PPt materi kuliah teknik pendingin pertemuan ke 5

materi ini menerangkan pebahasan tentang refrigenrant dan aplikasinya

1. TEKNIK PENDINGIN Dr. Eng. Rizal Mahmud, S.Pd ., M.T Teknik Mesin Institut Teknologi Adhi Tama Surabaya

2. Condensers Condensers and evaporators 2 ( G.F. Hundy , A.A. Trott. and le. Welch , Refrigeration and Air-Conditioning , 4 th edition, 2008) The purpose of the condenser in a vapour compression cycle is to accept the hot, high-pressure gas from the compressor and cool it to remove first the superheat and then the latent heat, so that the refrigerant will condense back to a liquid. In addition, the liquid is usually slightly subcooled. In nearly all cases, the cooling medium will be air or water. Heat to be Removed Apart from comparatively small heat losses and gains through the circuit, will be Heat taken in by evaporator + heat of compression

3. Condensers and evaporators 3 Again ignoring small heat gains and losses, will be the power input to the compressor, giving Evaporator load + compressor input power = condenser load Condenser load is stated as the rate of heat rejection. Some manufacturers give ratings in terms of the evaporator load, together with a ‘ de-rating ’ factor, which depends on the evaporating and condensing temperatures. Evaporator load x factor =condenser load

4. Condensers and evaporators 4 Air-cooled Condensers The simplest air-cooled condenser consists of a plain tube containing the refrigerant, placed in still air and relying on natural air circulation. Forced convection of the large volumes of air at low resistance leads to the general use of propeller or single-stage axial flow fans. The low specific heat capacity and high specific volume of air implies a large volume to remove the condenser heat. If the mass flow is reduced, the temperature rise must increase, raising the condensing temperature and pressure to give lower plant efficiency. As the condenser load increases the temperature difference between the air inlet (ambient) temperature and the condensing temperature will increase in order to reject heat at a faster rate with the same surface.

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6. Condensers and evaporators 6 Water-cooled Condensers Small water-cooled condensers may comprise two concentric pipes (double pipe), the refrigerant being in either the inner tube or the annulus. Configurations may be straight, with return bends or headers, or coiled. Larger sizes of water-cooled condenser require closer packing of the tubes to minimize the overall size, and the general form is shell-and-tube, having the water in the tubes. The supply of water is usually limited and requires the use of a cooling tower. Other possibilities are worth investigation; for example, in the food industries, large quantities of water are used for processing the product, and this could be passed first through the condensers if precautions are taken to avoid contamination.

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8. Evaporators Condensers and evaporators 8 ( G.F. Hundy , A.A. Trott. and le. Welch , Refrigeration and Air-Conditioning , 4 th edition, 2008) The purpose of the evaporator is to receive low-pressure, low-temperature fluid from the expansion valve and to bring it in close thermal contact with the load. The refrigerant takes up its latent heat from the load and leaves the evaporator as a dry gas. The flow pattern can be one of two types. Either the refrigerant flows continuously through the heat exchanger whilst it evaporates and becomes superheated, or alternatively it resides in a vessel at low pressure whilst it evaporates or from which it is taken to individual coolers, returning as liquid/vapour mixture. The most common type by far is the continuous flow type, referred to as a direct expansion evaporator.

9. Condensers and evaporators 9 ( G.F. Hundy , A.A. Trott. and le. Welch , Refrigeration and Air-Conditioning , 4 th edition, 2008) Air Cooling Evaporators Air cooling evaporators for display cases, cold rooms, blast freezers and air conditioning have finned pipe coils. In all but very small coolers such as domestic and small retail units there will be fans to blow the air over the coil. Construction materials are the same as for air-cooled condensers. Aluminium fins on copper tube are the most common for the halocarbons, with stainless steel or aluminium tube for ammonia. The size of the tube will be such that the velocity of the boiling fluid within it will cause turbulence to promote heat transfer. Tube diameters will vary from 9 mm to 32 mm, according to the size of coil.

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11. Condensers and evaporators 11 Liquid cooling evaporators may be direct expansion or flooded type. Flooded evaporators ( Figure 7.2 ) have a body of fluid boiling in a random manner, the vapour leaving at the top. In the shell-and-tube type, the liquid is usually in the pipes and the shell is some three-quarters full of the liquid, boiling refrigerant. A number of tubes is omitted at the top of the shell to give space for the suction gas to escape clear of the surface without entraining liquid. Gas velocities should not exceed 3 m/s and lower figures are used by some designers. Liquid Cooling Evaporators

12. Condensers and evaporators 12 A sectional arrangement of a flooded shell and tube type is shown in Figure 7.3. The speed of the liquid within the tubes should be about 1 m/s or more, to promote internal turbulence for good heat transfer. End cover baffles will constrain the flow to a number of passes, as with the shell-and-tube condenser.

13. Condensers and evaporators 13 Liquid cooling evaporators may comprise a pipe coil in an open tank, and can have flooded or direct expansion circuitry. Flooded coils will be connected to a combined liquid accumulator and suction separator (usually termed the surge drum), in the form of a horizontal or vertical drum (see Figures 7.2(c) and 7.4 ). The expansion valve maintains a liquid level in this drum and a natural circulation is set up by the bubbles escaping from the liquid refrigerant at the heat exchanger surface.

14. Condensers and evaporators 14 Overall heat-transfer coefficient of Condensers and Evaporators The overall heat-transfer coefficient, for an evaporator or condenser is the proportionality constant, which, multiplied by the heat-transfer area and the mean temperature difference between the fluids, yield the rate of heat transfer. If heat flows across a tube, as in Fig. 12-3, between refrigerant on the outside and water on the inside, for example, under steady-state conditions the rate of heat transfer q in watts is the same from the refrigerant to the outside surface of the tube, from the outside to the inside surface of the tube, and from the inside surface of the tube to the water. ( W. F. Stoecker and J. W. Jones le , Refrigeration and Air-Conditioning , 2 th edition)

15. Condensers and evaporators 15 where q = rate of heat transfer, W = heat-transfer coefficient on outside of tube, W/m2 • K = outside area of tube, m2 = refrigerant temperature, °C = temperature of outside surface of tube, °C k = conductivity of tube metal, W /m • K x = thickness of tube, m = temperature of inside surface of tube, °C = mean circumferential area of tube, m2 = heat-transfer coefficient on inside of tube, W/m2 • K = inside area of tube, m2 = water temperature, °C   (1) (2) (3)   The expressions for q in each of these transfers are, respectively,

16. Condensers and evaporators 16 (4) and (5)   To express the overall heat-transfer coefficient the area on which the coefficient is based must be specified. Two acceptable expressions for the overall heat-transfer coefficient are where = overall heat-transfer coefficient based on outside area, W/m2 • K = overall heat-transfer coefficient based on inside area, W/m2 • K From Eqs . (4) and (5) it is clear that = . The U value is always associated with an area. Knowledge of or facilitates computation of the rate of heat transfer q. To compute the U value from knowledge of the individual heat-transfer coefficients, first divide Eq. (1) by , Eq. (2) by , and Eq. (3) by ,  

17. Condensers and evaporators 17 leaving only the temperature differences on the right sides of the equations. Next add the three equations, giving Alternate expressions for are available from Eqs . (4) and (5)   Equating Eqs . (6) and (7) and canceling q provides an expression for computing the U values The physical interpretation of the terms in Eq. (8) is that and are the total resistances to heat transfer between the refrigerant and water. This total resistance is the sum of the individual resistances.   (6) (7) (8)

18. Condensers and evaporators 18 1. From the refrigerant to the outside surface of the tube 2. Through the tube 3. From the inside surface of the tube to the water .  

19. Condensers and evaporators 19 Equation (9) is applicable to turbulent flow, which typically prevails with the velocities and fluid properties experienced in most commercial evaporators and condensers. Liquid in tubes, heat transfer and pressure drop The expression for the heat transfer coefficient for fluids flowing inside tubes, is of the form where n and m are exponents. The constant C and exponents in the equation are where h = convection coefficient, W/m2 • K D = ID of tube, m k = thermal conductivity of fluid, W /m • K V = mean velocity of fluid, m/s p = density of fluid, kg/m3 μ = viscosity of fluid, Pa • s cp = specific heat of fluid, J /kg • K (9)

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