Mechanical engineer interviews questions

Shaft A shaft is a rotating machine element which is used to transmit power from one place to other place. Carbon steels of grade 40C8, 45C8, 50C4, 50C12 are normally used as shaft materials. Material properties It should have good heat treatment properties. It should have high strength It should have low notch sensitivity factor. It should have good machinability. It should have high wear resistance. Types of shaft Transmission shaft: These shafts transmit power between the source and machines absorbing power. The counter shafts, line shafts, overhead shafts all shafts are transmission shafts.

Copper is one of the earliest metals discovered by man. The boilers on early steamboats were made from copper. The copper tubing used in water plumbing in Pyramids was found in serviceable condition after more than 5,000 years. Cu is a ductile metal. Pure Cu is soft and malleable, difficult to machine. Atomic number of copper is 29. Atomic weight is 63.54. Copper has a density of 8.94 g/cm3 and melting temperature 1083°C.  Copper is found as native metal and in minerals cuprite, malachite, azurite, chalcopyrite and bornite. It is also often a by-product of silver production. Sulphides, oxides and carbonates are the most important ores.

Carbon steel is steel that has properties made up mostly of the element carbon, and which relies upon carbon content for its structure. The most perfect carbon structure in the world is a diamond, which is 100% carbon. Carbon is present in all steel and is the principal hardening element, determining the level of hardness or strength attainable by quenching. It raises tensile strength, hardness, resistance to wear and abrasion as the carbon content of steel is increased. It lowers ductility, toughness and machinability. Low-carbon steels contain up to 0.30% C. The largest category of this class of steel is flat-rolled products (sheet or strip), usually in the cold-rolled and annealed condition. The carbon content for these high-formability steels is very low, less than 0.10% C, with up to 0.4% Mn.

Bernoulli's Principle  It states that as the speed of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases. The phenomenon described by Bernoulli's principle has many practical applications. Bernoulli's principle thus says that a rise (fall) in pressure in a flowing fluid must always be accompanied by a decrease (increase) in the speed, and conversely, if an increase (decrease) in , the speed of the fluid results in a decrease (increase) in the pressure.  Bernoulli's principle can be explained in terms of the law of conservation of energy. As a fluid moves from a wider pipe into a narrower pipe or a constriction, a corresponding volume must move a greater distance forward in the narrower pipe and thus have a greater speed. At the same time, the work done by corresponding volumes in the wider and narrower pipes will be expressed by the product of the pressure and the volume. Since the speed is greater in the narrower pip...

Steam Traps are automatic valves designed to remove condensate from steam lines, but prevent steam loss by “trapping” the steam; hence the name Steam Trap . Air is usually present for some period of time in a steam line, either during start-up and/or operation. So, depending on the application, air is another fluid that usually requires removal from a steam line. Steam is formed when water vaporizes to form a gas. In order for the vaporization process to occur, the water molecules must be given enough energy that the bonds between the molecules break. This energy given to convert a liquid into a gas is called 'latent heat'. Steam-based heating processes use latent heat and transfer it to a given product.

A key is a piece of mild steel inserted between the shaft and hub to connect these together in order to prevent relative motion between them . It is always inserted parallel to the axis of the shaft. Keys aroused as temporary fastenings and are subjected to consider-able crushing and shearing stresses. Types of Keys: (a) Shunk keys, (b) Saddle keys, (c) Tangent keys, (d) Round keys, and (e) Splines.  The Function of Keys There are two main functions of keys when they are used to lock the transmission shaft. The primary function is to restrict relative rotational motion and axial movement between the shaft and the machine element.

To keep hydraulic components performing correctly, the hydraulic liquid must be kept as clean as possible. Foreign matter and tiny metal particles from normal wear of valves, pumps, and other components are going to enter a system. Strainers, filters, and magnetic plugs are used to remove foreign particles from a hydraulic liquid and are effective as safeguards against contamination. Strainers A strainer is the primary filtering system that removes large particles of foreign matter from a hydraulic liquid. Even though its screening action is not as good as a filter's, a strainer offers less resistance to flow. A strainer usually consists of a metal frame wrapped with a fine-mesh wire screen or a screening element made up of varying thicknesses of specially processed wire. Filters A filter removes small foreign particles from a hydraulic fluid and is most effective as a safeguard against contaminants. Filters are located in a reservoir, a pressure line, a return line...

Function of agitated Nutsche filter: Agitated Nutsche Filter (ANF) is filter used for solid and liquid separation which is aided with the application of vacuum and cake squeezing by mechanical means. Agitated Filter is a closed vessel designed to separate solid and liquid by filtration under pressure or vacuum. The closed operation ensures odorless, contamination free and nonpolluting working conditions maintaining product purity and hygiene. The advanced technology of agitation and hydraulics used in the equipment makes it versatile and user friendly. The resulting wet cake can be reslurried and washed thoroughly with water or solvents unlike in” NUTSCHE” type filters or centrifuges. Wash liquid quantity can be controlled and recycled, reducing effluent load. The discharge of wet cake is automatic. If the process demands filtration in chilled or hot condition, it is also possible. Drying of wet cake is also possible when drying features are incorporated. The equipment is fun...

The mechanism used for lowering or producing low temperature in a body or a space, whose temperature is already below the temperature of its surrounding, is called the refrigeration system . The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators. Refrigerators are cyclic device, and the working fluids used in the refrigeration cycles are called refrigerants. Here the heat is being generally pumped from low level to the higher one & is rejected at high temp. Another device that transfers heat from a low-temperature medium to a high-temperature one is the heat pump. Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only.

Sub cooling Definition: Sub cooling is defined as: The temperature of a liquid refrigerant when it is cooled below its condensing temperature. Sub cooling is the temperature decrease below the boiling point (same as the condensing point) in the condenser. Too high and refrigerant condenses too early in the condenser and ‘wastes’ most of the effect of the condenser. Too low and a mixture of gas and liquid can be delivered to the expansion valve, reducing efficiency. Super heating cooling Definition: Super heating is defined as: The temperature of vapor refrigerant above its saturation change of state (evaporation) temperature. Superheat is the temperature rise above the boiling point of the refrigerant after the evaporator. Too high, and the refrigerant boils off early in the evaporator, and ‘wastes’ most of the effect of evaporator. Too low and you risk liquid going into the compressor.

High grade heat comes from ‘ DE-superheating ’ the refrigerant between the compressor and the condenser. This heat can be between 60ºC and 90ºC. A heat exchanger (the DE-superheater ) is installed, with the refrigerant on one side and the fluid to be heated on the other. Not only does this DE-superheat the refrigerant, it reduces the cooling water or air needed by the condenser. Heat Recovery is a very useful energy saving technique where the heat that is removed by the refrigeration systems or other processes can be utilized for various applications as opposed to this heat just being wasted, as is normally the case. Considerable energy savings can be achieved by re-using heat from your refrigeration plant or any other process present on site. Depending on the specific temperature requirements of other cooling and heating loads throughout your facility, recaptured heat can be re-used directly, boosted through a heat pump to a higher temperature, used in an absorption chi...

Excessive Vibration and Noise in Pump Vibration from the pump’s feet and the base (where the pump rests) can cause the pump to be noisy.  A piece of old carpet or rubber between the pump’s base and equipment pad might reduce the noise. The bearings might be noisy due to age or high concentrations of chemicals and heat. A leaky pump seal could damage the front motor bearing.  It is very important to quickly repair any leaks around the pump to prevent extensive damage.  Many times a noisy pump can come from cavitation. Cavitation has two possible causes, which are both hydraulically related.  The first is that the suction piping is undersized.  If this pipe is undersized, the amount of water the pump requires to operate properly is reduced.  Increasing the pipe size or dropping the size of the impeller can quiet the pump. Ensure the pump is level and the suction lines are not higher than the pump. Other reason for excessive vibration and...

Work is the scalar product of the force acting on an object and the displacement through which it acts. When work is done on or by a system, the energy of that system is always changed. If work is done slowly, we say that the power level is low. If work is done quickly, the power level is high. Kinetic energy is the energy an object has because of its motion, and potential energy is the energy an object has because of its location or configuration. If the energy of a system remains constant throughout a process, we say that energy is conserved. Simple form: work = force x distance                            W = F x d Work is a measure of expended energy.

An economizer that recovers 5% of boiler input. A comprehensive tune-up with precision testing equipment to detect and correct excess air losses, smoking, unburned fuel losses, sooting and high stack temperatures can result in boiler fuel savings of 2% to 15%. An upgraded boiler maintenance program including optimizing air-to-fuel ratio, burner maintenance, and tube cleaning, can save about 2% of a facility’s total energy use. A 5 degree C Change in boiler exhaust temperature changes to save 1% to 2% of a boiler’s fuel use.

Hydrogen (H2) is a potentially emissions-free alternative fuel that can be produced from domestic resources. Hydrogen is the simplest element. An atom of hydrogen consists of only one proton and one electron. It's also the most plentiful element in the universe. In nature, hydrogen is never found on its own; it is always combined into molecules with other elements, typically oxygen and carbon (For example, is a combination of hydrogen and oxygen (H2O)). Hydrogen can be extracted from virtually any hydrogen-containing compound, including both renewable and non-renewable resources. Regardless of the fuel source, fuel cells utilize hydrogen with little to no polluting emissions, making hydrogen the ultimate clean energy carrier. Hydrogen is colorless, odorless, tasteless and non-toxic. Hydrogen is locked up in enormous quantities in water (H2O), hydrocarbons (such as methane, CH4), and other organic matter. Efficiently producing hydrogen from these compounds is one of the ...

CENTRIFUGAL PUMPS   A centrifugal pump is of very simple design. The only moving part is an impeller attached to a shaft that is driven by the motor. The two main parts of the pump are the impeller and diffuser.

Principle of Operation A centrifugal compressor accelerates the velocity of the gases (increases kinetic energy) which is then converted into pressure as the gas flow leaves the volute and enters the discharge pipe. Centrifugal compressors consist of stationary casing, Containing rotating impeller (imparts a high velocity of air), Fixed diverging passage (The air is decelerated with rise in static pressure). Impeller may be single or double-sided Basic Components Impellers, Vanes, Volutes, Suction Eyes, Discharge lines, Diffuser Plates, Seals, Shaft, Casing

In steam-using industries, two commonly referred to types of steam are dry steam (also called Saturated steam) and wet steam. Dry steam applies to steam when all its water molecules remain in the gaseous state. It's a transparent gas. Wet steam applies to steam when a portion of its water molecules have given up their energy (latent heat) and condense to form tiny water droplets. Take the example of kettle boiling water. Water is first heated using an element. As water absorbs more and more heat from the element, its molecules become more agitated and it starts to boil. Once enough energy is absorbed, part of the water vaporizes, which can represent an increase as much as 1600X in molecular volume. In a boiler, energy from the fuel is transferred to liquid water in order to create steam. At first, cold water gets warmer and receives energy in the form of “sensible heat”, right until the boiling point.

How to Recognize Fan & Blower Types There are two primary types of fans: Centrifugal fans, and Axial fans. Centrifugal fans use a rotating impeller to move air first radially outwards by centrifugal action, and then tangentially away from the blade tips. Incoming air moves parallel to the impeller hub and it turns radially outwards towards the perimeter of the impeller and blade tips. As the air moves from the impeller hub to the blade tips, it gains kinetic energy. This kinetic energy is then converted to a static pressure increase as the air slows before entering the tangential discharge path. Centrifugal fans are capable of generating relatively high pressures. They are frequently used in ‘‘dirty’’ airstreams (high moisture and particulate content), in material handling applications and in systems operated at higher temperatures.

Use back pressure steam turbines to produce lower steam pressures. Install PHE based hot water system in place of direct steam injection system Ensure that piping, valves, fittings and vessels are well insulated. Use more-efficient steam de - superheating methods. Optimize the deaerator vent rate. Ensure process temperatures are correctly controlled. Maintain lowest acceptable process steam pressures. Reduce hot water wastage to drain. Clean boiler heat transfer surfaces on regular intervals. Install pressure reducing station for low steam pressure.

Cooling Tower Types Cooling towers fall into two main categories: Natural draft and Mechanical draft. Natural draft towers use very large concrete chimneys to introduce air through the media. Due to the large size of these towers, they are generally used for water flow rates above 45,000 m3/hr. These types of towers are used only by utility power stations. Mechanical draft towers utilize large fans to force or suck air through circulated water. The water falls downward over fill surfaces, which help increase the contact time between the water and the air this helps maximize heat transfer between the two. Cooling rates of Mechanical draft towers depend upon their fan diameter and speed of operation . Mechanical draft towers are available in the following airflow arrangements: 1. Counter flows induced draft. 2. Counter flow forced draft. 3. Cross flow induced draft.

Vapour compression refrigeration is used in almost 80 % of the refrigeration industries in the world for refrigeration, heating, ventilating and air conditioning. The application of vapour compression system cycles, using R-12 and R-22 and other halocarbon compounds, had gone deep in the field of refrigeration and air-conditioning before the advent of ozone depletion. The common use of CFC is being phased out due to its impact on the environment, especially to Ozone layer. So, the concerned industry has developed two alternative Refrigerants: one based on Hydro chloro Fluorocarbon (HCFC) and another based on Hydro Fluorocarbon (HFC). The high-grade energy consumption of these devices is very high and the working substance creates environmental problems due to environmental unfriendly refrigerants such as chlorofluorocarbons, hydro chlorofluorocarbons and hydro fluorocarbons. The HCFCs have a 2 to 10% ozone depleting potential as compared to CFCs and also, they have an atmosph...

Is compressed air expensive? Compressed air is actually very expensive because only 10 to 30 percent of the electric energy input reaches the point of end-use. The remaining input energy converts to wasted heat or is lost through leakage. For example, to generate 8 M3/hr it takes 1 HP power. Compressed air leakage of 30 – 40 percent is not uncommon. Carry out periodic leak tests to estimate the quantity of leakage. Detection and repair can reduce leaks to less than 10 percent of compressor volume. Leak repair, when combined with adjustments to compressor controls, can reduce compressor run time, increase equipment life and reduce maintenance . Keep the minimum possible range between load and unload pressure settings. The compressor with the highest part-load efficiency is placed in trim service to handle variations in load. Retrofit with variable speed drives in big compressors, say over 100 kW,

Benefits of Waste Heat Recovery System: Reduce emissions   Recovery of waste heat has a direct effect on the efficiency of the process. This is reflected by reduction in the utility consumption & costs, and process cost. Improve heating system efficiency. Energy consumption can be reduced 5% to 25%. Reduction in the utility consumption & costs, and process cost. Reduction in pollution. A number of toxic combustible wastes such as carbon monoxide gas, sour gas, carbon black off gases, oil sludge.

Heat Energy and Heat Transfer Heat is a form of energy in transition and it flows from one system to another, without transfer of mass, whenever there is a temperature difference between the systems. The process of heat transfer means the exchange in internal energy between the systems and in almost every phase of scientific and engineering work processes, we encounter the flow of heat energy.  Modes of Heat Transfer The heat transfer processes have been categorized into three basic modes: Conduction, Convection and Radiation.

Combustion is a chemical process which involves the burning or rapid oxidation of a fuel. Complete combustion reaction means when a hydrocarbon reacts with sufficient amount of oxygen forming carbon dioxide and water. Incomplete combustion reaction means when a hydrocarbon reacts with insufficient amount of oxygen forming carbon monoxide and water. Incomplete combustion cause environmental pollution. Complete combustion results in more energy than incomplete combustion. Combustion is the process of burning or oxidation, which may be accompanied by light and heat. Oxygen itself does not burn, but it supports combustion. The rate of combustion is influenced by both oxygen concentration and its partial pressure.

Specific Power Consumption Specific power consumption is the ratio of the power consumed by an engine to a certain force such as the amount of output the engine (equipment) produces. It allows engines (equipments) of all different sizes to be compared to see which is the most power efficient. It allows manufacturers to see which engine (equipment) will use the least power while still producing a high amount of output. Specific power consumption of HVAC system The ratio of power consumption (in KW) to the Tons of refrigeration effect produced. Specific power consumption = KW/TR rating = Actual power consumed by the HVAC (KW input)/Tons refrigeration effect =Compressor power, KW/ Net refrigeration capacity (TR)

Incorrect amount of air in fuel combustion accounts for the largest losses in combustion systems. All fuels consist mostly of atomic Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Sulfur (S), minerals (ash) and water (H2O). In practice, mixing is never perfect, a certain amount of excess air is needed to complete combustion and ensure that release of the entire heat contained in fuel oil. One kg of fuel requires a certain minimum of ambient air to be fully combusted. We call this minimum amount of air the   stoichiometric  air. When fuel and oxygen in the air are in perfectly balance - the combustion is said to be  stoichiometric . If an insufficient amount of air is supplied to the burner, unburned fuel, soot, smoke, and carbon monoxide exhausts from the boiler - resulting in heat transfer surface fouling, pollution, lower combustion efficiency, flame instability and a potential for explosion .

The purpose of this page is to cover the basic concepts and principle of heat pump . The heat pump technology In recent years, the air source heat pump technology is the focus of new energy technology in the world. People is familiar with the "pump" is a kind of can increase the potential of mechanical equipment, such as pumps are mainly water from low to high pumping. And air source heat pumps is a kind of energy from the nature of the air, water or soil to obtain the low grade heat energy power, after work, to provide people with high grade heat energy device. Heat must flow from a system at high temperature to one at a lower temperature. As a natural phenomenon, as the water flow from a low height, the heat is always from a high temperature zone to low temperature zone. But people can create machines, like the water from low to high and the water pump, using the heat pump can take the heat from low temperature to high temperature and suction.

Deaeration is the process of removal of oxygen, Carbon dioxide and other non condensable gases from boiler feed water thereby reducing the risk of corrosion in the pressure parts of the boiler. Deaeration of two types namely Mechanical Deaeration and Chemical Deaeration. Mechanical deaeration works on the principle of Henry’s law of physics. Removal of oxygen, carbon dioxide and other non-condensable gases from boiler feed water is vital to boiler equipment longevity as well as safety of operation. The deaerator capacity rating shall exceed the capacity of the steam system it is servicing and have a minimum of 10 minutes storage capacity to the overflow. The deaerator shall be designed for oxygen removal to 0.005 cc/l (7 ppb) or less and carbon dioxide removal to a zero measurable level in the effluent throughout all load conditions between 0% and 100% of rated capacity.

What is specific speed of the turbine? It is defined as the speed of a turbine which will develop unit power under unit head. What are the unit quantities? Unit quantities are the quantities which are obtained when the head on the turbine are unity. Explain about characteristic curves of a hydraulic turbine Characteristic curves of a hydraulic turbine are the curves with the help of which the exact behavior and performance of the turbine under different working conditions can be known. What do you mean by pipe flow?             1. Pipe flow has no free surface and subjected to hydraulic pressure only.             2. The cross section of the flow is fixed

Comparison between fire tube boiler and water tube boiler? The reason is to be found in thermodynamic nature. To get a high efficient water-steam-cycle a high temperature and pressure of the steam is required. The higher pressure and temperature the more effective process you get.  Water tube boilers are better suited for the high pressures and temperatures requirements. Higher pressures require greater wall thickness, but this reduces heat transfer. The practical limit is about 16bar (240psi). Fire / smoke tube boilers are also more prone to catastrophic failure. The water - and energy - in a Scotch boiler, say at a modest pressure of 10 bar, is enough to launch the boiler through bulkheads and accident in the boiler room. Water tube boilers lend themselves to modern modular construction techniques more than fire tube boilers. Even in field-erected water-tube boilers, most of the fabrication - for instance, water-wall panels - can be done in a workshop under contro...

Heat exchangers are devices designed to transfer heat from one fluid to another without the fluids coming into contact. There are a wide variety of applications for heat exchangers, for example: radiators, air conditioning and power plants. Mechanism of Heat Transfer by Conduction The transfer of heat energy by conduction takes place within the boundaries of a system. The rate equation which describes this mechanism is given by Fourier Law Where    = rate of heat flow in X-direction by conduction in J/S or W, k = thermal conductivity of the material. It quantitatively measures the heat conducting ability and is a physical property of t he material that depends upon the composition of the material, W/mK, A = cross-sectional area normal to the direction of heat flow, m2, dT/dx = temperature gradient at the section, Thermal Conductivity of Materials Thermal conductivity is a physical property of a substance and In g...

Specific energy consumption of furnaces is very useful for comparing with design values or best practical norms. There are many factors affecting furnace specific energy consumption such as capacity utilization of furnaces, final heating temperature, use of inefficient burner, excess air ratio, improper insulation/refractories, high stack loss etc... Specific energy consumption test is the key for assessing current performance of furnaces and finding the scope for improvements and productivity. Specific Energy Consumption  = Amount of Energy consumed in Kcal/hr / Quantity of material processed, Ton/Hr If Amount of fuel consumed is 370 Kg/hr and amount of billet processed is 6 Ton/hr.. Then Specific fuel consumption of furnace 

Evaporation ratio of boiler reduces with time, due to poor combustion, poor operation and maintenance and heat transfer fouling. Deterioration of fuel quality and water quality also leads to poor performance of boiler. Hence for performance test it is necessary to find out current evaporation ratio of boiler. Any observed abnormal deviations could therefore be investigated to pin point the problem area for necessary correction action. Evaporation Ratio = Quantity of steam generation/ Quantity of fuel consumption Boiler evaporation ratio also known as steam to fuel ratio. Boiler evaporation ratio monitoring is best for any boiler when its own performance is compared on day to day basis as a performance indicator. A drop in boiler evaporation ratio indicates a drop in boiler efficiency. Values of evaporation ratio for different type of fuels are as below: Coal Fired Boiler              -  ...

Specific Power Consumption   Specific power consumption is the ratio of the power consumed by an engine to a certain force such as the amount of output the engine (equipment) produces. It allows engines (equipments) of all different sizes to be compared to see which is the most power efficient. It allows manufacturers to see which engine (equipment) will use the least power while still producing a high amount of output. Specific power consumption of Air compressor The ratio of power consumption (in KW) to the volume delivered at ambient conditions. Specific power consumption= Actual power consumed by the compressor/Measured free air delivery If the measured flow is 250 m3/hr and actual power consumption is 30KW. Then Specific power requirement = 20 KW/250 m3/hr

Explain impulse turbine? If at the inlet of the turbine the energy available is only kinetic energy  the turbine is known as impulse turbine. Explain Reaction turbine? if at the inlet of the turbine the water possesses kinetic energy as well as pressure energy the turbine is known as reaction turbine. Explain tangential flow turbine. If the water flows along the tang ent of the runner, the turbine is known as the tangential flow turbine. Explain radial flow turbine. If the water flows in the radial direction through the runner the turbine I called radial flow turbine. Explain inward flow radial turbine. If the water flows from outwards to inwards radially the turbine is called inward radial flow turbine.