0% 0 Chemical Heat Transfer Quiz 6 1 / 50 Pick out the wrong statement? Thermal conductivity of a dry material is more than that of the damp material Ratio of its capacity to economy equals the steam consumption in kg/hr in an evaporator Vaporisation of organic substances in evaporator mostly causes foam formation Bubble size increases with the dynamic viscosity of the liquid in case of nucleate pool boiling 2 / 50 Nusselt number is the ratio of the temperature gradient at the wall to__________? That across the entire pipe Temperature difference None of these Heat flux 3 / 50 Pick out the wrong statement? The maximum is the emissive power of a surface at a temperature T1 occurs at a wavelength of ?1. If the surface temperature is halved, the maximum in the emissive power would occur at a wavelength of 0.5 ?1 In process heat exchangers, saturated steam is preferred over the superheated steam A body at 925 K emits an energy of 1.42 x 1011?W/m2 (? is the Stefan-Boltzmann constant) in the wavelength band between 3 ?m to 4 ?m. The fraction of this energy in the total energy emitted over the entire wavelength range is equal to emissivity When a vertical plate is heated in infinite air environmental under natural convection conditions, the velocity profile in air, normal to the plate, exhibits a maximum 4 / 50 Overall heat transfer co-efficient for cooling of hydrocarbons by water is about__________? 1000 ? 1500 BTU/hr. ft.2?F 50 -100 W/m2.?K 50 -100 BTU/hr. ft.2?F 50 -100 Kcal/hr.m2.?C 5 / 50 Pick out the wrong statement/ The emissivity of a surface decreases, if it gets corroded by atmospheric environment The emissivity of a non-metallic surface decreases with increase in the temperature The emissivity of a surface increases with increase in surface roughness The emissivity of a polished surface is quite low 6 / 50 Pick out the wrong statement ? Sherwood number in mass transfer corresponds to Nusselt number in heat transfer and Schmidt number to Prandtl number Hot gases at moderate pressure are usually in the shell side of shell and tube heat exchangers. At higher pressure, however, it is customary to put gas in the tube side Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger In drying a solid containing moisture above the critical moisture content the number of degrees of freedom is 2 7 / 50 Multipass heat exchangers are used________? To obtain higher heat transfer co-efficient and shorter tube Because of simplicity of fabrication For low heat load To reduce the pressure drop 8 / 50 Prandtl number is given by___________? CP ?/a ?/h CP CP ?/k hD/k 9 / 50 Planck?s distribution law is valid for ______ bodies? Black Coloured White All A., B. & C. 10 / 50 Pick out the wrong statement ? The controlling resistance in case of heating of air by condensing steam is in the air fil In case of a 1 ? 2 shell and tube heat exchanger, the LMTD correction factor value increases sharply, when a temperature cross occurs The log mean temperature difference (LMTD) for counter flow and parallel flow can be theoretically same when any one of the fluids (hot or cold fluid) passes through the heat exchanger at constant temperature Phase change in case of a pure fluid at a given pressure from liquid to vapor or vice-versa occurs at saturation temperature 11 / 50 Overall thermal resistance for conductive heat transfer through a series of flat resistances is equal to the______? Sum of all resistances Minimum resistance presents in the series Average of all resistances Maximum resistance in the series 12 / 50 Mechanical recompression evaporation is used in the production of______? Fruits jam Alcohol Salt Distilled water 13 / 50 Pick out the wrong statement ? Finned tube heat exchangers are suitable for heating air by steam Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature Economy of a multiple effect evaporator is not influenced by the boiling point elevation Double pipe heat exchangers are mostly used in the field of refrigeration 14 / 50 Pick out the wrong statement ? The amount of heat involved in the condensation or vaporisation of 1 kg of a fluid is the same Propagation velocity for travel of heat radiation through vacuum is equal to the velocity of the light Heat transfer by radiation cannot occur across an absolute volume In case of a shell and tube heat exchanger, the pressure drop through the shell is proportional to the number of times the fluid crosses the bundle between baffles 15 / 50 Multiple effect evaporators ar used to_____? Increase the steam economy & decrease the capacity Decrease the steam economy & increase the capacity Increase the steam economy & the capacity Decrease the steam economy & the capacity 16 / 50 Prandtl and Reynold?s analogies are identical for Prandtl number value of___________? 0.5 1 0 5 17 / 50 Pick out the wrong statement? Orifice baffles are never used in a shell and tube heat exchanger Pressure drop on the shell side of a heat exchanger depends upon tube pitch also Split ring type and pull through type floating heads are two commonly used floating heads is heat exchangers In a horizontal tube evaporator, surface blanketing by air is avoided 18 / 50 Mode of heat transfer involved in the cooling of air cooled internal combustion engine is__________? Forced convection Natural convection Conduction None of these 19 / 50 Prandtl number is the ratio of_________? Momentum diffusivity to thermal diffusivity Thermal diffusivity to mass diffusivity Momentum diffusivity to mass diffusivity Thermal diffusivity to momentum diffusivity 20 / 50 Multiple effect evaporation accounts for_____? Lower operating costs Investment economy Steam economy None of these 21 / 50 Pick out the wrong statement ? In case of heat transfer by purely forced convection, GR/Re 2 ? 1 Distillation process is not the same as evaporation The equivalent diameter of heat transfer for a duct of square cross-section (having each side as ?x?) is equal to 4x The effectiveness of nucleate boiling depends basically on the ease with which the bubbles are formed and detached from the heating surface 22 / 50 Nusselt number (for forced convection heat transfer) is a function of the ______ number? Both A. & B. Reynolds Prandtl Neither A. nor B. 23 / 50 Nucleate boiling is promoted _____? On polished surfaces On rough surfaces None of these In the absence of agitation 24 / 50 Pick out the correct statement ? Higher is the temperature of the radiating body, higher is the wavelength of radiation The wavelength corresponding to maximum mono-chromatic emissive power increases with rise in temperature Solid angle subtended by the finite surface at the radiating element is called the angle of incidence Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder 25 / 50 Pick out the wrong statement? The liquor left after the removal of crystals from a solution is called mother liquor Super saturation of the solution is the driving potential for a crystal growth Swenson-Walker crystalliser is a batch crystalliser The first stage of crystal formation is called nucleation 26 / 50 Pick out the wrong statement ? Longitudinal fins are used in extended surface heat exchangers, when the direction of fluid flow is parallel to the axis of the tube S.I. unit of fouling factor is Watt/m2.?K In a shell and tube heat exchanger, the shell pressure drop is maximum for orifice baffles Superheated steam is preferably not used for process heating because of its low heat transfer film co-efficient 27 / 50 Prandtl number for most of dry gases is about_________? 150 70 0.72 0.001 28 / 50 Pick out the wrong statement ? In industrial practice, sub-cooling of condensate is required, when the condensate is a volatile liquid and is to be transferred for storage The condensing film co-efficient is about 3 times lower for vertical condenser as compared to the equivalent horizontal condenser for identical situation Overall heat transfer co-efficient in a heat exchanger is controlled by the value of the film coefficient, which is higher Film co-efficient for vaporisation decreases as a result of vapor binding 29 / 50 Pick out the wrong statement ? None of these The boiling point of a solution is affected by liquid head as well as boiling point elevation Filmwise condensation takes place on non-wettable surfaces Heat transfer from a hot body to cold body by the emission of heat waves is called radiation 30 / 50 Pick out the wrong statement? Corrosive liquid is normally passed through the tubes in a shell and tube heat exchanger Steam jet ejector is used for vapor compression in a thermal recompression evaporator The capacity of an evaporator is reduced by the boiling point elevation Heat sensitive materials should be concentrated in high pressure evaporators 31 / 50 Out of the following four assumptions used in the derivation of the equation for LMTD [LMTD = (?t1 ? ?t2)/ln(?t1/?t2)], which one is subject to the largest deviation in practice ? No partial phase change in the system Constant overall heat transfer co-efficient. Constant rate of fluid flow Constant specific heat 32 / 50 Pick out the wrong statement? None of these Unit of thermal conductance is W/?K Thermal conductance of a wall of thickness ?L?, thermal conductivity ?k? and heat flow area ?A? is kL/A Reciprocal of the resistance to heat flow is called thermal conductance 33 / 50 Pick out the wrong statement? The Colburn jH factor for heat transfer is given by Nst Npr The ratio NNu/NRe . Npr is called the Stanton number The Peclet number is a measure of the ratio of energy transport by convection to that by conduction Fluid movement under the influence of buoyant forces resulting from change in density takes place in case of natural convection 34 / 50 Pick out the wrong statement ? ?Solvates? are chemical compounds formed by solute with their solvents. When water is the solvent, then it is called a ?hydrate? In case of short tube vertical evaporators, area of central downtake is about 50 to 100% of the total tube cross-sectional area In heat exchanger calculations (?t) weighted is used in place of ?t, when it involves more than one sequence of heating or cooling i.e., desuperheating & condensation or condensation & sub-cooling Heat transfer co-efficient during nucleate boiling is not influenced by the agitation imparted 35 / 50 Prandtl number is the reciprocal of______? Thermal diffusivity/Momentum diffusivity Thermal diffusivity ? Mass diffusivity Thermal diffusivity ? Momentum Mass diffusivity ? Momentum diffusivity 36 / 50 Multiple effect evaporation is generally recommended, when the________? Large scale evaporation of liquor is needed Fuel is cheaply available Corrosive liquids are to be concentrated Evaporation on small scale is to be done 37 / 50 Pick out the correct statement ? In steady state heat conduction, the only property of the substance which determines the temperature distribution, is the thermal conductivity In heat transfer by forced convection, Grashoff number is very important 1 kcal/hr.m.?C is equal to 1 BTU/hr. ft.?F In unsteady state heat conduction, heat flows in the direction of temperature rise 38 / 50 Prandtl and Reynolds analogy are same, when Prandtl number is______? 1 > 2 1.5 0.5 39 / 50 Open pan evaporators are preferred to be used, when the solution to be concentrated is__________? Scaling Corrosive Highly viscous Salty 40 / 50 Overall heat transfer co-efficient of a particular tube is U1. If the same tube with some dirt deposited on either side has coefficient U2, then__________? U1 > U2 U2 > U1 U1 = dirt factor ? U2 U1 = U2 41 / 50 Pick out the wrong statement ? Absorptivity of a body approaches unity in case of diffuse reflection With change in temperature, the radiant energy emitted by a black body remains unchanged Absorptivity of a perfectly black body is unity Value of Stefan-Boltzmann constant is 4.876 ? 10-8 KCal/m2.hr.?K4 42 / 50 Prandtl number is the ratio of_________? Thermal diffusivity to momentum diffusivity Momentum diffusivity to thermal diffusivity Thermal diffusivity to mass diffusivity Mass diffusivity to thermal diffusivity 43 / 50 Peclet number (Pe) is given by______? Pe = Re.Pr Pe = Pr/Re Pe = Re/Pr Pe = Nu.Re 44 / 50 Minimum recommended baffle spacing in a shell and tube heat exchanger is about (where, D = shell diameter)? 0.5 D 0.66 D 0.2 D 0.80 D 45 / 50 Natural convection is characterised by___________? Reynolds number Peclet number Grashoff number Prandtl number 46 / 50 Nusselt number is the ratio of the ______? Temperature gradient of the wall to that across the entire pipe Temperature difference to the temperature gradient at the wall None of these Heat flux at the wall to that across the entire pipe 47 / 50 Pick out the correct equation ? jH = (St)1/3(Pr) = f/2 jH = (St)(Pr)2/3 = f/2 jH = (St)(Pr)1/3 = f/2 jH = (St)2/3(Pr) = f/2 48 / 50 Nusselt number is a function of Prandtl number and _______ number of fluid in natural convection heat transfer? Grashoff Reynolds Stanton Biot 49 / 50 Out of 100 kcal/second of incident radiant energy on the surface of a thermally transparent body, 300 kcal/second is reflected back. If the transmissivity of the body is 0.25, the emissivity of the surface will be__________? 0.55 0.35 0.45 0.85 50 / 50 Mode of heat transfer in which the fluid moves under the influence of changes in fluid pressure produced by external work is called________? Radiation Forced convection Natural convection Conduction Provide Your Information Your score is The average score is 0% LinkedIn Facebook Twitter 0% Restart quiz Send feedback Share on facebook Facebook Share on twitter Twitter Share on whatsapp WhatsApp PrevChemical Heat Transfer Quiz 5 Chemical Heat Transfer Quiz 7Next
