Mechanical engineer interviews questions

What are the possible types of flow in open channel with respect to space and time? A.      steady and unsteady flow B.      uniform and non uniform flow What is Compressibility? It is the property by virtue of which fluids undergoes a change in volume under the action of external pressure. What is Newton’s law of Viscosity? According to Newton’s law of viscosity the shear force F acting between two layers of fluid is proportional to the difference in their velocities du and area A of the plate and inversely proportional to the distance between them. What is cohesion and adhesion in fluids? Cohesion is due to the force of attraction between the molecules of the same liquid. Adhesion is due to the force of attraction between the molecules of two different liquids or between the molecules of the liquid and molecules of the solid boundary surface.

Purpose - The purpose of this tutorial is to provide information about  asbesto s, the potential health effects associated with exposure, and safety procedures that should be followed to reduce exposure and protect the health of employees. Types of Asbestos - Asbestos can be defined as friable or non-friable.   Friable means that the material can be crumbled with hand pressure and is therefore likely to emit fibres. The fibrous or fluffy sprayed-on materials used for fireproofing, insulation, or sound proofing are considered to be friable and they readily release airborne fibres if disturbed.  Materials such as vinyl-asbestos floor tile or roofing felts are considered non-friable and generally do not emit airborne fibres unless subjected to sanding or sawing operations. Asbestos cement pipe or sheet can emit airborne fibres if the materials are cut, abraded or sawed, or if they are broken during demolition operations. Asbestos related Terms Asbestos containing mate...

PD pumps captures confined amounts of liquid and transfers it from the suction to the discharge port. The main reason positive displacement pumps are not more common than dynamic pumps is because they cannot achieve the high flow rates often required for industrial applications. Flow is constant with changing pressure.  A Positive Displacement Pump must not be operated against a closed valve on the discharge side of the pump because it has no shut-off head like centrifugal pumps.  A Positive Displacement Pump operating against a closed discharge valve will continue to produce flow until the pressure in the discharge line has increased until the line bursts or the pump is severely damaged - or both.     Progressing cavity pumps handling high solids content sludge in a waste treatment plant and reciprocating pumps are applied on coal slurry pipeline with solids contents as high as 40 percent by weight. This is sometimes a surprising PD pump characteristic, but wi...

There are two components linked with safety valve. 1. Tonnage 2. Pressure. When the inlet static pressure rises above the set pressure of the safety valve, the disc will begin to lift off its seat. However, as soon as the spring starts to compress, the spring force will increase; this means that the pressure would have to continue to rise before any further lift can occur, and for there to be any significant flow through the valve. The additional pressure rise required before the safety valve will discharge at its rated capacity is called the overpressure. For steam boilers, a small overpressure is required, usually 3% or 5%. For most other applications, 10% overpressure is required. The allowable overpressure depends on the standards being followed and the particular application. For compressible fluids, this is normally between 3% and 10%, and for liquids between 10% and 20%. Generally up to 5 TPH boiler set and reset pressure difference should be below 5% and over ...

Preventive maintenance is the process of systematic inspection, detection, correction, and prevention of incipient failures, before they become actual or major failures. Preventive maintenance (PM) is a schedule of planned maintenance actions aimed at the prevention of breakdowns and failures. The primary goal of preventive maintenance is to prevent the failure of equipment before it actually occurs. Preventive maintenance offers the maintenance team an opportunity to service the equipment, lubricate bearings and other parts to reduce friction and allow the machine to work at optimum capacity. it also enables one to checkmate failures before they finally occur thereby preempting the possibility of downtime with its attendant cost of complete replacements, labor maximization etc. The ANSI standards for Preventive Maintenance help you determine the type and frequency of inspections and maintenance procedures, define the minimum requireme...

Refrigeration & air conditioning

What is adiabatic saturation temperature? It is the temperature at which the outlet air can be brought into saturation state by passing through the water in the long insulated duct (adiabatic) by the evaporation of water due to latent heat of vaporization. What is psychrometer? Psychrometer is an instrument which measures both dry bulb temperature and wet bulb temperature. What is psychrometric chart? It is the graphical plot with specific humidity and partial pressure of water vapour in y axis and dry bulb temperature along x axis. The specific volume of mixture, wet bulb temperature, relative humidity and enthalpy are the properties appeared in the psychrometric chart. What is sensible heat and latent heat? Sensible heat is the heat that changes the temperature of the substance when added to it or when abstracted from it. Latent heat is the heat that does not affect the temperature but change of state occurred by adding the heat or by abstracting the heat....

Define Molecular mass. Molecular mass is defined as the ratio between total mass of the mixture to the total number of moles available in the mixture. What is isothermal compressibility? Isothermal compressibility is defined as the change in volume with change in pressure per unit volume keeping the temperature constant. What is psychrometry? The science which deals with the study of behavior of moist air (mixture of dry air and water vapour) is known as psychrometry. What is difference between absolute humidity and relative humidity? Absolute humidity is the mass of water vapour present in one kg of dry air. Relative humidity is the ratio of the actual mass of water vapour present in one kg of dry air at the given temperature to the maximum mass of water vapour it can with hold at the same temperature. Absolute humidity is expressed in terms of kg/kg of dry air. Relative humidity is expressed in terms of percentage.   What is effective temperature...

What is partial pressure? The partial pressure of each constituent is that pressure which the gas would exert if it occupied alone that volume occupied by the mixtures at the same temperature.   What is compressibility factor? The gas equation for an ideal gas is given by (PV/RT) = 1, for real gas (PV/RT) is not equal to 1 (PV/RT) = Z for real gas is called the compressibility factor. How does the Vander Waal’s equation differ from the ideal gas equation of state? The ideal gas equation pV=mRT has two important assumptions, 1. There is little or no attraction between the molecules of the gas. 2. That the volume occupied by the molecules themselves is negligibly small compared to the volume of the gas. This equation holds good for low pressure and high temperature ranges as the intermolecular attraction and the volume of the molecules are not of much significance. As the pressure increases, the inter molecular forces of attraction and repulsion increases an...

When coal is burned in a power plant, only 3% of the energy in the coal is converted into light in a light bulb. What happens to the other 97% of the energy? Explain your answer using the laws of thermodynamics.   The majority of energy is released as heat. The first law of thermodynamics states that energy cannot be created or destroyed. The burning of coal simply converts the potential energy in coal to eclectic and heat energy. The second law of thermodynamics states that when energy is converted from one form to another, some of it is degraded into heat. This explains why no process requiring energy is 100% efficient. What is Ideal gas? It is defined as a gas having no forces of intermolecular attraction. These gases will follow the gas laws at all ranges of pressures and temperatures. What is Charle’s law? It states that if any gas is heated at constant pressure, its volume changes directly as its absolute temperature. What is equation of state? The relation between ...

According to the second law of thermodynamics, when energy is converted from one form to another, some of it is degraded into heat, a less usable form that disperses into the environment. Examples of the second law of thermodynamics: A 1000 Watt electrical heater uses 1000 J/sec of electrical energy.  If it is 60% efficient, then the heater converts 60% of the electrical energy into heating water and 40% is wasted by being transformed into heat. -The 60% efficiency (40% waste) increasing disorder. -This is the law of increasing entropy. It states that the entropy of the universe increases with every physical process (change) that occurs. Entropy refers to the level of disorder, randomness, or chaos, of a system. The higher the randomness of a system, the higher its entropy. The more organized a system, the lower its entropy. As a result of the second law of thermodynamics, no process requiring an energy conversion is ever 100% efficient because much of the energy ...

According to the first law of thermodynamics, energy cannot be created or destroyed, although it can change from one form to another. Examples of the first law of thermodynamics:-         1)   A 1000 Watt electrical heater uses 1000 J/sec of electrical energy.  If it is 60% efficient, then the heater converts 60% of the electrical energy into heating water and 40% is wasted by being transformed into heat. -Electric energy transformed into light and heat energy -This is the law of the conservation of energy. It states that energy can neither be created, nor can it be destroyed. This means that the total amount of energy in the universe always remains conserved, or constant. However, energy can be changed from one form to another.           2)  When an exothermic reaction occurs some of the molecular enthalpy (energy in the molecules) of the system is converted into heat or light. This energy is then rele...

Is water stored behind a dam an example of potential or kinetic energy? What would cause the water to convert to the other form of energy? Water behind a dam is potential energy because it is stored energy. As water is release from the dam via a spillway or break in the dam it is converted to kinetic energy-energy in motion. What is chemical energy? Chemical energy is energy stored in the structure of molecules within the bonds between atoms.  Energy can be added or released from a molecule by changing the arrangement of electrons (rearranging chemical bonds). What is an Endothermic and Exothermic reaction? Endothermic Heat energy is absorbed from the surroundings. The energy level of the products is higher than the energy level of the reactants. An endothermic reaction would result in products with a higher energy (less stable). Exothermic Heat energy is released to the surroundings. The energy level of the products is lower than the energy level of ...

What is Heat? Heat is not a thing, heat is a process. Heat is the transfer of energy between two objects due to temperature differences. Notice that the name of the transfer process is heat. What gets transferred is energy. Heat is NOT a substance although it is very convenient to think of it that way. In fact, it used to be thought that heat was a substance. There is a circular nature to the definitions used: (a) energy does work or produces heat, but (b) heat is a transfer of energy. Ultimately, energy is expressed in the motion of substances. If it is moving, it has energy. If it has the capacity to move, there is some potential energy stored away. What is Clausius statement? It is impossible for a self-acting machine working in a cyclic process, to transfer heat from a body at lower temperature to a body at a higher temperature without the aid of an external agency. What is Kelvin Planck Statement? It is impossible to construct a heat engine to pro...

What is Process? A description in the manner in which a change in state occurs. What is Isobaric process? Any process in which the pressure of a gas remains constant What is Isochoric process? Any process in which the volume of a gas remains constant What is Isothermal process? Any process in which the temperature of a gas remains constant What is Adiabatic? The expansion or compression of a gas without a gain or loss of heat. What is a steady flow process? Steady flow means that the rates of flow of mass and energy across the control surface are constant. What is the term source and sink?

What is First law of thermodynamics? The heat lost by a system is equal to the heat gained by the system minus any work done by the system; conservation of energy. Or According to the first law of thermodynamics, energy cannot be created or destroyed, although it can change from one form to another. What are the limitations of first law of thermodynamics? First law of thermodynamics has the limitation that it merely indicates that in any process there is an exact equivalence between various forms of energy. It provides no information concerning the spontaneity or feasibility of the process. What is Second law of thermodynamics? Heat flows naturally from a region of higher temperature to a lower temperature. All natural systems tend toward a state of higher disorder. Or According to the second law of thermodynamics, when energy is converted from one form to another, some of it is degraded into heat, a less usable form that disperses into the environment. Wha...

What is meant by thermodynamic equilibrium? When a system is in thermodynamic equilibrium, it should satisfy the following three conditions. (a) Mechanical Equilibrium :- Pressure remains constant (b) Thermal equilibrium :- Temperature remains constant (c) Chemical equilibrium : There is no chemical reaction. What do you understand by pure substance? A pure substance is defined as one that is homogeneous and invariable in chemical composition throughout its mass. What is the difference between a heat pump and a refrigerator? Heat pump is a device which operating in cyclic process, maintains the temperature of a hot body at a temperature higher than the temperature of surroundings. A refrigerator is a device which operating in a cyclic process, maintains the temperature of a cold body at a temperature lower than the temperature of the surroundings. What is the characteristic gas equation?  Characteristic gas equation is pV = mRT Where,  ...

What is Carnot principle? No irreversible engine operating between two reservoirs at constant temperatures can have a greater efficiency than a reversible engine operating between the same temperatures. Furthermore, all reversible engines operating between the same temperatures have the same efficiency. What are the limitations of carnot cycle? 1. No friction is considered for moving parts of the engine. 2. There should not be any heat loss. What is Specific heat capacity at constant pressure? It is defined as the amount of heat energy required to raise or lower the temperature of unit mass of the substance through one degree when the pressure kept constant.   It is denoted by Cp. What is Specific heat capacity at constant volume? It is defined as the amount of heat energy required to raise or lower the temperature of unit mass of the substance through one degree when volume kept constant. What is meant by surroundings? Any other matter outsid...

What is thermal engineering? Thermal engineering is the science that deals with the energy transfer to practical applications such as energy transfer power generation, refrigeration, gas compression and its effect on the properties of working substance. What is Thermodynamics? The study of heat transfer What is System? A system is some portion of the universe selected for study.  Strictly speaking a system may, or may not, contain matter or energy.  For atmospheric applications, systems will always contain both matter and energy.  The remainder of the universe is defined as the "surroundings" of the system.  Selection of the system is primarily a matter of convenience.  Systems may be classified as isolated, closed or open. Isolated system:  A system which cannot exchange either matter or energy with its surroundings.  The universe is defined by the First Law as an isolated system. Closed system:  A system which can exc...

Generally bumping clearance depends on the manufacturer but as a thumb rule it should be between 0.5% to 1%

Reciprocating compressors are generally cooled with air or water. The cylinders in air cooled compressors often include large external fins that increase the surface area available for heat transfer. In water cooled compressors, freshwater is circulated through jackets that are built into the walls of the cylinders and cylinder heads. when comparing with a single stage compressor work can be reduced by inter cooling in a multistage compressor. Work can be reduced further by increasing number of stages and inter cooling, but as the number of stages increases design becomes complex, constructional cost increases, maintenance cost also increase, which may nullify the effect of work saved during operation. This is the limiting factor for more number of stages. Isothermal Compression During the process of compression, whatever heat produced is taken away by a cooling medium. In other words, it is the compression , keeping temperature of the gas constant. For a process to ...

Bumping clearance is checked by putting a lead ball or plastic gauges over the piston and then turning the compressor one revolution by hand. By doing this the lead ball will compress and the thickness obtained is the clearance volume. This thickness is measured with vernier caliper or micrometer and is then compared with the manufacturer’s value. Adjustments are made in case there is an offset in the value.

Some clearance space is required otherwise piston hits on the cylinder head as it travels. Also expansion of piston occurs as it travels and very little clearance may cause the same problem. Also increased clearance space reduces compressor efficiency and increases its running hours.

Reciprocating Compressor Crankcase and body – Cast Iron Crankshaft – Spheroidal graphite cast iron or stainless steel Connecting rod – Forged Steel Piston – Aluminium Alloy or Cast / Ductile Iron Piston Rings – Cast Iron Screw Compressor Casing – Cast or Ductile Iron

Bumping clearance can be adjusted with the help of inserting shims (thin metallic plates) in the bottom bearings. Inserting shims will move the connecting rod and the piston which will change the clearance.   The bumping clearance once found to be incorrect would have to be adjusted. The methods of adjusting the bumping clearances are as follows: 1.The cylinder head gaskets can be changed to a different thickness thus altering the bumping clearance. 2.The shims between the foot of the connecting rod and the bottom end bearing can be changed thus changing the bumping clearance. However after adjusting the bumping clearance the clearance should be checked once again to make sure that there is no error and the clearance is within the range as specified by the manufacturers. It must be stressed that compressors are unforgiving and incorrectly maintained compressors have claimed many a lives Compressors are used to increase the pressure of a gas. Like pumps, compressors can be class...

The bumping clearance can be checked by the following methods: 1.In case a suitable opening is available the piston can be barred to the top dead centric and then feeler gauges can be put inside and the clearances checked at two three points. 2.The more convenient method is to take lead wire from the engine store and make a small ball based on the expected clearance and put it between the piston and the head from the valve opening. Then the piston is slowly turned to the top dead centre with the help of a Tommy bar. After that the piston is again turned down and the lead wire ball is extracted and the thickness measured with the help of a micrometer. This measurement would give the bumping clearance.

Bumping clearance as the name signifies is a clearance given so that the piston of the reciprocating compressor would not bump into its cylinder head. Routine jobs like lifting the cylinder head to change the low pressure or first stage valves can change the bumping clearance if the correct thickness gaskets are not used or if the head is over tightened thus squeezing out the gaskets.

Clearance volume or bumping clearance is the space between the top of the piston and the cylinder head of an air compressor. This clearance is an important aspect of the compressors and should be as less as practically possible to improve the volumetric efficiency of the compressor. The clearance volume should not be too less or too more. Moreover, it affects the efficiency of the machinery and thus should be checked at regular intervals of time. The adjustment of Bumping Clearance is a very critical adjustment of the clearance volume. If more the volumetric efficiency of the compressor suffers and if less the unloaded piston may hit the cylinder head and damage both.

The bumping clearance must be adjusted properly otherwise there is risk of damage and loss of efficiency. If the bumping clearance is less the volumetric efficiency would increase but there is risk of the piston hitting the cylinder head, especially when the compressor is unloaded during start and stopping. In an air compressor, when the discharge valve closes in the end of the compression cycle, a small amount of high pressure air is trapped in the clearance volume. Before again taking suction, the air trapped in the clearance volume must expand below the suction pressure i.e. below the atmospheric pressure. The expansion of this trapped air in the clearance volume causes effective loss of stroke due to which the volumetric efficiency of compressor drops. Therefore, the clearance volume has a significant effect on the efficiency of the compressor. On the other hand to play safe, the engineer gives few millimeters of extra clearance, the volumetric efficiency of t...

During overhauls of the air compressor, if the gasket fitted between the cylinder head joints is of the wrong type, then the bumping clearance will increase, resulting in wear down of bottom bearings or wrong bearings are put in place. The bumping clearance in a new machine is set properly by the manufacturers during construction but over a period of time the clearance changes because of the following reasons: 1. Wear at the crank pin bearing. The crank pin bearing wears down due to use and this clearance can travel right up to the piston and an unloaded piston can hit the cylinder head. This type of wear can be recognized when the compressor makes impact sounds running unloaded at the starting and stopping operations. This type of wear would also be accompanied by a slow decrease in oil pressure over a period of time. 2. Opening up of cylinder heads. In certain types of reciprocating compressors the cylinder head have to be removed for the changing of the first stage suction...

Cavitation occurs when the pressure on a liquid drops below the vapor pressure for the liquid or when gas is otherwise introduced to a pump such that it disrupts the flow of the liquid through the pump. Positive displacement pumps trap a quantity of liquid (or gas) and then increase the pressure by pushing (displacing) it into the discharge pipe. External pressure pushes the liquid into the pump chamber which then is made smaller before the pump chamber discharges its contents. If any gas enters the pump chamber along with a liquid it is simply compressed along with the liquid. Positive displacement pumps work just fine with both gases and liquids - although when gases are being compressed it is usually necessary to cool the pump chamber which heats up quite a bit during the almost adiabatic compression step of the pump. Cavitation does not occur in part because the pressure in the pump chamber is almost completely uniform. If the compression chamber drops below the vapor pressure...

What is the slip in reciprocating pump? Slip is the difference between the theoretical discharge and actual discharge of the pump.                         Slip= Qth-Qact. What is meant by Priming? The delivery valve is closed and the suction pipe, casing and portion of the delivery pipe up to delivery valve are completely filled with the liquid so that no air pocket is left. This is called as priming. What is the main parts of reciprocating pump? ü   A cylinder with a piston, Piston rod, connecting rod and a crank. ü   Suction pipe, Delivery pipe. ü   Suction valve and ü   Delivery valve. How will you classify the reciprocating pump? The reciprocating pump may be classified as, 1. According to the water in contact with one side or both sides of the piston. 2. According to the number of cylinders provided. Classificati...

PSA stands for pressure swing adsorption. Absorption is actually a process which involves a chemical change. Generally accepted use of the term "absorb" implies a sponging effect. The term has been misused for a long time. Since nothing we do is chemical, only physically separating molecules, the process is correctly called adsorption. It’s the process of separating molecules of gas from each other. We use compressed air to pressurize a vessel filled with a material, either carbon or zeolites, which actually sift molecules by physical or structure. By forcing air into this pressure vessel or sieve bed filled with sieve, we cause N2 or O2 molecules to be trapped in the sieve, while the other gas floats free in the bed. After a period of time, we release a bit of the pressure in the sieve bed to draw off the O2 or N2 molecules and collect them in a surge/storage tank for use by the application. We then open a valve in the sieve bed and release all of the pressure in the sie...

Membranes are a series of hollow fibers that have holes in the walls. Compressed air is forced through these hollow fibers. The holes in the wall of the hollow fiber are small enough that CO, CO2 and O2 will exit through them under pressure. These same holes, however, are too small to accept N2 or Argon. By forcing air down the fiber, we permeate (force to escape) the smaller molecules and capture the Ar and N2 at the other end. We generally heat the air in the membrane to excite the molecules in the hollow fiber and increase the chance that they will permeate out through the holes. This process does not decrease the life of the membrane.