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ISAT 413 - Module V: Industrial Systems. Topic 3: Run-around Coil Systems, Regenerative Heat Exchangers, Pinch Technology. Run-around Coil Systems : Definition of Run-Around Coil Systems Design Factors Examples. Run-around coil system of heat recovery.
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Topic 3: Run-around Coil Systems, Regenerative Heat Exchangers, Pinch Technology
A run-around coil heat recovery system is the name given to a linking of two recuperative heat exchangers by a third fluid which exchanges heat with each fluid in turn as shown diagrammatically below.
A run-around coil heat recovery system similar to that on slide 4 is used for a room in which the presence of bacteria rules out any possibility of air re-circulation or a direct recuperative heat exchanger. Air enters the room at 24oC and leaves at 20oC; the average outside air temperature during the annual period of use is 5oC. Assuming that the mass flow rate of air is 2 kg/s, mean specific heat 1.005 kJ/kg-K, that (UA)H = (UA)C = 4 kW/K, and that the specific heat of the secondary fluid is 2.5 kJ/kg-K, calculate:
(i) the required mass flow rate of secondary fluid;
(ii) the temperature of the air leaving the run-around coil;
(iii) the percentage energy saving by using the run-around coil.
(use e - NTU method to analyze run-around coil heat recovery system)
A corrosive gas at a flow rate of 30 kg/s from a process at 300oC is to be used to heat 20 kg/s of water entering 10oC using a run-around oil as shown on slides 8 & 12. Calculate using the data given:
(i) the mass flow rate of secondary fluid required;
(ii) the effectiveness of the overall heat transfer;
(iii) the exit temperature of the water;
(iv) the temperatures of the secondary fluid.
Data: Mean specific heat of gases, 1.2 kJ/kg-K; mean specific heat of water, 4.2 kJ/kg-K; mean specific heat of secondary fluid, 3.8 kJ/kg-K; (UA) for the gas to secondary fluid heat exchanger, 40 kW/K; (UA) for the secondary fluid to water heat exchanger, 200 kW/K.
In a regenerative heat exchanger (sometimes called a capacitance heat exchanger) the hot and cold fluids pass alternately across a matrix of material; the matrix is heated up by the hot fluid then cooled down by the cold fluid so that the process is cyclic.
In (a) matrix B is hot and heats up the cold fluid while matrix A is heated by the hot fluid; in (b) the cold fluid is now heated by matrix A while the hot fluid re-heats matrix B; the valves are then switched over and the cycle commences again as in (a) .
A matrix of material is mounted on a wheel which is rotated slowly through the hot and cold fluid streams as shown above. It is known as the thermal wheel, and Ljungstrom rotary regenerator after its Danish inventor.
A rotary regenerator is used to recover energy from a gas stream leaving a furnace at 300oC at a mass flow rate of 10 kg/s. Heat is transferred to a mass flow rate of air of 10 kg/s entering at 10oC. The wheel has a diameter 1.5 m, giving an approximate face area of 1.6 m2, and a width of 0.22 m; the matrix has a surface area to volume ratio of 3000 m2/m3 and a mass of 150 kg; the rotational speed of the wheel is 10 rev/min. The heat transfer coefficient for both fluid streams is 30 W/m2-K and the mean specific heats at constant pressure for the gas and air are 1.15 kJ/kg-K and 1.005 kJ/kg-K; the specific heat of matrix material is 0.8 kJ/kg-K.
(i) the effectiveness of the heat exchanger;
(ii) the rate of heat recovery and the temperature of the air at exit;
(iii) the air temperature at exit if the rotational speed of the wheel is increased to 20 rev/min;
(iv) the air temperature at exit if the rotational speed of the wheel is reduced to 5 rev/min;
In (a) the hot gases are fed back through the burner and through a matrix to exhaust; while in (b) air is drawn through the matrix and supplied with gas to the burner where combustion takes place; two burners are used in tandem so that continuous combustion can take place.
Double Accumulator Regenerative Heat Exchanger
A double accumulator as shown above is to be installed to recover energy from the air leaving a building. The air leaves the building at 20oC at a rate of 2 kg/s and the mean outside air temperature for the heating season is 5oC. Calculate the rate of the recovery, etc...
For many years the approach to a large network of heat exchangers was either by ‘rule of thumb’ or a systematic mathematical examination of all possible configurations to try to achieve the best layout.
Another approach to network design is given the name Process Integration, or Pinch Technology.
The design of the heat exchanger is based on the minimum allowable temperature difference between the two streams being 20K. Additionalheating and cooling are required to achieve the desire temperatures.
The two lines representing the streams are positioned so as to show a region of overlap which represents the action of the HX in transferring 140 KW. The minimum temperature difference occurs where the two lines are nearest together - this point is called the Pinch point.
The effect of increasing the Pinch temperature difference is twofold; the amount of heat exchange between the two fluids is reduced and the external duties are increased. Note that the slope of the cold stream line is determined by the value of CC, which is 1/CC = 0.5 K/kW.
Considering the design of a system of heat recovery between two (or more) hot streams and two (or more) cold streams to illustrate some fine points of Pinch Technology.
The heat flow capacities and temperatures of four streams are shown in the table below. For the purpose of definition, a hot stream is defined as one which requires cooling to reach its final temperature and a cold stream is one which requires heating to reach its final temperature. The minimum allowable temperature difference between the streams is 20 K.
(Hot Stream Composite)
(Cold Stream Composite)
Cooling load will exceed the heating load by ? kW.
Cooling load will exceed the heating load by 50 kW.