SCHOOL of ENGINEERING  

BENG2 APPLIED THERMODYNAMICS (THER 205)

Heat Transfer : Heat Exchangers - design

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1. Exhaust gases flowing through a tubular heat exchanger at 0.3 kg/s are cooled from 400°C to 120°C by water (C_p = 4.186 kJ/kgK) entering at 10°C. The specific heat capacity of the gases is 1.13 kJ/kgK. The overall heat transfer coefficient is 140 W/m²K. Calculate the surface area required for a water flow rate of 0.4 kg/s under conditions of:-
(a) parallel flow, and
(b) counter flow. [3.95 m², 3.39 m²]

2. Dry saturated steam at 2.7 bar condenses on the outside of a tube of outer diameter 49 mm and negligible thickness. The external surface heat transfer coefficient is 20 kW/m²K. The cooling water flow rate of 2 kg/s enters the tube at 20°C and leaves at 90°C. Assuming the condensate is saturated liquid determine the condensate mass flow rate and the length of tube required. [0.269 kg/s, 13.75 m]

3. A tubular cross-flow condenser is used to condense 1.2 kg of steam per second. Cooling water enters the tubes 10°C and leaves at 32°C. The mean diameter of each tube is 13 mm and the mean velocity of the water is 3 m/s. Calculate the number of tubes required if the steam enters the condenser dry saturated at 34 kPa and there is no subcooling of the condensate. Calculate also the length of the tubes if the heat transfer coefficient for the condensing steam is 30 kW/m²K.
For turbulent flow in a tube use:-

and

All properties are to be evaluated at the mean bulk temperature. Neglect the thermal resistance of the wall and base calculations on a mean tube diameter of 13 mm. [78; 1.98 m]

4. A counterflow heat exchanger is to be specified for a heat recovery system using waste water to pre-heat an air supply for a warm air central heating unit.
The following conditions apply:

The air flow rate (at atmospheric pressure) is 0.08 kg/s and preliminary considerations have suggested the use of 15 mm dia. copper tubes with air flowing inside the tubes. If the pressure drop on the air side is limited to 0.10 kPa, calculate the number of tubes in parallel and their length, assuming no thermal resistance on the water side and in the tube walls.
You may assume without proof that, on the air side: &
What is the minimum water flow rate to satisfy the worst condition? [43, 1.03 m, 0.0116 kg/s]