1 Lec 14: Heat exchangers

2 Heat Exchangers and mixing devices Heat exchangers are devices which transfer heat between different fluids Mixing devices (also called open heat exchangers) combine two or more fluids to achieve a desired output, such as fluid temperature or quality

3 Heat exchangers are used in a variety of industries Automotive - radiator Refrigeration - evaporators/condensers Power production - boilers/condensers Power electronics - heat sinks Chemical/petroleum industry- mixing processes

4 Heat exchangers can take a variety of shapes

5 Condenser/evaporator for heat pump

6 Cooling towers are a type of heat exchanger.

7 Something a little closer to home..

8

9 Heat Exchangers In a closed heat exchanger, the fluids do not mix. This is a shell-and-tube heat exchanger.

10 Heat Exchangers Your book has very simple examples of heat exchangers. One is counterflow where the fluids flow in opposite directions in the heat exchanger:

11 Heat Exchangers Another type is parallel flow, where the fluids flow in the same direction:

12 Heat Exchangers Yet another type is cross-flow, shown below. These are common in air conditioning and refrigeration systems.

13 With heat exchangers, we have to deal with multiple inlets and outlets

14 TEAMPLAY At steady flow, what is the relationship between

15 What assumptions to make? Ask yourself: See any devices producing/using shaft work? What about potential energy effects? What about kinetic energy changes? Can we neglect heat transfer?

16 Apply conservation of mass on both streams... If we have steady flow, then: AndFluid A Fluid B

17 Conservation of energy can be a little more complicated... I’ve drawn the control volume around the whole heat exchanger. Implications: No heat transfer from the control volume. Fluid A Fluid B

18 Conservation of energy looks pretty complicated: We know from conservation of mass:

19 Conservation of energy equation for the heat exchanger Apply what we know about the mass flow relationships:

20 Heat Exchangers Generally, there is no heat transfer from or to the heat exchanger, except for that leaving or entering through the inlets and exits. So, And, because the device does no work, Also, potential and sometimes kinetic energy changes are negligible.

21 Heat Exchangers - apply assumptions

22 Heat Exchangers After throwing away a bunch of terms, we’re left with: The energy change of fluid A is equal to the negative of the energy change in fluid B.

23 TEAMPLAY How would the energy equation differ if we drew the boundary of the control volume around each of the fluids?

24 Heat Exchangers Now if we want the energy lost or gained by either fluid we must let that fluid be the control volume, indicated by the red.

25 Heat Exchangers The energy equation for one side: Or dividing through by the mass flow:

26 Example Problem Refrigerant 134a with a mass flow rate of 5 kg/min enters a heat exchanger at 1.2 MPa and 50C and leaves at 1.2 MPa and 44C. Air enters the other side of the heat exchanger at 34 C and 1 atmosphere and leaves at 42 C and 1 atmosphere. Calculate: a) the heat transfer from the refrigerant in (kJ/min) b) the mass flow rate of the air (kg/min)

27 Draw diagram R134a INLET T 1 =50C P 1 = 1.2 MPa R134a OUTLET T 2 =44C P 2 = 1.2 MPa AIR INLET T 3 =34C P 3 = 101 kPa AIR OUTLET T 3 =42C P 3 = 101 kPa

28 State assumptions Steady state, steady flow No work Air is ideal gas Kinetic energy change is zero Potential energy change is zero

29 Start analysis with R134a Apply assumptions We can get h 1 and h 2 from tables. The refrigerant mass flow is given.

30 From R134a tables h 1 = kJ/kg h 2 = kJ/kg Plugging back into energy equation:

31 On to part (b) of the problem. We want to get the mass flow of the air... Start by writing the energy equation for the air side: Simplify 000

32 Sample Problem, Con’t If air is an ideal gas, then we can rewrite the enthalpy difference as: Rearrange to solve for mass flow: How do we get the heat transfer rate to/from the air?

33 Almost there!!!!! We can write: so Get specific heat from table: Plug in numbers from here:

34 TEAMPLAY Work problem 5-100E

35 Open heat exchangers (mixers) In an open heat exchanger, the fluids mix.

36 The mixer may look more like a tank

37 Apply conservation equations for steady state, steady flow Mass Energy

38 Look at some typical assumptions Steady state, steady flow No work No heat transfer - not always true Kinetic energy change is zero - usually Potential energy change is zero - usually

39 Look at the impact on the energy equation Physically, what does this equation tell you?

40 Sample Problem Water is heated in an insulated tank by mixing it with steam. The water enters at a rate of 200 lbm/min at 65°F and 50 psia. The steam enters at 600°F and 50 psia. The mixture leaves the tank at 200°F and 48 psia. How much steam is needed?

41 Draw system with knowns: T 1 = 65°F P 1 = 50 psia Water Steam T 2 = 600°F P 2 = 50 psia T 3 = 100°F P 3 = 50 psia Outlet

42 Assumptions Steady state, steady flow No work No heat transfer Kinetic energy change is zero Potential energy change is zero

43 Applying assumptions gives us: Mass Energy We have two unknowns (m 2 and m 3 ) and two equations.

44 Get some properties and complete the solution h 1 = 33.1 btu/lb m h 2 = btu/lb m h 1 = 33.1 btu/lb m

45 TEAMPLAY Work problem 5-96