Food Engineering Lessons 4 & 5 PDF

the state of entropy of the entire universe, FOOD ENGINEERING LESSON 4 o as an isolated system, will always increase...

the state of entropy of the entire universe, FOOD ENGINEERING LESSON 4 o as an isolated system, will always increase over time Thermodynamics - The second law also states that the - Thermodynamics provides a foundation for studying o changes in the entropy in the universe can common phenomena in food processing. never be negative - Typical approach in food process study: - The second law of thermodynamics can also be stated o Observe a phenomenon. that o Make experimental measurements to confirm o all spontaneous processes produce an observations. increase in the entropy of the universe o Develop a mathematical basis. o Apply knowledge to the engineering process. - Entropy explains: - This method is like the thermodynamic approach to o Why is it that when you leave an ice cube at examining physical systems. room temperature, it begins to melt? - In food engineering, many of the processes of concern o Why do we get older and never younger? to a food engineer are applications of thermodynamics. o Why is it whenever rooms are cleaned, they - For example, we may need to: become messy again in the future? o calculate heat and work effects associated with a given process. - Certain things happen in one direction and not the other, o know the maximum work obtainable from a this is called the "arrow of time" and it encompasses process that may be a key calculation. every area of science. o determine how to carry out a process with o The thermodynamic arrow of time (entropy) is minimum work. the measurement of disorder within a system. o determine relationships between variables of a system when it is at equilibrium. - To understand entropy changes, consider both the entropy change of the system and its surroundings Laws of Thermodynamics o The total entropy change of the universe is the 1. Zeroth Law of Thermodynamics sum of these two changes: 2. First Law of Thermodynamics 3. Second Law of Thermodynamics 4. Third Law of Thermodynamics Laws of Thermodynamics – Zeroth Law - The Zeroth Law of Thermodynamics states that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each Gibb’s Free Energy, ∆𝑮 other. - It measures the maximum reversible work that can be o If system A is in thermal equilibrium with performed by a system at constant temperature and system C, and system B is also in thermal pressure. equilibrium with system C, then systems A and - It combines the enthalpy (𝐻) of a system with its entropy B are in thermal equilibrium with each other. (𝑆) through the equation: - Two systems that are in thermodynamic equilibrium will ∆𝑮 =∆𝑯+𝑻∆𝑺 not exchange any heat. - Systems in thermodynamic equilibrium will have the o If 𝜟𝑮 < 𝟎, the reaction is spontaneous same temperature (exergonic) o If 𝜟𝑮 > 𝟎, the reaction is non-spontaneous Laws of Thermodynamics – First Law (endergonic) - The first law of thermodynamics is a statement of the o If 𝜟𝑮 = 𝟎, the system is at equilibrium. conservation of energy. The law states: o “The energy of an isolated system remains constant.” - Stated in other words: o “Energy can be neither created nor destroyed but can be transformed from one form to another.” Laws of Thermodynamics – Third Law - Energy can be stored within an object or transferred to - The third law of thermodynamics states that the entropy other forms of energy (kinetic, potential, thermal, of a pure crystalline substance approaches zero as the mechanical, electrical, chemical, etc.). temperature approaches absolute zero (0 Kelvin or - o Increasing an object's elevation increases its 273.15°C). potential energy, which remains stored until o When we are considering a perfect (100% the object is moved again. pure) crystalline structure, at absolute zero (0 o Thermal energy increases when heat is Kelvin), it will have no entropy (S). transferred into an object, raising its - Through the third law: temperature. o “Absolute zero” is defined. o In a hydroelectric plant, the potential energy of o It serves as a reference point for entropy falling water is converted to mechanical energy calculations. and then to electrical energy. o Limits of the cooling process are understood. o Electrical energy is converted to other useful forms, like thermal energy in heaters, when Heat transmitted to homes or factories - Heat transfer between a system and its surroundings is o Energy conversion or transmission often probably the most prevalent form of energy that we generates heat, which is sometimes observe in many food engineering systems. misinterpreted as "loss" of energy. - Heat plays a major role in cooking, preservation, and ▪ Example: In an electric motor, 10- creating new food products with unique properties – due 15% of electrical energy may convert to its association with temperature differences. to heat due to friction, termed as o Heat moves from hot to cold objects due to "loss.“ temperature differences. o All mechanical energy can convert to heat, but o Heat transfer is temperature-driven not all heat can convert to work, highlighting - Denoted by Q, measured in joules (J). the second law of thermodynamics. o Negative Q: heat transfers from the system to the surroundings. Laws of Thermodynamics – Second Law o Positive Q: heat transfers into the system (e.g., - The Second Law of Thermodynamics states that: heating a potato) Thermal Energy FOOD ENGINEERING LESSON 5 Fluid flow - Movement of liquid foods is essential in commercial food processing plants. - Systems are used for moving both raw/unprocessed and processed liquid foods before packaging - Liquid foods in processing plants have a wide range of flow properties, from milk to tomato paste. - Design of transport systems in food processing must prioritize sanitation to maintain product quality. Some points: - Fluids move from high to low-pressure regions and from higher to lower elevations due to gravity. - Fluid moving to a lower elevation decreases in potential energy and increases in kinetic energy. - Heated fluids experience a decrease in density, causing lighter fluid to rise and denser fluid to take its place. - Inside moving fluids, imaginary layers slide over each other with viscous forces acting tangentially, opposing Energy Balance flow. - High-viscosity fluids (e.g., honey) move slower than low-viscosity fluids (e.g., milk). Liquid Transport Systems - A typical transport system consists of four basic components, namely tanks, pipeline, pump, and fittings. Pipes for Processing Plants - Fluids in food processing plants are mostly transported in closed conduits: o Pipes – used in high-pressure applications (liquids). o ducts – used in lower-pressure applications (gases). - Pipelines for liquid foods use stainless steel as it provides: o Smoothness o cleanability o corrosion resistance Pipes for Processing Plants - Essential components of a pipeline system include: o Straight length of pipe (diameter 2-10 cm). o Elbows and tees for changing direction. o Valves for controlling flow rate, which can be remotely operated based on preset signals. - All pipeline components must ensure sanitary handling of the product. o Cleaning-in-place (CIP) is a common method for cleaning these systems, which must be considered in the system's initial design. Types of Pumps - Mechanical energy is needed to move liquid products unless gravity is used. o Pumps provide the necessary mechanical energy. - Pumps can be classified into two main types: o Centrifugal pumps o Positive displacement pumps Centrifugal Pumps - Centrifugal pumps use centrifugal force to increase liquid pressure. - They consist of a motor-driven impeller enclosed in a case. - Advantages: o Efficient with low-viscosity liquids (e.g., milk, fruit juices). o Suitable for high flow rates and moderate pressure requirements. o Provide a steady discharge flow. o Can handle both clean/clear and dirty/abrasive liquids, as well as liquids containing solid particles - Liquid enters at the center of the impeller and moves to the periphery, where it experiences maximum pressure and exits into the pipeline. - High-viscosity liquids (e.g., honey) are difficult to transport with centrifugal pumps.  - Flow rates are controlled by a valve on the discharge The Continuity Equation end of the pump - The principle of conservation of matter is used to solve fluid flow problems. Positive Displacement Pumps - Consider fluid flowing in a pipeline with different cross- - Positive displacement pumps apply direct force to a sectional areas at two points. confined liquid to produce the pressure needed for - In time step Δ𝑡, fluid moves from space 𝑋𝑌 to space 𝑋′𝑌′. movement. - Distance between 𝑋 and 𝑌 is Δ𝑥1, and between 𝑋′ and - Flow rates are accurately controlled by the speed of the 𝑌′ is Δ𝑥2. pump's moving parts. - Cross-sectional area at 𝑋 is 𝐴1, and at 𝑋′ is 𝐴2. - They can transport liquids with high viscosities - Mass contained in space 𝑋𝑌 must equal mass in space - Rotary pumps are a type of positive displacement 𝑋′𝑌′ for matter conservation pump. o They work by enclosing a pocket of liquid between the rotating part of the pump and the housing. o Deliver a set volume of liquid from the inlet to the outlet. o At least one moving part must withstand rubbing action to ensure tight seals. o Can reverse flow direction by reversing rotor rotation. o Deliver a steady discharge flow. Viscosity - Fluids can be visualized as composed of different layers. Reynolds Number - The relative movement of fluid layers is due to a - A simple experiment with dye in a flowing liquid can shearing force applied parallel to the surface. illustrate flow characteristics. - According to Newton's second law, the fluid offers o At low flow rates, dye moves in a straight line, resistance to movement in the opposite direction of the showing laminar flow. shearing force. o At intermediate flow rates, dye begins to blur, o This resistance force measures the fluid's indicating transitional flow. viscosity. o At high flow rates, dye blurs immediately and - Different fluids exhibit varying levels of resistance to spreads randomly, indicating turbulent flow. movement (viscosity). - Reynolds conducted experiments to define the: o Honey, more viscous than water or milk is o Inertial forces as a function of liquid density (𝜌), more difficult to pour or stir tube diameter (𝐷), and average fluid velocity (𝑣). Shear Rate and Shear Stress – Newtonian Fluid o Viscous forces are a function of liquid viscosity - Shear rate is the relative change in velocity divided by (𝜇). the distance between the plates. 𝜸=𝚫𝒗 𝚫𝒚 - The Reynolds number, a dimensionless number, is - Newton observed that if the shearing stress, 𝝈, is defined as the ratio of inertial forces to viscous forces. increased (by increasing force), then the shear rate, 𝛾, will also increase in direct proportion. - Shear stress then becomes: 𝝈=𝝁 𝚫𝒗 𝚫𝒚 where 𝜇 is the viscosity of the fluid. - Liquids that follow this, are called Newtonian fluid - When shear stress is plotted against shear rate, a straight line passing through the origin is obtained, with the slope representing viscosity. o Newtonian liquids include water, honey, fluid milk, and fruit juices. - The Reynolds number quantitatively describes fluid flow - Fluids that do not follow this are called non-Newtonian characteristics in pipes or on surfaces of various fluids. shapes. - It provides insight into energy dissipation caused by Dynamic viscosity vs Kinematic viscosity viscous effects. - Dynamic viscosity (𝜇) is a measure of a fluid's - When viscous forces dominate energy dissipation, the resistance to deformation or flow under shear stress. 𝝁 Reynolds number is small, indicating laminar flow = 𝝈 / 𝜸 shear stress, shear rate - SI Unit: Pascal-second (𝑃𝑎 𝑠). - It determines how fast a layer of fluid will move in response to a force. - Kinematic viscosity (𝑢) is the ratio of dynamic viscosity to fluid density. 𝒖 =𝝁 / 𝝆 dynamic viscosity, fluid density Entrance Region and Fully Developed Flow - SI Unit: Square meter per second (m²/s). - When a liquid enters a pipe, there is an initial length - It is a measure of how easily a fluid flows relative to its called the entrance region. mass – or fluids motion is influenced by their density - The velocity profile of the liquid is uniform at the pipe entrance. Handling Systems for Newtonian Liquids - Friction at the pipe wall reduces velocity to zero near the - Liquid foods in food processing plants are transported wall, increasing towards the center. using pumps or gravity systems. - The boundary layer develops from X to Y, affecting the - Flow characteristics depend on liquid velocity, internal velocity profile. forces (viscous and inertial), and energy required for - Beyond Y, flow is fully developed with a parabolic pumping under different conditions. velocity profile across the pipe's cross section - Stream lines are imaginary curves in fluid flow where fluid moves without crossing the curve. - Velocity along a streamline is tangential to the curve. - Streamlines grouped together form a stream tube  - Assumptions: o Locations 1 and 2 are on the same stream line. o The fluid has a constant density; therefore, the fluid is incompressible. o The flow is inviscid (viscosity is zero). o The flow is steady. o No shaft work is done on or by the fluid. o No heat transfer takes place between the fluid and its surroundings. Pump Performance Characteristics - Pump performance characteristics are essential for designing liquid transport systems involving pumps. - It requires quantitative data about pump performance and energy requirements for flow through system components. - Pump performance is typically provided by manufacturers in the form of a pump characteristic diagram Forces Due to Friction - Testing procedures: - Pumping a liquid through a pipe requires overcoming o Pump performance is tested under controlled viscous forces between fluid layers and friction forces conditions using standard procedures between the liquid and pipe wall, which cause (Hydraulic Institute, 1975). mechanical energy dissipation (frictional energy loss). o Testing involves running the pump at a - Friction forces vary with flow rates (described by the constant speed on a test stand. Reynolds number) and surface roughness. o Measurements include: - The friction factor (𝑓) expresses the influence of friction ▪ heights of suction and discharge forces ports, ▪ pressures at suction and discharge ▪ volumetric flow rates ▪ torque on the pump shaft. Flow Measurement - Flow rate measurement is crucial for liquid transport systems, aiding in design calculations and ensuring Moody chart operational efficiency. - also known as the Moody diagram, is a graphical - Various flow measurement devices provide direct representation used in fluid mechanics to determine the quantification of mass flow rate or velocity. friction factor for flow in a pipe. - Common methods include: o Pitot tube - Key components: o Orifice meter o Reynolds Number – x-axis. o Venturi tube o Relative Roughness – right y-axis; it is the ratio - These methods involve measuring pressure of the average height of surface irregularities differences, often using a U-tube manometer. to the pipe diameter. - A U-tube manometer consists of a small diameter tube o Friction Factor – left y-axis. shaped like a "U," partially filled with a different fluid (manometer fluid) to measure pressure, such as Frictional Energy Loss mercury. - The frictional energy loss for a liquid flowing in a pipe is composed of major and minor losses. 𝐸𝑓 = 𝐸𝑓,𝑚𝑎𝑗𝑜𝑟 The Pitot Tube +𝐸𝑓,𝑚𝑖𝑛𝑜𝑟 - Pitot tube is a sensor used to measure fluid velocity. - Major losses (𝐸𝑓,𝑚𝑎𝑗𝑜𝑟) in pipes occur due to the - It works based on the difference between stagnation friction of the liquid as it flows through straight sections, pressure (point directly facing the flow) and static leading to a pressure drop relative to the fluid's density pressure (measured on the side of the tube). - Minor losses (𝐸𝑓,𝑚𝑖𝑛𝑜𝑟) are due to components like - The Pitot tube has two small tubes: valves, tees, elbows, and changes in fluid direction or o Inner tube with an inlet facing the flow to entry/exit from tanks. measure stagnation P. - Three components of minor losses: o Outer tube with holes on the circumference to o Frictional energy loss due to sudden measure static P. contraction. - The difference between these two measurements is o Frictional energy loss due to sudden called dynamic pressure – this is what is used to expansion. estimate fluid velocity o Frictional energy loss due to pipe fittings. The Orifice Meter Bernoulli’s Equation - Orifice meter is a device used to measure flow rate of - Bernoulli's equation can describe the behavior of a fluid fluids in a pipe. moving along a streamline. - It works by placing a restriction (orifice plate) inside the - It comes from the idea that energy is conserved, pipe, reducing the flow area. meaning the total energy of the fluid stays the same if - The pressure difference created, exhibited through the there is no friction causing energy loss manometer, is measured. - This pressure difference is related to the flow rate of the fluid. The Venturi Meter - A Venturi meter is used for measuring fluid flow rate in o Bingham Plastic a pipe. ▪ Bingham plastics behave like solids - It consists of a pipe with a gradually tapering section below a certain yield stress and flow (Venturi) in the middle. like viscous liquids above this stress o This tapering reduces the cross-sectional area level. of the pipe temporarily, causing the fluid ▪ These liquids require a minimum velocity to increase. stress (yield stress) to start flowing. - As fluid passes through the narrowest part of the Once the yield stress is exceeded, Venturi, its velocity increases, the pressure decreases. they flow at a constant viscosity - The pressure difference between the narrower and ▪ Examples include toothpaste wider sections of the Venturi is measured using pressure sensors. o Herschel-Bulkley Fluids - This difference is then used to calculate the flow rate of ▪ Herschel-Bulkley fluids exhibit a yield the fluid stress like Bingham plastics but have a shear-thinning behavior above the Measurement of Viscosity yield stress. - Viscosity of a liquid can be measured using a variety of ▪ They require a minimum stress (yield approaches and methods. stress) to start flowing, and their - Common types of instruments used: viscosity decreases as the shear rate o Capillary Tube Viscometer increases beyond this point. o Rotational Viscometer - Time-dependent Non-Newtonian liquids Capillary Tube Viscometer o They exhibit viscosity changes over time when - It measures viscosity based on the time it takes for a subjected to shear stress. fluid to flow through a narrow capillary tube under o They can be further categorized into two main gravity or applied pressure. types: o It consists of a narrow tube (capillary) through ▪ Rheopectic fluids which the fluid flows. ▪ Thixotropic fluids o The time taken for the fluid to pass through the capillary is measured. o Rheopectic fluid ▪ Rheopectic fluids exhibit an increase Rotational Viscometer in viscosity over time under constant - It measures viscosity by rotating a spindle in the fluid shear stress. and measuring the torque required to overcome viscous ▪ Rheopectic fluids return to their resistance. original viscosity state gradually once o It consists of a spindle immersed in the fluid. the shear stress is removed, typically o The spindle is rotated at a constant speed, and showing a delayed recovery. the torque required to maintain the rotation is ▪ Examples include some types of measured. cream Flow Characteristics of Non-Newtonian Fluids o Thixotropic - Non-Newtonian fluids are a special class of fluids whose ▪ Thixotropic fluids exhibit a decrease flow behavior differs from that of typical Newtonian in viscosity over time under constant fluids, like water. shear stress. - These fluids do not follow Newton's law of viscosity, ▪ Thixotropic fluids return to their meaning their viscosity (resistance to flow) changes original viscosity with time after the under different conditions. shear stress is removed, with - A key feature of non-Newtonian fluids is that their recovery time varying based on the viscosity is not constant, and they do not retain their fluid and conditions. shape once the force causing them to move is removed. ▪ Examples are yogurt and honey. - Non-Newtonian fluids can be classified as time- independent or time-dependent based on their viscosity behavior over time. - Time-independent Non-Newtonian liquids o They respond immediately to applied shear stress with a flow rate that is not directly proportional to the applied force. o The relationship between shear stress and shear rate is nonlinear. ▪ This means that as the shear rate changes, the viscosity of the fluid changes as well. o Shear-Thinning (Pseudoplastic) ▪ These fluids exhibit a decrease in viscosity as the shear rate increases. ▪ When sheared, the structure of shear thinning fluids breaks down, allowing them to flow more easily. ▪ Examples include fruit purées and mayonnaise. o Shear-Thickening (Dilatant) ▪ These fluids increase in viscosity as the shear rate increases. ▪ This behavior occurs when the suspended particles within the fluid start to interact more as shear stress is applied, making the fluid "stiffer. ▪ Examples include suspensions like cornstarch in water.