Gandhinagar Institute of Technology Mechanical Engineering (Thermal Engineering) Semester II. Design of Heat Exchange Equipments [ ]

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1 Experiment 1 Study of fundamentals of Fluid Flow and Heat Transfer associated with Heat Exchangers Review questions (1) Significance of dimensionless numbers. (2) Define overall heat transfer coefficient. Derive equation of it without and with fouling on both sides. (3) What are the main selection criteria of a heat exchanger? State requirements of good heat exchanger. (4) Explain rating and sizing in heat exchanger. (5) Explain Heat Exchanger Design Methodology with neat sketch. (6) According to construction features classify heat exchange equipments with neat sketch.

2 Review questions: Experiment 2 Design of heat exchange equipment by using method of LMTD 1) Explain effect of scale formation. 2) Explain importance of LMTD method for Heat Exchanger design. 3) Derive LMTD equation for (i) Counter flow & (ii) Parallel flow Heat Exchanger. Problem: 1) In a double pipe heat exchanger C h = 0.5 C c. The inlet temperature of hot and cold fluids is th1 and tc1. Deduce and expression in terms of t h1, t c1, and t h2 for the ratio of the area of the counter-flow heat exchanger to that of parallel flow heat exchanger which will give the same hot fluid outlet temperature th2. Find this ratio if t h1 = 150 o C, t c1 = 30 o C, and t h2 = 900 o C. 2) A simple counter flow heat exchanger cools kg/s of oil of c p J/kg C from 90 C to 60 C. The cooling water supplied at 20 C with flow rate of kg/s. To increase the capacity of the heat exchanger the cooling water flow rate is increased to kg/s keeping the cooling water inlet temp as previous. Due to increased flow rate the overall heat transfer coefficient increases by 25%. The mass flow rate of oil and inlet temperature of oil are constant as previous. Determine % increase in capacity of heat exchanger due to increase in cooling water flow rate. 3) Feed water heater of a steam generator is in the form of a single pass, shell and tube heat exchanger consisting of 100 tubes (25mm ID, 31mm OD). 500 Lpm of water is heated from 30 C to 70 C by condensing steam at standard atmosphere pressure on shell side. The shell side heat transfer coefficient is 5000W/m 2 K. On waterside, heat transfer coefficient is 1270W/m 2 K and fouling factor is m 2 K/W. Neglect fouling factor on shell side and resistance offered by tube wall. Calculate the required length of the tubes. 4) Calculate for the following cases, the surface area required for a heat exchanger which is required to cool 2320 kg/h of engine oil (c p = 2.4kJ/kg K) from 72 C to 42 C. The cooling water at 15 C has a flow rate of 2200kg/h. For each configuration, the Overall heat transfer coefficient may be taken as 0.28 kw/m 2 C. 1. Single pass parallel flow 2. Single pass counter flow exchanger (one shell pass and four tube passes) exchanger (two shell pass and eight tube passes) 5. Cross flow single pass with both water and oil unmixed 6. Cross flow single pass with water mixed and oil unmixed

3 5) Find the length of counter flow heat exchanger to heat 4000 kg/hr of oil from 10 C to 20 C using hot water at 70 C. The hot water flows with velocity of 75 cm/sec through a copper pipe of ID = 1.8 cm and OD = 2.1 cm. The oil flows through the annulus between this pipe and 3 cm ID steel pipe. Neglect the tube wall resistance and scale resistance. The following properties of oil and water may be used. Use Colburn correlation: h ρvcp Cp μ k 2/3 = Re 0.2 Water Oil Cp (kj/kg K) k (W/m K) ρ (kg/m 3 ) ν (m 2 /sec) X X 10-5

4 Review questions Experiment 3 Design of heat exchange equipment by using method of ε NTU 1) Derive an equation for effectiveness in case of counter flow heat exchanger. 2) Derive an equation for heat transfer effectiveness from first principle, for a simple parallel flow heat exchanger. 3) What is the heat capacity rate? What can you say about the temperature changes of the hot and cold fluids in heat exchangers if both fluids have the same capacity? What does a heat capacity of infinity for a fluid in a heat exchanger mean? 4) Compare the LMTD and ε NTU approach for analysis and design of heat exchangers. Problems: 1) Consider a very long, concentric tube heat exchanger having hot and cold water inlet temperatures of 85 and 15 C. The flow rate of the hot water is twice that of the cold water. Assuming equivalent hot and cold water specifies heats; determine the hot water outlet temperature for the following modes of operation (a) Counter flow, (b) Parallel flow. 2) In an open heart surgery under hypothermic conditions, the patient s blood is cooled before the surgery and rewarded afterwards. It is proposed that a concentric tube flow heat exchanger of length 0.5m is to be used for this purpose with a thin walled inner tube having a diameter of 55mm. If water at 60 C and 0.1kg/s is used to heat blood entering the heat exchanger at 18 C and 0.05kg/s, what is the temperature of blood leaving the heat exchanger and the heat flow rate. Take U o = 500 W/m 2 K,c p of blood = 3.5 kj/kg K. 3) 2.5kg/s of air at 105 C is cooled by passing though a single pass cross flow inter cooler with water flowing through tubes at a rate of 2kg/s entering at 25 C. The overall heat transfer coefficient has been determined to be 150W/m 2 k based on the outside tube surface area of 20.45m 2. To what temperature is the air cooled? Use effectiveness NTU method. Also find outlet temperature of water.

5 4) A chemical of specific heat 3.6 kj/kg K flowing at the rate of 30,000 kg/h enters a parallel flow heat exchanger at 100 C. Cooling water enters the heat exchanger at the rate of 50,000 kg/h at 10 C. The effective heat transfer area is 10m 2 and overall heat transfer coefficient is 1000 W/m 2 K. Calculate (a) Outlet temperature of water and chemical (b) Effectiveness of heat exchanger (c) Area required for counter flow heat exchanger under identical conditions (d) The maximum drop in temp of the hot fluid possible with parallel flow arrangement 5) A liquid (Cp=0.8 KJ/Kg K) is entering a counter flow heat exchanger at 25 C at rate of 2.5 kg/s. It is heated at 750 C by another (Cp=1.0 KJ/Kg K) with flow rate of 2 kg/sec entering at 1000 C. With these things remaining same, what will be percentage change in the area of heat exchanger if liquid is heated up to 600 C instead of 750 C. 6) A heat exchanger has a total outsider surface area of 17.5m 2. It is to be used for cooling oil at 200 C with a mass flow rate of 10,000 kg/hr having a specific heat of 1900 J/kg K water at a flow rate of 3000 kg/hr is available at 20 C as a cooling agent. If the overall heat exchanger coefficient is 300 W/m 2 K based on the outside area, estimate the temperature of oil as it exist from the heat exchanger if (i) the heat exchanger is operated in a parallel flow mode (ii) in a counter flow mode. 7) A simple counter flow heat exchanger is used to cool 2000 lph of oil of specific gravity 0.8 and specific heat J/kg C from 90 C to 60 C. The cooling water at 30 C is supplied at the rate of 600 lph. Calculate the capacity of the unit in Watts. The same unit now operated with parallel flow with t ci = 30 C and m c = 800 lph. The m h & t hi remains constant, while U increases by 20% in this case. Determine the new capacity of the unit in Watts.

6 Experiment 4 Design and analysis of Double-Pipe Heat Exchanger Review Questions: 1) Explain thermal and hydraulic analysis of inner tube and annulus. 2) Explain double pipe heat exchanger and derive the expression for hydraulic diameter and equivalent diameter for hairpin heat exchanger. 3) What is hairpin heat exchanger? Give classification of double-pipe heat exchanger. 4) Explain overall heat transfer coefficient, hydraulic diameter and equivalent diameter for hairpin heat exchanger with multi tube-finned inner tubes. 5) Explain following for tube and tube heat exchanger: i. Pumping power ii. Pressure drop Problems: 1) Engine oil with a flow rate of 5 kg/s will be cooled from 60 to 40 C by sea water at 20 C in a double pipe heat exchanger. The water flows through the inner tube, whose outlet is heated to 30 C. The inner tube outside and inside diameters are do = inches and d i = inches respectively. For the annulus, D o = 4.5 inches and D i = inches. The length of the hairpin is fixed at 3 m. The wall temperature is 35 C. The number of the annulus is 3. The thermal conductivity of the tube wall is 43 W / m.k. Calculate: (1) Heat transfer coefficient in the annulus (2) Overall heat transfer coefficient (3) Calculate pressure drop in the annulus and in the inner tube. 2) Water at a flow rate of 5000 kg/h will be heated from 20 to 35 o C by hot water at 140 C. A 15 C hot water temperature drop is allowed. A number of 1.5 m hairpins of 3 in (annulus inner diameter, Di= m) by 2 in (inner tube do= m and di = m) double pipe heat exchangers with annuli and pipes each connected in series will be used. Hot water flows through inner tube. Fouling factors are Ri = m2-k/w and Ro = m2-k/w. Assume that pipe is made of carbon steel (k = 54 W/m-K). Calculate: (1) Heat transfer coefficient for the fluid flowing through inner tube and annulus (2) The number of hairpins required. Use following properties date and correlation for calculation

7 Properties: Inner tube Annulus ρ = kg/m 3 ρ = kg/m 3 Cp= kj/kg. K Cp= 4.179kJ/kg. K k = W/m.K k = W/m.K μ= x 10-3 Pa.s μ= x 10-3 Pa.s Pr = 1.28 Pr = 5.77 Dittus- Boelter correlation for both the fluids Nu D = 0.023Re D 0.8 Pr n where n= 0.4 for heating and 0.3 for cooling. 3) A double pipe heat exchanger is designed as engine oil cooler. The flow rate of oil is 5 kg/s, and it will be cooled from 60 to 40 C through annulus (ID = m, OD = m). Sea water flows through the tubes (ID = m, OD = m) and is heated from 10 to 30 C. The number of bare tubes in the annulus is 3, and the length of the hairpin is 3 m. Assume that the wall temperature is 35 C. Design calculation give the number of hair pins as 85. Allowable pressure drop in the heat exchanger for both streams is 20 psi. Is this design acceptable? Outline your comments. 4) The objective of this problem to design an oil cooler with seawater. The decision was made to use a hairpin heat exchanger. Fluid Annulus Fluid Tube-side Fluid - Engine Oil - Seawater Flow rate (kg/s) 4 - Inlet temperature ( C) Outlet temperature ( C) Density (kg/m 3 ) Specific heat (kj/ kg. C) Viscosity (kg/m.s) x 10-4 Prandtl number (Pr) Thermal conductivity (W/m.K)

8 Length of the hairpin = 3 m Annulus nominal diameter = 2 in. Nominal diameter of inner tube = 3/4 in. Fin height H f = m Fin thickness δ = 0.9 mm Number of fins = 18 Material throughout = carbon steel Thermal conductivity = 52 W/m.K Number of tubes inside the annulus = 3 Select proper fouling factors. Calculate: (1) Overall heat transfer coefficient for clean and fouled heat exchanger (2) Total heat transfer area of the heat exchanger with and without fouling; OS design (3) Surface area of a hairpin and number of hairpin (4) Pumping powers for both streams

9 Experiment 5 Design and analysis of Shell and tube type heat exchanger Review question: 1. Write brief note on TEMA-Standard for the design of shell and tube heat exchangers. 2. Explain in details type of Baffle as per allocation of streams and its Geometry. 3. Explain rating procedure step by step in shell and tube heat exchanger. 4. Explain design consideration for shell and tube heat exchanger. 5. Explain various TEMA-Standard Shell designs for the shell and tube heat exchangers. 6. What is impingement plate? How does it affect the number of tube count? Draw the four commonly used pitch lay-out patterns. Problem: 1. A shell and tube heat exchanger is to be designed for heating 9000 kg/hr of water from 15 0 C to 85 0 C by hot engine oil (Cp=2.35 kj/kg K) flowing through the shell of the heat exchanger. The oil makes a single pass in the shell, entering at C and leaving at 95 0 C with an average heat transfer coefficient of 400 W/m 2 K. The water flows through 10 thinwalled tubes of 25 mm diameter with each tube making 8 passes through the shell. Calculate the length of tube required for the heat exchanger to accomplish the specified water heating. The heat transfer coefficient on the water side is 3000 W/m2K. 2. A heat exchanger is to be designed to heat raw water by the use of condensed water at 67 o C and 0.2 bar, which will flow in the shell side with a mass flow rate of 50,000 kg/hr. The heat will be transferred to 30,000 kg/hr of city water coming from a supply at 17 0 C (cp = 4184 J/kg K). A single shell and a single tube pass is preferable. A fouling resistance of m2 K /W is suggested and the surface over design should not be over 40 %. A maximum coolant velocity of 1.5 m/s is suggested to prevent erosion. A maximum tube length of 5 m is required because of space limitations. The tube material of carbon steel (k=60 W/m K). Raw water will flow inside straight tubes whose outer diameter is 19 mm and inner diameter is 16 mm. Tubes are laid out on a square pitch with a pitch ratio of The baffle spacing is approximated by 0.6 of shell diameter and the baffle cut is set to 25%. The water outlet temperature should not be less than 40 o C. Consider shell side heat transfer co-efficient 5000 W/m 2 K and tube side it is 4000 W/m2 K. Perform Preliminary analysis.

10 Experiment 6 Design and analysis of Plate type & compact heat exchanger Review question: 1. List applications and advantages of plate type heat exchanger. 2. Compare welded plate heat exchanger and plate and frame heat exchanger. 3. Explain passes and flow arrangements in Gasketed plate heat exchangers. State its application also. 4. Explain typical designs of plate fin heat exchangers used in industries. 5. What is extended surface heat exchanger? Discuss plate-fin and tube-fin heat exchangers with their applications.

11 Experiment 7 Design of evaporator and condenser. Review question: 1. With neat sketch explain forward feed system for multiple effect evaporators and discuss its design considerations. 2. Explain horizontal and vertical shell-side condensers with neat sketch. 3. State design and operational considerations while selecting, as well as design practices of condensers. 4. Explain water cooling evaporators and air cooling evaporators with neat sketch. 5. Explain evaporator and condenser for refrigeration system. 6. Give classification of evaporators. How they are different than other heat exchangers? Explain any one of them in detail. 7. Explain in detail construction and design of industrial condensers. 8. Explain briefly about the type of evaporators used in refrigeration and air-conditioning systems.

12 Experiment 8 Design and analysis of compact heat exchanger Review question: 1. Write brief note on compact heat exchanger. 2. Explain the following: J-factors, fouling factor, Economic analysis of compact heat exchanger. 3. Classify plate fin heat exchangers and tube fin heat exchangers. 4. Explain design procedure of cross flow plate fin compact heat exchanger. Problems: 1. Air at 1 atm and 400 K and with a velocity of U = 10 m/s flows across a compact heat exchanger matrix has A min / A fr = and D h = cm. Calculate heat transfer coefficient and frictional pressure drop for the air side. Take length of the matrix is 0.6 m and h Pr ( / ) /(G cp ) = Properties of air is ρ = kg/m μ = 2.29 X 10 kg/m.s Cp = 1013 J/kg K and Pr = Air at 2 atm and 500 K with the velocity (u ) of 20 m/s flow across a compact heat exchanger matrix (where, ratio of minimum free-flow area to frontal area is 0.78). Calculate the heat transfer coefficient and the frictional pressure drop. Length of the matrix is 0.8 m. Refer table 1 if required for appropriate property selection. Table 1. Effect of Reynolds number on heat transfer and pressure drop characteristics Re h GCp Pr / f Density Ratio at Inlet and Outlet Air at 1 atm and 400 K and with a velocity of U =10 m/s flow across a compact heat exchanger matrix having 8.0-3/8 T geometrical configuration. Calculate the heat transfer coefficient h and frictional pressure drop for air side. The length of the matrix is 0.6m.Use following geometrical data and fluid properties. Refer Table 1 for j and f factors. Tube O.D. = 1.02 cm; fin pitch = 3.15/cm; fin thickness = cm; fin area/total area = 0.839; air passage hydraulic diameter = cm; free-flow area/frontal

13 area, σ=0.534; Heat transfer area/total volume = 587 m /m Properties of air at 400 K P= Kg/m3; μ=2.29 x 10 Kg/m.s; Cp=1013 J/Kg.K; Pr=0.719 Table 1. j and f value for 8.0-3/8 T geometrical configuration Re j f

14 Experiment 9 Design and analysis of regenerative type heat exchanger for low temperature applications Review question: 1. Explain rotary and fixed matrix regenerators with their applications. 2. Explain typical constructional aspects of regenerative heat exchangers. 3. Explain rotary and fixed matrix regenerators with their applications.

15 Experiment 10 Design of Furnaces. Review question: 1. Explain with neat sketch design considerations of heavy diesel fired furnace. 2. State design considerations for a coal based furnace. 3. What are the different methods available in open literature for furnace design?

Mechanical Engineering Department Sheet (1)

Benha University Heat and Mass Transfer Faculty of Engineering at Shoubra 3 rd Year (Power) Mechanical Engineering Department Sheet (1) (1) What is heat exchanger? Mention with brief description and sketches