Mechanical engineer interviews questions

Raw or filtered makeup water contains dissolved minerals and insoluble matter that pose a serious threat to efficient cooling. Microbiological organisms, dirt or silt, dissolved minerals and gases, if left untreated, can concentrate and cause serious reductions in heat transfer efficiency, increased maintenance problems, or even a total system failure. As the cooling water evaporates, contaminants are allowed to concentrate in the system. Contaminants enter the system either through the makeup water or from the air via the cooling tower. If left untreated, high concentrations of impurities in open recirculating systems can lead to a number of serious problems, including: 1. Scale 2. Fouling 3. Microbiological growth 4. Corrosion 1) Scale:- The most serious side effect of scale formation is reduced heat transfer efficiency. Loss of heat transfer efficiency can cause reduced production or higher fuel cost. If heat transfer falls below the critical level the entire s...

Approach It is the difference between cold-water temperature (sump) and wet bulb temperature. Approach = Outlet temp - Wet Bulb Temp. Range It is the difference between Inlet water temperature and outlet water temperature. Range = Inlet temp. - Outlet temp. Fill or Packing The structural system, which keeps the water evenly, distributed as it falls through the tower. Wet Bulb temperature The wet bulb temperature of the air is the lowest temperature at which water can be cooled by evaporation. The wet bulb temperature is also the dew point of the ambient air. Dew point temperature The dew point of the air, which is also the coldest temperature to which water can be cooled by passing it through air. Wet bulb temperature is normally determined by using a psychrometer that contains a thermometer in contact with a water wetted wick. Blow down    It is the controlled discharge of recirculating water to waste that is necessary to limit the solids in the ...

The natural draft or hyperbolic cooling tower is designed to take advantage of the temperature differences between the ambient air and the hotter air inside the tower. The design creates a chimney effect that causes the cold air at the bottom of the tower to push the warmer air out the top. The natural draft-cooling tower further divided in two types. Cross flow:- In a cross flow tower, air is drawn across the falling water. In this design the fill is located outside the tower. Counter flow:- The fill is contained within a counter flow tower since the air is drawn up and through the failing water. Design selection depends upon conditions at the particular site.

Maintenance :- maintenance is a set of organized activities that are carried out in order to keep an item in its best operational condition with minimum cost acquired.

Predictive Maintenance Definition:- Predictive Maintenance as the name implies predicting the failure before it occurs, identifying the root causes for those failures symptoms and eliminating those causes before they result in extensive damage to the equipment.

Corrective Maintenance system:- 1. Organized maintenance work intended to restore a failed unit. 2. It includes different types of actions like typical adjustments to redesign equipment's. 3. It is a one time job and each corrective maintenance activity undertaken should be completed fully.

Preventive Maintenance systems:- 1. Proper identification of all the items, their documentation and coding. 2. Inspection of plant and equipment at regular intervals. 3. Proper cleaning and lubrication of equipment's. 4. It helps to upkeep the machines through minor repairs.

The main problem associated with the use of hard water in the different applications is scale. Scale is a precipitate deposited on surfaces (e.g. inner pipe surfaces) that are in contact with water.             Scale is formed by degree of super saturation high saturation index enhances scale formation           Ca2+ + 2 HCO3- CaCO3 + H2O + CO2 Scale is formed by Temperature           Increase temperature, decreases solubility of CaCO3 and enhances scale formation   Scale is formed by PH

How hard water is formed in nature? When the concentration of calcium and bicarbonate increases due to the intensive reactions of rain water when it falls on carbonate rocks this may cause water hardness.

As ocean water evaporates, it form clouds, which condenses and precipitates rain. As rain water falls it encounters carbon dioxide and reacts with it to form a mild acid (Carbonic Acid). CO2 + H2O = H2CO3 Thus as rain water comes into contact with limestone in the earth, the limestone dissolves and goes into solution with the water. CaCO3 + H2CO3 = Ca + 2 HCO3 Surface waters also encounter carbon dioxide from the decay of organic materials.

What Is Hard Water? Water with higher content of alkali earth (Ca, Mg), bicarbonates is called hard water.

In areas of deposit formation, dissolved solids, specifically Calcium and magnesium hardness constituents can precipitate from cooling water as the temperature increases. Deposits accumulate on the heat transfer surfaces as sulphates and carbonates, the magnitude of which is dependent on the water hardness, the dissolved solid content, local temperatures and local flow characteristics. Scales can reduce heat transfer rates and lead to loss of mechanical strength of component parts, this can be exacerbated by the presence of oils and metal oxides. Calcium Carbonate Appears as a pale cream, yellow deposit formed by the thermal decomposition of calcium bi-carbonate ; Ca(HCO3)2 + Heat becomes CaCO3 + H2O + CO2 Magnesium Silicate A rough textured off-white deposit found where sufficient amounts of Magnesium are present in conjunction with adequate amounts of silicate ions.Silicate deposit is a particular problem for systems which utilize silicate additives for corrosion p...

Sensible heat When an object is heated, its temperature rises as heat is added. The increase in heat is called sensible heat. Similarly, when heat is removed from an object and its temperature falls, the heat removed is also called sensible heat. Heat that causes a change in temperature in an object is called sensible heat. Latent heat All pure substances in nature are able to change their state. Solids can become liquids (ice to water) and liquids can become gases (water to vapor) but changes such as these require the addition or removal of heat. The heat that causes these changes is called latent heat. Latent heat however, does not affect the temperature of a substance - for example, water remains at 100°C while boiling. The heat added to keep the water boiling is latent heat.

Steam pipes IBR steam pipe means any pipe through which steam passes from a boiler to a prime mover or other user or both if pressure at which steam passes through such pipes exceeds 3.5 kg/cm2 above atmospheric pressure or such pipe exceeds 254 mm in internal diameter and includes in either case any connected fitting of a steam pipe. Steam boiler Steam boilers under IBR means any closed vessel exceeding 22.75 liters in capacity and which is used expressively for generating steam under pressure and includes any mounting or

Temperature is the measure of the relative warmth or coolness of an object. For measurement of Temperature there are two scales of measurements, one is “Fahrenheit” and the other is “Centigrade” or “Celsius”. The arbitrary reference taken is the freezing point of water under atmospheric conditions. This point at which water freezes to a solid state is considered as ZERO in Celsius or Centigrade Scale. In MKS systems, the unit of temperature is degree Centigrade (C) In FPS system the unit of temperature is degree. Fahrenheit (F) In SI system the unit of temperature is degree Celsius (C) 10 Centigrade = 10 Celsius.

The ideal gas law is a combination of all the gas laws. The ideal gas law can be expressed as PV = mRT        P is the pressure in atm pre.       V is the volume in liters        m is the mass of the gas considered        R is a constant          T is the temperature in Kelvin

Charles's Law can be stated as the volume occupied by any sample of gas at a constant pressure is directly proportional to the absolute temperature. V / T =constant V is the volume T is the absolute temperature (measured in Kelvin) Charles's Law can be expressed into two other useful equations. 1) V1 / T1 = V2 / T2 2) V2 = V1 (T2 / T1)

STP is Standard Temperature and Pressure.  STP is 0 o Celcius and 1 atmosphere of pressure. Gases properties can be compared using STP as a reference. 

Boyle's Law states the volume of a definite quantity of dry gas is inversely proportional to the pressure, the temperature remains constant. Mathematically Boyle's law can be expressed as P 1 V 1 = P 2 V 2

The manometer is one of the simplest tools for measuring gas pressure differences. A manometer is a u-tube.  One side of the "U" is open to atmosphere and the other side is connected to a closed container. The "U" is filled with a fluid. If both sides of the "U" have the same liquid levels then the pressure inside and the pressure outside are the same. The difference between the liquid levels equals the pressure difference between inside and outside. The mercury level will be lower on the side with greater pressure. The higher pressure "pushes" the mercury down

The barometer is the simplest instrument for measuring atmospheric pressure. The earth's atmosphere at sea level has a weight of 14.7 pounds over a square inch of surface. This weight changes as the temperature and composition of the air mass changes. A barometer uses a substitute column of mercury fluid in place of the air.

Pressure is defined as ‘Force per Unit Area’. Pressure is defined as the force exerted by an object over the surface of unit Area.                                                           i.e. pressure =     force /  area In practice it is expressed or measured in following units,   N/m 2 , Kg/cm 2  , pascal (1Pascal= 1 N/m 2 ), 

A part per million is one of aty in one million of another quantity. Parts per million is abbreviated to "ppm". To convert a concentration from v/v to ppm the concentration must be in ml/ml, L/L, etc. then multiply by 10 6 to get ppm v/v. In the same way w/w concentration can only be converted to ppm using the factor "10 6 " if the concentration is expressed in grams per gram (g/g),

Density of a substance signifies how densely it is packed with mass. Mathematically it is expressed as “mass per unit volume”  i.e kg/m 3 it also termed as “Mass density” and denoted by r. Density or Mass density, r = m/v Depending upon the Units of Measurements, the Density is expressed in various units such as “gm/cc”or “kg/ m 3

The specific volume of a substance is its volume per unit mass i.e m3/kg. The unit of specific volume is m3/kg. One kilogram of air at 0 0 C and under an absolute pressure of 1.0332 kg/cm2 (760 mm of Hg) has volume of 0.7734 m 3.

For Air compressors that have start/stop or load/unload controls, there is an easy way to estimate the amount of leakage in the system. The Leak test method involves starting the compressor when there are no demands on the system. A number of measurements are taken to determine the average time it takes to load and unload the air compressor. Total leakage in percentage in air network can be calculated as:  Leakage (%) = (T x 100) / (T + t )  where,  T = on-load time,    t = off-load time. Leakage quantity will be expressed in terms of the percentage of compressor capacity lost. The percentage lost to leakage should be less than 10% in a well maintained system. 

Volumetric efficiency = Free air delivered m3/min / Compressor displacement  Compressor Displacement = Π/4 x D2 x L x N x S x n Where, D = Cylinder bore, meter;  L = Cylinder stroke, meter;  N = Compressor speed rpm; 

Isothermal efficiency is calculated as follows:  Isothermal Efficiency=Actual measured input power / Isothermal Power    Isothermal power (kW) = P1 x Q1 x loger/36.7    Where   P1 =  Absolute intake pressure kg/ cm2;   Q1 =  Free air delivered m3/hr; and   r  =  Pressure ratio P2/P1.  

Isolate the compressor along with its individual receiver that are to be taken for a test from the main compressed air system by tightly closing the isolation valve or blanking it, thus closing the receiver outlet. Open the water drain valve and drain out water fully and empty the receiver and the pipeline. Make sure that the water trap line is tightly closed once again to start the test. Start the compressor and activate the stopwatch. Note the time taken to attain the normal operational pressure P2 (in the receiver) from initial pressure P1. Free Air Delivery is to be corrected by a factor (273 + t1) / (273 + t2) Calculate the capacity as per the below formula given. Q = P2 - P1/P0 x V/T NM3/Minute

The basic types of mechanical agitators are: Paddle Agitators This is one of the most primary types of agitators with blades that reach up to the tank walls. Paddle agitators are used where an uniform laminar flow of liquids is desired. Anchor Agitators This simple agitator consists of a shaft and an anchor type propeller and can be mounted centrally or at an angle. It is mainly used in reactors. Radial Propeller Agitators Radial agitators consist of propellers that are similar to marine propellers. They consist of two to four blades that move in a screw like motion, propelling the material to be agitated parallel to the shaft.

The substances which are used to decrease the force of friction between the moving parts of machine in contact are known as Lubricants and the process of decreasing the force of friction between the moving parts of machine in contact is known as Lubrication. When on surface of machinery moves over the another surface, resistance to relative motion of the surfaces arises. When we look at the solid surface it appears smooth to naked eye , but this smooth surface shows irregularities of projections and cavities when viewed under high power microscope. cavities in surfaces: When one such surface is placed over another, its projections fall into the cavities of the other and get interlocked .Due to this interlocking , there is resistance to the relative motion of the surfaces. This is called the frictional forces or frictional resistance of friction. In due course of motion, the old projections get broken and deformities arise. Friction: Friction may be defined as the opposing fo...

There are 3 types of lubrication mechanisms 1) Hydrodynamic or Fluid Film or Thick Film Lubrication 2 )Boundary Lubrication or Thin Film Lubrication 3) Extreme Pressure Lubrication 1) Hydrodynamic or Fluid Film or Thick Film Lubrication:-   This type of lubrication is done in those cases where liquid lubricants having minimum viscosity under working conditions are used and at the same time, it should remain in place and separate the moving or sliding parts of the machine. In the fluid film lubrication , the liquid lubricant fills the grooves or irregularities of the sliding surfaces, and forms a thick layer in between them so that there is no direct contact between the sliding surfaces. In this way, the lubricant reduce the wear and tear by reducing the frictional resistance produced by the movement of sliding parts. 2) Hydrodynamic or Fluid Film or Thick Film Lubrication:- The frictional resistance to the movement of the sliding parts is only due to the internal re...

Autonomous maintenance culture the operator shall acquire the ability to look at the quality of the product and the performance of the equipment and notice when something is wrong. The ability of the operator will depends on the following skills below: Ability to detect equipment abnormalities and make improvement in timely fashion (This depends on setting standards for comparison) Ability to understand equipment function and mechanism and the ability to detect causes of abnormalities. Ability to understand the relationship between equipment and quality and ability to predict problems in quality and detect their causes. Ability to make repairs Development of Autonomous maintenance Means Developing Operators Strong in Equipment Related Skills : Operator can detect deterioration of equipment Operator can prevent downtime caused by equipment mis-operation. Operator performs machine set up quickly, efficiently and autonomously. Operator can do simple cleaning, repairs & ...

MTTF could be defined as the expected value of the function of the statistical distribution of failures, which is ∫t f(t)dt; assuming that the distribution of failure is approximated by the exponential distribution (that means having a failure rate constant in time),  the MTTF could be approximated by the reciprocal of the failure rate λ.  again, failure rate for a system , under some approximations, can be obtained testing the component and adding number of failures / time of testing  so MTTF = time in service / number of failures

Material of Shaft: Cast Iron is very rare for shafting nowadays, though a century ago, things were different. The primary problem with Cast Iron is brittleness. This makes it unsuitable as there are few applications where failure would not have serious secondary consequences.  Further, wrought material is readily available and shafts are easily machined from it without much loss of material. This cancels out the advantage of Cast Iron that it is already close to final form when cast and needs less machining. For large shafts with significant turning material loss, forgings are economical and very strong.  A fairly modern development of Cast Iron is Malleable Iron (SG Iron, Spheroidal Graphitic Iron). However, even this versatile material is not much used for shafts The most commonly used shafting materials are EN-8, EN-9 or EN-24 in rolled condition. For higher strength forged material can be used. The residual stresses during the process of rolling or forgi...

Proactive Maintenance - is focusing on eliminating what causes failure. It is also everything that you do before the equipment goes into service. By going to a Proactive maintenance program you are attacking the root cause and that is what takes a program world class. Predictive Maintenance - is based on the condition of lubricant or machine. it is monitoring and predicting machine failure. It focuses on recognizing the symptoms of a failure. Predictive maintenance is generally condition based maintenance. Preventive maintenance  - is generally interval/time based maintenance and sometimes can be an opportunity based maintenance (shutdowns, TAs, etc). Proactive maintenance is generally maintenance tasks generated via studies, analysis, RCA, etc and the purpose is to improve the performance. Of course the studies start in the first place if you feel that there is a need to improve something.

There are two areas of concern in a well designed Pipe Line. One is internal and the second one is external. For external maintenance, again there are two types, one is sub-surface and surface Piping. The internal damage control remains a common issue for both sub-surface and surface Piping which are normally met depending on the Product flow and its corrosive effect.  The usual method of chemical injections to neutralize the corrosive effect has to be seriously looked in to. For External side of sub-surface Piping, Good wrapping technique and selective soil filling with clear sign posts is a good practice for long lasting effect. For external side surface piping, a regular interval painting practice after inspection /grit blasting. . In all cases condition monitoring at regular intervals using intelligent PIG , detection tests and un interrupted cathodic protection will improve the life span of Pipe line.

Machine Maintenance  involves keeping the machine in a healthy state. This involves proper understanding of maintenance models and their applications. Machine maintenance is about about reducing the downtime of the machine or equipment by performing every maintenance task right the first time. The reasons for machinery maintenance are: Most machines are designed with wear components that will have a fixed service life and will require change out at the end of the component's design life. Typically in a bush bearing the shaft is harder than the bearings to allow the replaceable bush to wear than the expensive shaft. Machinery maintenance is all about identifying such components and renewing them at the end of its run before the components fails in service, which can result in secondary damages.  The machines also have lubrication and cooling systems to enhance the life of the components and maintenance is to make such systems work without fail. The machines do handle flu...

Both semi open and closed impellers are used in slurry services. The control of leakage back into suction is usually accomplished with a combination of clearing or expelling vanes on the impeller and close axial clearances. Because these axial clearances increase with wear, pumps should be arranged to allow simple clearance adjustments to maintain performance. Close radial clearances wear quickly when solids are present and cannot be conveniently corrected with external adjustment, and should only be used on low concentrations of fine slurries. An axial clearance arrangement between the impeller inlet diameter and liner is common for providing leakage control for high-wear services. Impeller attachment methods vary by manufacturer and service requirements. Various bolted designs and threaded designs are used successfully. When pumping highly abrasive slurries, the impeller attachment should be protected from wear to optimize service life. An internally threaded impeller is typic...

For greater efficiency air compression should be isothermal as this requires the minimum work input. In practice Isothermal compression is not possible, an ideal Isothermal cycle requires sufficient time to allow all the required heat to be transferred out of the cylinder, practicality dictates that the piston must have a relatively high speed to give a reasonable output, Cylinder cooling on a single stage compressor gives better efficiency but there is a limitation in the surface area to cylinder volume that can be used for cooling effect, but multistage compressors with an efficient extended surface inter stage cooler gives cycle improved compression efficiency better approaching that of the isothermal.

The use of CFCs is now beginning to be phased out due to their damaging impact on the protective tropospheric ozone layer around the earth. The Montreal Protocol of 1987 and the subsequent Copenhagen agreement of 1992 mandate a reduction in the production of ozone depleting Chlorinated Fluorocarbon (CFC) refrigerants in a phased manner, with an eventual stop to all production by the year 1996. In response, the refrigeration industry has developed two alternative refrigerants; one based on Hydrochloro Fluorocarbon (HCFC), and another based on Hydro Fluorocarbon (HFC). The HCFCs have a 2 to 10% ozone depleting potential as compared to CFCs and also, they have an atmospheric lifetime between 2 to 25 years as compared to 100 or more years for CFCs (Brandt, 1992). However, even HCFCs are mandated to be phased out, and only the chlorine free (zero ozone depletion) HFCs would be acceptable. Until now, only one HFC based refrigerant, HFC 134a, has been developed. HCFCs are comparativel...

The cooling effect produced is quantified as tons of refrigeration .(TR). 1 TR of refrigeration = 3024 kCal/hr heat rejected. The refrigeration TR is assessed as  TR = Q x Cp x (Ti – To) / 3024 Where, Q is mass flow rate of coolant in kg/hr Cp is coolant specific heat in kCal /kg deg C Ti is inlet, temperature of coolant to evaporator (chiller) in 0C To is outlet temperature of coolant from evaporator (chiller) in 0C. The above TR is also called as chiller tonnage. The specific power consumption kW/TR

In case of air conditioning units, the airflow at the Fan Coil Units (FCU) or the Air Handling Units (AHU) can be measured with an anemometer. Dry bulb temperatures and wet bulb temperatures are measured at the inlet and outlet of AHU or the FCU and the refrigeration load in TR is assessed as ; TR = Q x Rho x (H in – H out) / 3024 Where,  TR is the Ton of refrigeration Q is the air flow in m3/h  Rho is density of air kg/m3 H in is enthalpy of inlet air kCal/kg H out is enthalpy of outlet air kCal/kg

Bearings are classified in  main two categories: 1) Plain or slid ing contact bearings  2) Rolling or anti-friction bearings Further classification are given below: 1)Plain or slid ing bearings : - In which the rotating shaft has a sliding contact with the bearing which is held stationary . Due to large contact area friction between mating parts is high requiring greater lubrication. Further classified in two categories:   - Metallic sliding bearings - Non-metallic sliding bearings Then further classified based on their loading, lubrication and configurations: Based on loading: - Axial Loading bearings - Radial Loading bearings

Roller Bearings are a type of roller bearing that uses cylinders (rollers) to maintain the separation between the moving parts of the bearing. The purpose of a roller bearing is to reduce rotational friction and support radial and axial loads.They can operate at moderate to high speeds. Compared to ball bearings, roller bearings can support heavy radial loads and limited axial loads (parallel to the shaft). Tapered roller bearings (TRB): Tapered Roller Bearing can take both radial and axial loads and used for gear boxes for heavy trucks, bevel-gear transmission, lathe spindles, etc. Tapered roller bearings are made of high carbon chrome steel. They consist of four basic components including: the cone (inner ring), the cup (outer ring), tapered rollers, and a cage (roller retainer). The cone, cup and rollers carry the load while the cage spaces and retains the rollers on the cone. Needle roller bearings: It use small diameter of rollers. They are used for radial load at s...

The main function of a rotating shaft is to transmit power from one end of the line to the other. It needs a good support to ensure stability and frictionless rotation. The support for the shaft is known as “bearing” . The shaft has a “running fit” in a bearing.

Advantages Plain bearing are cheap to produce and have noiseless operation. They can be easily machined, occupy small radial space and have vibration damping properties. Also they can cope with tapped foreign matter. Disadvantages They require large supply of lubricating oil, they are suitable only for relative low temperature and speed; and starting resistance is much greater than running resistance due to slow build up of lubricant film