liquid discharged from the tubes returns to the pool so that only part of the total evaporation occurs in one pass. 
• All forced circulation evaporators are operated in this way; climbing- film evaporators are usually circulation units.
• These are adapted to single-effect evaporation. 
• These are not suited for heat-sensitive materials.

EVAPORATION

Evaporation

• Evaporation is when a liquid becomes a gas without forming bubbles inside the liquid volume.
• During evaporation, only the molecules near the liquid surface are changing from a liquid to vapor.
• Evaporation is the process that occurs when the surface of a liquid is converted into a gas
• For example, evaporation occurs when a glass of water is left out overnight and the water level is found to drop.
• Another example is when steam flows through a tube that is submerged in a pool of liquid, minutes bubbles of vapor form at random points on the surface of the tube. The heat is passing through the tube surface where no bubbles form enters the surrounding liquid by convention. Then some of the heat in the liquid then flows toward the bubbles, causing evaporation from its inner surface into itself.
• The objective of evaporation is to concentrate a solution consisting of a non volatile solute and volatile solvent.
• Evaporation is conducted by vaporizing a portion of the solvent to produce a concentrated solution of thick liquor.
• Normally, in evaporation thick liquor is the voluble product and the vapor is condensed and discarded.
• Hence, Evaporation is a widely used method for the concentration of aqueous solutions, involves the removal of water from a solution by boiling the liquor in a suitable vessel, an evaporator, and withdrawing the vapor. 
• If the solution contains dissolved solids, the resulting strong liquor may become saturated so that crystals are deposited. 
• Evaporation is achieved by adding heat to the solution to vaporize the solvent. 
• The heat is supplied principally to provide the latent heat of vaporization, and, by adopting methods for recovery of heat from the vapor, it has been possible to achieve a great economy in heat utilization. 
• Whilst the normal heating medium is generally low-pressure exhaust steam from turbines, special heat transfer fluids or flue gases are also used. 
• Evaporation is differ from drying in that the residue is the liquid, sometimes a highly viscous one rather than solid.
• Evaporation is differ from distillation in that vapor usually is a single component, and even when vapor is a mixture, separation of vapor into a smaller fraction is not carried out in evaporation.

• Single effect Evaporator
• Multiple effect evaporator
• once through and circulation evaporators
• Capacity
• Economy
• Steam consumption
• Heat transfer in Evaporators
• Boiling point elevation
• Boiling point rise
• Comparison of single and multiple effect evaporators