085

'8 10 20 30 40 50 60 70 80 90 100 Figure 7-7 Multiplying factor for the pressure drops read from Fig. 7-6 in order to correct for temperature. '8 10 20 30 40 50 60 70 80 90 100 Figure 7-7 Multiplying factor for the pressure drops read from Fig. 7-6 in order to correct for temperature. Table 7-4 Equivalent lengths in straight pipe of several fittings, metersf_ Table 7-4 Equivalent lengths in straight pipe of several fittings, metersf_

0487

2-19 Heat-transfer processes used by the human body The primary objective of air conditioning is to provide comfortable conditions for people. Some principles of thermal comfort will be explained in Chap. 4. Since some thermodynamic and heat-transfer processes such as those discussed in this chapter help explain the phenomena presented in Chap. 4, these processes will be explored now.6 From the thermal standpoint the body is an inefficient machine but a remarkably good regulator of its own...

Refrigeration And Air Conditioning

Professor of Mechanical Engineering University of Illinois at Urbana-Champaign Associate Professor of Mechanical Engineering University of Texas at Austin New York St. Louis San Francisco Auckland Bogot Caracas Lisbon London Madrid Mexico City Milan Montreal New Delhi San Juan Singapore Sydney Tokyo Toronto 2-10 Isentropic Compression - 22 2-11 Bernoulli's Equation 23 2-16 Thermal Resistance 28 2-17 Cylindrical Cross Section 33 2-18 Heat Exchangers 33 2-19 Heat-Transfer Processes Used by the...

078

Figure 7-11 Piping arrangements (a) direct return and (b) reversed return. ment. Some major tasks in the design process are to decide on the location of the components, select the pipe size, select the pump, and choose the size of the expansion tank. Two basic piping arrangements are the direct-return and the reversed-return plans (Fig. 7-11). A drawback of the direct-return system is that the pressure difference available to the various heat exchangers is nonuniform. Heat exchanger A in Fig....

0022

However, if a building does not have mechanical ventilation, or if the fans in the system are not operating, infiltration will occur. The volumetric flow rate of infiltration air is rather difficult to determine with any measure of precision. It will vary with the quality of construction, wind speed and direction, indoor-outdoor temperature difference, and internal pressure in the building. One procedure that is often used in load calculations is to estimate the infiltration in...

Cooling Towers And Evaporative Condensers 37

Figure 19-10 Coil section of an evaporative condenser. (Baltimore Air coil Company, Inc.) Figure 19-10 Coil section of an evaporative condenser. (Baltimore Air coil Company, Inc.) section showing the pipe coils, the water spray heads, and the eliminator plates on top that reduce the carryout of water droplets from the condenser. The discharge gas from the compressor condenses inside a bank of tubes over which water is spraying. The air flowing upward through the water spray eventually carries...

Pm 196

The magnitude of hc AA cDm for the n to n + 1 section is h AA 1 . c 4.19L At I- (19-7) cpm hi,n+hi,n + _ha,n+ha,n+l The expression in the parentheses in Eq. (19-7) is 1 Jii - ha)m a quantity calculated in the stepwise integration as in Table 19-1. Example 19-2 Calculate the dry-bulb temperature of the air as it passes through the cooling tower of Example 19-1 if the air enters at 35 C. Solution For section 0-1, hi - ha)m 0.05328. Dividing Eq. (19-7) by 2G yields h A A (4.19)18.8 (0.5) (0.05328)...

0026

K thermal conductivity, W m*K The thermal conductivity is a characteristic of the material, and the ratio kjL is referred to as the conductance. The thermal conductivity, and thus the rate of conductive heat transfer, is related to the molecular structure of materials. The more closely packed, well-ordered molecules of a metal transfer energy more readily than the random and perhaps widely spaced molecules of nonmetallic materials. The free electrons in metals also contribute to a higher...

134

Velocity shows that the final wet-bulb temperatures are approximately the same for a given number of rows of tubes. Figure 8-9 shows a comparison of the condition curves when air enters a given coil at different dry-bulb temperatures but at the same wet-bulb temperature. The essentially identical final wet-bulb temperatures in Table 8-2 and the corresponding section of Table 8-1 can be explained by recalling the following facts. The enthalpy difference between the air and the wetted surface...

85

Figure 9-28 Reset of warm-air supply temperature based on zone load. Figure 9-28 Reset of warm-air supply temperature based on zone load. The control pressures from all zones are connected to a pressure selector that selects the highest pressure and uses it to reset the hot-dugt temperature in a schedule typically as follows Highest control pressure, kPa 50 55 60 Hot-duct setting, C 30 35 40 When at least one zone is calling for full heating, the hot-duct setting moves to its design value of 40...

00

Figure 11-6 Work of compression and power required by an ideal compressor, Refrigerant 22, 4.5 percent clearance, 50 L s displacement rate, and 35 C condensing temperature. value of Afy is large at low evaporating temperatures and drops to zero when the suction pressure equals the discharge pressure_(when the evaporating temperature equals the condensing pressure). The curve of the power requirement in Fig. 11-6 therefore shows a zero value at two points, where the evaporating temperature...

50

0 10 20 30 40 50 60 70 80 90 100 Ethylene glycol, by mass Figure 15-1 Freezing point of ethylene glycol solutions. One of the most important properties of antifreeze solutions is the freezing point, shown in Fig. 15-1. The freezing points form the classical phase diagram shown in skeleton form in Fig. 15-2. The curves of the freezing points show that the solution of the two constituents has a lower freezing point than either substance individually. Figure 15-2 shows possible phases and mixtures...

12

Figure 19-8 Analyzing a crossflow tower by dividing it into sections. Figure 19-8 Analyzing a crossflow tower by dividing it into sections. In Fig. 19-8, for example, the calculation can proceed in turn through section 1, 2, 3, and 4, calculating t1 and h , t2 and h2, t3 and and Drop next to the middle row of sections to calculate sections 5, 6, 7, and 8, and finally compute the sections in the lower row, sections 9, 10, 11, and 12, to obtain the values of i9, f10, fjj, and r12. The procedure...

References

Thermal Environmental Conditions for Human Occupancy, Standard 55-81, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, Ga., 1981. 2. Standard for Ventilation Required for Minimum Acceptable Indoor Air Quality, ASHRAE Standard 62-81, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, Ga., 1981. 3. Handbook and Product Directory, Equipment Volume, American Society of Heating, Refrigerating, and Air-Conditioning Engineers,...

075

At an outdoor temperature of 5 C the performance factor of the heat pump in Fig. 18-5 is 15.5 3.3 4.70. The electric energy to the compressor motor for 1 GJ of heat is For equal costs the electricity rate would be An electric rate lower than 6.86 cents per kilowatthour makes heating by heat pump more attractive than heating by gas. 18-6 Matching heating capacity to the heating load The heating capacity of an air-source heat pump depends upon the outdoor temperature, as demonstrated in Fig....

D C

Example 2-9 Using the data given in Fig. 2-10, determine the heat transfer in watts per square meter through the wall section and the temperature, of the outside surface of the insulation if tQ 0 C and ti 21 C. In the insulated portion of the wall assume that 20 percent of the space is taken up with the structural elements, which are wood studs. Solution From the data given, for each 1 m2 of surface ti-tQ 21 -0 . q j-t- - 9.37 W m2 The temperature at the outside surface of the insulation is...

10

j 10-12(10ILi 10) and I2 1012(10IL2 10) (21-15) Substituting Eq. (21-15) into Eq. (21-13) gives SPL SPLj + 10 log (1 + 10(ILi il2> 10) (21-16) The combined SPL is therefore the SPL due to the source contributing the higher SPL of the two plus a quantity dependent upon the difference SPLj - SPL2. The quantity to be added to SPLj is plotted in Fig. 21-6 from values calculated from Eq. (21-16). The graph shows that when two sources of equal intensity are combined, the total is 3 dB higher than...

5075

Similar computations are performed for other values of te, and the results are shown in Table 14-3. The refrigerating capacity, evaporating temperatures, and condensing temperatures correspond to those derived from the graphical method shown in Fig. 14-6. Users of the successive-substitution method of system simulation should be aware that not all calculation sequences will converge. Other information-flow diagrams than the one used in Fig. 14-7 can be devised to relate the four equations and...

Q

Q The essential differences between the actual and the standard cycle appear in the Q pressure drops in the condenser and evaporator, in the subcooling of the liquid leaving 3 the condenser, and in the superheating of the vapor leaving the evaporator. The stan- dard cycle assumes no drop in pressure in the condenser and evaporator. Because of 2 friction, however, the pressure of the refrigerant drops in the actual cycle. The result of these drops in pressure is that the compression process...

60

Figure 11-7 Refrigerating effect and capacity of ideal compressor, Refrigerant 22, 4.5 percent clearance, 50 L s rate of displacement, and 35 C condensing temperature. 11-8 Refrigeration capacity The refrigeration capacity q is where hl and h4 are the enthalpies in kilojoules per kilogram of the refrigerant leaving and entering the evaporator, respectively. The refrigerating effect hl - h4, increases slightly with an increase in suction pressure, as Fig. 11-7 shows, provided that the enthalpy...

13

Figure 9-18 (a) Outdoor-air control and (b) desired damper position in Example 9-4. Figure 9-19 Control diagram for Example 9-4. Figure 9-19 Control diagram for Example 9-4. 9-15 Humidistats and humidifiers Without humidiflcation many buildings would experience low humidities during the winter. The outdoor air brought in for ventilation has a low humidity ratio, and relative humidities in unhumidified buildings of 10 percent are not uncommon. Humidistats are usually located in the conditioned...

Lfo

Equations (21-7) and (21-8) relate the sound intensity to the amplitude of the pressure fluctuation, while Eq. (21-9) relates the intensity to the sound power, at least for direct radiation from a nondirectional source. The importance of relating these quantities is that sound-measuring instruments are capable of measuring pjms, which is proportional to intensity. Conversion from an intensity measurement to power also demands knowledge of the acoustic characteristics of the room, discussed...

Info

Course, the air entering must also leave by natural means, i.e., exfiltration, or be exhausted by mechanical means. - In commercial and institutional buildings it is considered advisable to control the entry of outside air to assure proper ventilation and minimize energy use. As infiltration is uncontrolled, these buildings are designed and constructed to limit it as much as fpossible. This is done by sealing the building envelope where possible, using vestibules *or revolving doors, or...

Nc

Figure 9-17 Some control valves and switches (a) manual pressure-selecting switch, (b) pressure-electric switch, (c) electric air switch, (d) pneumatic air switch, (e) pressure selector, (f) diverting relay, (g) reversing relay, (h) switching valve. Pneumatic air switch (Fig. 9-17d). Three-way, two-position valve that selects one of two different air pressures dependent upon the control pressure. Pressure selector (Fig. 9-1 le). A high-pressure selector chooses the highest of two incoming...

K

The equation for the mean condensing coefficient for vapor condensing on the outside of horizontal tubes is where N number of tubes in vertical row D OD of tube, m Several investigators have found that the constant 0.725 in Eq. (12-24) agrees closely with experimental results. White9 found the constant to be 0.63, and Goto et al.10 found it to be approximately 0.65. Equations (12-22) and (12-24) are beautiful examples of how equations of motion and energy are combined. They are expressions of...

1

The temperature of one or both fluids may vary as they flow through the heat exchanger, analysis is difficult unless a mean temperature difference can be determined which will characterize the overall performance of the heat exchanger. The usual practice is to use the logarithmic-mean temperature difference (LMTD) and a configuration factor which depends upon the flow arrangement through the heat exchanger. The LMTD is defined as where AtA- temperature difference between two fluids at position...

224

In Example 2-9 if the structural element has been neglected, the total resistance calculated would have been Rtot 3.67 and q would have been 5.73 W, which indicates that the presence of structural elements has a significant effect on the heat-transfer calculation. where U overall heat-transfer coefficient, W m2 K A surface area, m2 Comparing this equation with Eq. (2-12), we see that

Fo 126

Alternate expressions for tQ - ti are available from Eqs. (124) and (12-5) Equating Eqs. (12-6) and (12-7) and canceling q provides an expression for computing the U values o o ii o o m ii The physical interpretation of the terms in Eq. (12-8) is that l U0A0 and fUiAi are the total resistances to heat transfer between the refrigerant and water. This total resistance is the sum of the individual resistances 1. From the refrigerant to the outside surface of the tube jhQA0 3. From the inside...

J

1 2 Figure 13-4 Incremental length of capillary tube. 13-4 Analytical computation of pressure drop in a capillary tube The equations relating states and conditions at points 1 and 2 in a very short length of capillary tube in Fig. 13-4 will be written using the following notation A - cross-sectional area of inside of tube, m2 D ID of tube, m friction factor, dimensionless h enthalpy, kJ kg hj enthalpy of saturated liquid, kJ kg hg enthalpy of saturated vapor, kJ kg AL - length of increment, m p...

Hqq

E power emitted by source, W Eq reference level, W The expression log (E Eq) has units of bels, and the multiplying factor of 10 converts the units into decibels (dB), in which the magnitudes are somewhat more convenient. The reference level Eq can be chosen arbitrarily and is usually 1 pW. Example 21-1 Calculate the sound power level of (a) a whisper that emits a power of 1 nW and (b) a rocket engine that emits 10 MW. 21-7 Intensity level and sound pressure level The sound intensity level IL...

335

Gypsum or plaster board, 15 mm 0.08 Plaster materials, cement plaster 1.39 Gypsum plaster, lightweight, 16 mm 0.066 Wood, soft (fir, pine, etc.) 8.66 Hardwood (maple, oak, etc.) 6.31 Table 4-4 Thermal resistance of unit areas of selected building materials at 24 C mean temperature (cont.) Table 4-4 Thermal resistance of unit areas of selected building materials at 24 C mean temperature (cont.)

1200

Periphery of the wheel and directs it toward the outlet. A fan may have a single or double inlet, depending upon whether the air enters the impeller from one or from both sides. The usual direction of discharge is horizontal, but certain applications call for discharge to be nonhorizontal. Four types of blading are common in centrifugal fans, radial, forward-curved, backward-curved, and airfoil. The forward-curved blade fan is commonly used in low-pressure air-conditioning systems and is the...

1979

Passive Solar Systems, ASHRAE SympASHRAE Trans., vol. 85, pt. 1, pp. 443-477, 1979. 5. Solar Orientation in Home Design, Univ. III. Small Homes Counc. Circ. C 3.2, Urbana, 1945 reissued 1977. 6. F. Trombe et al. Some Characteristics of the CNRS Solar House Collectors, CNRS Solar Laboratory, Font Romeu Odeillo, France, 1976. 7. J. A. Duffie and W. A. Beckman Solar Engineering of Thermal Processes, Wiley, New York,

O G

Figure 17-7 Conditions of solution in Example 17-2. Solution The computation of the mass flow rate incorporates material balances using applicable concentrations of the LiBr in the solution. Two different pressures exist in the system a high pressure prevails in the generator and condenser, while the low pressure prevails in the absorber and evaporator. Since a saturated condition of pure water prevails in the condenser due to simultaneous existence of liquid and vapor, the condensing...

L

T refrigerant temperature Water flow . , . Figure 12-3 Heat transfer be-t water temperature _'__tween refrigerant and water 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...

1406

Te 15 C and tc 40 C, Table 14-4 shows the values of the operating variables as iterations through the calculation loops proceed. The converged values of capacity and evaporating temperature, 109.0 kW and 8.2 C, respectively, from Table 14-4 check with the balance point for 20 C entering-water temperature from Fig. 14-9. In addition, the mathematical simulation shows tc 50.0 C, which checks with the condensing temperature shown in Fig. 14-6 at the evaporating temperature of 8.2 C. 14-9 Some...

S

Figure 12-14 Temperature distributions in a condenser. It is common practice to use Eq. (12-26) anyway with the following justification. Although the temperature difference between the refrigerant and cooling fluid is higher in the desuperheating section than calculated from Eq. (12-26), the convection coefficient in this section is normally lower than the condensing coefficient. The two errors compensate somewhat for each other, and the application of Eq. (12-26) along with the condensing...

Rotary Screw Compressors

Figurell-14 Cross section of the two rotors of a screw compressor. O Figure 11-15 Exploded view of main elements of a screw compressor. (Sullair Refrigeration, Inc.) - compressor. The refrigerant vapor enters one end of the compressor at the top and leaves the other end at the bottom. At the suction position of the compressor avoid is created into which the inlet vapor flows. Just before the point where the interlobe space leaves the inlet port, the entire length of the cavity or gully is...

D 2

friction factor, dimensionless L - length of tube, m Since the pressure drop in the straight tubes in an evaporator or condenser may represent only 50 to 80 percent of the total pressure drop, experimental or catalog data on the pressure drop as a function of flow rate are desirable. If the pressure drop at one flow rate is known, it is possible to predict the pressure drop at other flow rates. The expression applicable to straight tubes, Eq. (12-10), indicates that the pressure drop is...

Vlp 1 2 rl615

The contraction coefficient is a function of the ratio of the areas, it was deter mined experimentally by Weisbach5 in 1855 and is shown in Table 6-3. The form of Eq. (6-15) again shows the pressure drop to be calculated by the V2p 2 group multiplied by a geometry factor. The geometry factor reaches a maximum of approximately -y, which may be compared with the maximum value of the Table 6-3 Contraction coefficients in sudden contractions Table 6-3 Contraction coefficients in sudden contractions

Vc vxvv vW

Therefore r vc 100 - m - 1 (11-4) If an isentropic expansion is assumed between Vc and Fj, where specific volume of vapor entering compressor vdis specific volume of vapor after isentropic compression to pd Values of the specific volumes are available from the pressure-enthalpy diagram of the refrigerant or from tables of properties of superheated vapor.

Vane Compressors

11-19 Vane compressors The two basic-types of vane compressors are the roller or single-vane type and the multiple-vane type. Vane compressors are used mostly in do- Figure 11-18 Roller-type vane compressor. mestic refrigerators, freezers, and air conditioners, although they can also be used as booster compressors in the low-pressure portion of large multistage compression systems. In the roller type (Figs. 11-18 and 11-19) the centerline of the shaft is the same as the centerline of the...

C

Figure 9-22 Master-submaster control for programming a hot-water temperature using outdoor-air temperature. Temperature, Pressure, Pressure Temperature Temperature, Pressure, Pressure Temperature temperature transmitter temperature transmitter Figure 9-23 Reset through the outdoor-air temperature, controller of temperature transmitter temperature transmitter Figure 9-23 Reset through the outdoor-air temperature, controller of Next turn to the reset capabilities. The outdoor-air temperature...

Additional Readings On Compressors

Methods of Testing for Rating Positive Displacement Refrigerant Compressors, Standard 23-78, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, Ga., 1978. W. D. Cooper Refrigeration Compressor Performance as Affected by Suction Vapor Superheating, ASHRAE Trans., vol. 80, pt. 1, pp. 195-204,1974. J. Brown and S. F. Pearson Piston Leakage in Refrigeration Compressors, J. Refrig., vol. 6, no. 5, p. 104, September-October 1963. E. H. Jensen Effect of Compressor...

No

Figure 9-5 Throttling valves (a) normally open, (b) normally closed, (c) symbol. Figure 9-6 (a) Two-way and (b) three-way valves. 28- to 55-kPa spring range on a normally open valve, for example, is that the valve is fully open if the control pressure is 28 kPa or below. As the control pressure increases from 28 to 55 kPa, the valve moves from an open to a closed position. Control pressures above 55 kPa keep the valve in a closed position. The symbols for two types of three-way valves are shown...

Airconditioning Systems

5-1 Thermal distribution systems Chapter 4 explained how to compute heating and cooling loads in conditioned spaces. In order to compensate for these loads energy must be transferred to or from the space. In most medium-sized and large buildings the thermal energy is transferred by means of air, water, and occasionally refrigerant. The transfer of energy often requires conveying energy from a space to a central heat sink (refrigeration unit) or conveying heat from a heat source (heater or...

Compressors

11-1 Types of compressors Each of the four components of a vapor-compression system the compressor, the condenser, the expansion device, and the evaporator-has its own peculiar behavior. At the same time, each component is influenced by conditions imposed by the other members of the quartet. A change in condenser-water temperature, for example, may change the rate of refrigerant the compressor pumps, which in turn may require the expansion valve to readjust and the refrigerant in the evaporator...

Qa w7h7 w4h4 wlhl

0.148(2520) + 0.452(-l 16) - 0.6(-168) 421.3 kW The coefficient of performance of the system that incorporates the heat exchanger is This COP is an improvement over the value of 0.736 applicable to the simple system without a heat exchanger. Another comparison of interest is that of the absorption cycle to the ideal heat-operated cycle whose COP is expressed by Eq. (17-4). Equation (17-4) expects just one temperature of heat rejection, Ta, while in the absorption cycles just analyzed there are...

Problems

21-1 A tube 1.5 m long has a speaker at one end and a reflecting plug at the other. The frequency of a pure-tone generator driving the speaker is to be set so that standing waves will develop in the tube. What frequency is required Ans. 229 Hz, 458 Hz, etc. 21-2 The sound power emitted by a certain rocket engine is 107 W, which is radiated uniformly in all directions. (a) Calculate the amplitude of the sound pressure fluctuation 10 m removed from the source. Ans. 2540 Pa (b) What percentage is...

Pct0Ph

In Eq. (7-5) the term (Av vc)Vs is the volume change in the system as the water is heated from its filling temperature to operating temperature thus The difference in the pressure ratios, assuming a constant temperature of the air in the tank, is PilPc-PjPh VBlVt-VclVt VB'VC The product of the terms in Eqs. (7-6) and (7-7) yields the tank volume Vt. Example 7-6 What is the size of an expansion tank for a hot-water system with a volume of 7.6 m3 if the highest point, in the system is 12 m above...

4 5 6

Figure 13-8 Correction factors to be applied to Fig. 13-7 for other diameters and lengths of capillary tubes. the entering pressure to the capillary tube for a tube that is 1.63 mm in diameter and 2.03 m long. The various curves in Fig. 13-7 represent performance at a variety of inlet conditions magnitudes of subcooling and fractions of vapor. The companion graph to Fig. 13-7 is the one in Fig. 13-8, presenting correction factors to the flow rate of Fig. 13-7 for other lengths and diameters....

Co

Air Cooled Heat Exchanger Wind Skirt

Figure 10-13 (a) Refrigeration system with a heat exchanger to subcool the liquid from the condenser. (b) Pressure-enthalpy diagram of the system using a heat exchanger shown in (a). from the evaporator. The arrangement is shown in Fig. 10-13a and the corresponding pressure-enthalpy diagram in Fig. 10-136. Saturated liquid at point 3 coming from the condenser is cooled to point 4 by means of vapor at point 6 being heated to point 1. From a heat balance, z3 - h4 hx - h6. The refrigerating effect...

R T R T

To establish points on a line of constant specific volume, 0.90 m3 kg for example, substitute 0.9 for v, the barometric pressure for pt, and at arbitrary values of Tsolve for ps. The pairs ofp and t values then describe the line of constant v, as in Fig. 3-7. Example 3-3 What is the specific volume of an air-water-vapor mixture having a temperature of 24 C and a relative humidity of 20 percent at standard barometric pressure Figure 3-8 Air passing over a wetted surface. Solution The water-vapor...

W

Expressions have been developed for particular flow configurations so that the relationship between the Nusselt number and the Reynolds and Prandtl numbers can be expressed as with values of the constant C and the exponents n and m being determined experimentally. Alternatively these relationships can be illustrated graphically, as in Fig. 2-6, Figure 2-6 Typical data correlation for forced convection in smooth tubes, turbulent flow. Figure 2-6 Typical data correlation for forced convection in...

M

Figure 2-14 Pressure-reducing valve in Problem 2-1. (a) What is the state of the water entering the valve (subcooled liquid, saturated liquid, or vapor) (b) For each kilogram that enters the pressure-reducing valve, how much leaves the separating tank as vapor Ans. 0.0375 2-2 Air flowing at a rate of 2.5 kg s is heated in a heat exchanger from -10 to 30 C. What is the rate of heat transfer Ans. 100 kW 2-3 One instrument for measuring the rate of airflow is a venturi, as shown in Fig. 2-15,...

3

Out of the condenser and evaporator, so enthalpies at these points can be determined from a table of properties of water. In the generator and absorber, LiBr-water solutions exist for which the enthalpy is a function of both the temperature of the solution and the concentration. Figure 17-8 presents enthalpy data for LiBr-water solutions. The data are applicable to saturated or subcooled solutions and are based on a zero enthalpy of liquid water at 0 C and a zero enthalpy of solid LiBr at 25 C....

J L

Figure 3-9 Condition of air drives toward saturation line at temperature of wetted surface. Equilibrium of temperature and vapor pressure Equilibrium of temperature and vapor pressure vapor pressure than the liquid at temperature tw, will transfer mass by condensing some water vapor and dropping the humidity ratio of the air. What is unique is that the rates of heat and mass transfer are so related that the path is a straight line driving toward the saturation line at the wetted-surface...

Expansion Devices

13-1 Purpose and types of expansion devices The last of the basic elements in the vapor-compression cycle, after the compressor, condenser, and evaporator, is the expansion device. The purpose of the expansion device is twofold it must reduce the pressure of the liquid refrigerant, and it must regulate the flow of refrigerant to the evaporator. This chapter explains the operation of the common types of expansion devices, the capillary tube, the superheat-controlled expansion valve, the float...

3280

Figure 19-7 Path of air through a coun-terflow cooling tower. The path of the air conditions will generally appear similar to that shown on the psychrometric chart of Fig. 19-7. The curve assumes a direction dictated by the straight-line law (Sec. 3-8) in that the curve drives toward the saturation line at the temperature of water tw in contact with the air at that position. It is not uncommon for the temperature of the air first to drop as it passes up through the tower, reach a minimum, then...

In The Double-effect Absorption Unit Shown Solution

Figure 17-13 Reducing the refrigerating capacity by increasing the condensing temperature and reducing the concentration of LiBr leaving the generator. 17-12 Double-effect system A LiBr absorption unit with improved efficiency is the double-effect system shown schematically in Fig. 17-14. The major distinguishing feature of the double-effect system is that it incorporates a second generator, generator II, which uses the condensing water vapor from generator I to provide its supply of heat....

35

Additional information covering variations in fixtures, floor mass, and operating periods is available.4 6 For heat-producing equipment it is also necessary to estimate the powei ased along with the period and or frequency of use in a manner similar to that used for lighting. For equipment having little radiant-energy transmission the CLF can be assumed equal to 1.0. Table 4-7 shows loads from occupants as a function of their activity. The greatest uncertainty in...

54

From Fig. 17-8 the condition of solution at point 4, which is h -120 kJ kg, x 0.69, is found to be crystallized. Some of the solution has thus solidified, and there is danger of blocking the flow and causing refrigeration to cease. Example 17-5 illustrates two facts (1) the position in the system where crystallization is most likely to occur is where the solution from the generator leaves the heat exchanger, and (2) an operating condition conducive to crystallization is at low condensing...

34

Solution From Table 20-1 U is chosen as 3.5 W m2 K for a value of Fr 0.9, Eq. (20-12) yields (55 - 10) (3.5) 77 (0.87(0.87) (0.9) - - -J 0.9 0.435 For the purpose of selecting collectors, designers often use a graph of collector efficiencies, as shown in Fig. 20-9. The trends shown in Fig. 20-9 are predictable from Eq. (20-12). The efficiency is a function of the optical and thermal properties of the cover plate and the absorber and also the term (tai -1 ) . As the absorber tem- Figure 20-9...

H 93625 17861j 001135 tj 000098855184

Which is applicable between 2 and 30 C. Substitution of Eq. (8-4) into Eq. (8-3) yields a nonlinear equation i-i-h + 9.3625+ 1.786U. +0.01135f +0.00098855f 0 (8-5) R R a 1 1 1 ' Example 8-1 At one position in a cooling and dehumidifying coil which has an R value of 0.22, ha 85.5 kJ kg and tr 9.0 C. What are the values of tt and hf. Solution One technique for solving the nonlinear equation (8-5) uses the Newton-Raphson technique. If a function of x is written in such a form that (jc) 0, the...

5

The pressure drop of liquid flowing through the shell across tube bundles is also difficult to predict analytically, but when an experimental point is available for one flow rate, predictions of the pressure drop at other flow rates can be made quite accurately. Figure 12-5 shows the water pressure drop taken from catalog data of a water-chilling evaporator. The applicable exponent in the pressure-drop-flow-rate relationship here is 1.9. 12-5 Extended surface fins Equation (12-8) expresses the...

Condensers And Evaporators

12-1 Condensers and evaporators as heat exchangers Since both the condenser and evaporator are heat exchangers, they have certain features in common. One classification of condensers and evaporators (Table 12-1) is according to whether the refrigerant is on the inside or outside of the tubes and whether the fluid cooling the condenser or being refrigerated is a gas or a liquid. The gas referred to in Table 12-1 is usually air, and the liquid is usually water, but other substances are used as...

What Is Superheated Horn In Refrigeration

Expansion Compression Condesation Cycle

Figure 10-6 Refrigeration cycle when a gas is the refrigerant. 10-8 Using vapor as a refrigerant Because the Carnot refrigeration cycle is the most efficient cycle, every attempt should be made to reproduce it with actual equipment. Certainly the reversible processes cannot be duplicated, but at least the rectangular shape of the cycle on the temperature-entropy diagram should be maintained. Doing so means that all the heat can be received at one temperature level and rejected at another. If a...

Rmh

Figure 8-8 Coil condition curve of a partially dry coil. Example 8-3 The airflow rate through a direct-expansion coil is 0.32 kg s, and the entering conditions of the air are 30 C dry-bulb temperature and 20 C wet-bulb temperature. The refrigerant temperature is 10 C, hr 2400 W m2 K, hc 100 W m2 K, and the ratio of external to internal surface area is 18.0. (a) What is the dry-bulb temperature of the air when condensation begins (b) How much surface area of the coil is dry Solution (a) From the...

Schematic Diagram Of Terminal Reheat System

Dual Duct Air Conditioning

Figure 5-8 Psychrometric chart showing properties of air in Example 5-2. tions leaving the coil and the required temperature of the supply chilled water, and (c) the cooling capacity of the coil. Solution (a) On the psychrometric chart in Fig. 5-8, four parts of return air at point 1 (24 C and 50 percent relative humidity, h 47.5 kJ kg, W 0.0093 kg kg) mix with one part of outdoor air at point 2 (35 C dry-bulb temperature, 25 C wet-bulb temperature, h 76.0 kJ kg, W 0.016 kg kg). From energy and...

66 Refrigeration And Air Conditioning

Bulb temperature is the temperature exceeded by 2.5 percent of the hours during June to September. The mean coincident wet-bulb temperature is the mean wet-bulb temperature occurring at that 25 percent dry-bullf tempe rature. Tables 4-10 to 4-12, which provide additional data relative to the solar load, will be discussed when the solar load for windows and the thermal transmission for walls and roof are studied. Example 4-2 Select outside and inside design temperatures for a building to be...

Calculations

4-1 Introduction Buildings are built to provide a safe and comfortable internal environment despite variations in external conditions. The extent to which the desired interior conditions can be economically maintained is one important measure of the success of a building design. Although control of inside conditions is usually attributed to the active heating and cooling system, the design of heating, ventilating, and air conditioning (HVAC) must start with an examination of the thermal...

2 Compressor 2 Evaporator P-h Diagram

Civil Engineering Diagram

Figure 16-II Pressure-enthalpy diagram for single-compressor system for conditions in Example 16-3. Figure 16-12 (a) Two compressors and two evaporators operating with intercooling and flash-gas removal. (b) The pressure-enthalpy diagram corresponding to the system in (a). Solution Sketch the pressure-enthalpy diagram of the cycle as in Fig. 16-12 b. The discharge pressure of the low-stage compressor and the suction pressure of the high-stage compressor are the same as the pressure in the 5 C...

Nominal 75 Mm Diameter The Lower Branch 35 Mm

Figure 7-14 Piping system in Prob. 7-3. 7-2 Compute the pressure drop in pascals per meter length when a flow rate of 8 L s of 60 C water flows through a Schedule 40 steel pipe of nominal diameter 75 mm (a) using Eq. (7-1) and (b) using Figs. 7-6 and 1-1 .Ans. (a) 334 Pa m 7-3 In the piping system shown schematically in Fig. 7-14 the common pipe has a nominal 75 mm diameter, the lower branch 35 mm, and the upper branch 50 mm. The pressure of water at the entrance is 50 kPa above atmospheric...

V

Figure 5-2 (a) Heating and humidification and (b) cooling and dehumidification with reheat. In Fig. 5-2b the air is cooled, and if the temperature of the metal surface of the coil is below the dew point of the air, moisture will condense. The heating coil may be operated simultaneously with the cooling coil so that the combination of the cooling and reheat processes provides a steep slope between A and the condition entering the space. The combination of reheat with the cooling and...

Heat Pumps

18-1 Types of heat pumps All refrigeration systems are heat pumps, because they absorb heat energy at a low temperature level and discharge it to a high temperature level. The designation heat pump, however, has developed around the application of a refrigeration system where the heat rejected at the condenser is used instead of simply being dissipated to the atmosphere. There are certain applications and occasions where the heat pump can simultaneously perform useful cooling and useful heat...

Thermal Principles

2-1 Roots of refrigeration and air conditioning Since a course in air conditioning and refrigeration might easily be titled Applications of Thermodynamics and Heat Transfer, it is desirable to begin the technical portion of this text with a brief review of the basic elements of these subjects. This chapter extracts some of the fundamental principles that are important for calculations used in the design and analysis of thermal systems for buildings and industrial processes. The presentation of...

Cooling And Dehumidifying Coils

8-1 Types of cooling and dehumidifying coils One of the frequent assignments of a refrigeration or air-conditioning system is to reduce the temperature of an airstream. A natural concomittant of dropping the temperature of the air is removing moisture from it. In cooling the air in a low-temperature refrigerated warehouse the dehumidifi-cation process forms frost on the coil, which is usually an undesirable by-product of the temperature-reduction process. In a comfort or industrial...

T

With these definitions of thermal resistance it is possible by analogy to apply certain concepts from circuit theory to heat transfer. Recall that the conductance C is the reciprocal of the resistance, C 1 R*, and that in series circuits the resistances sum but for parallel circuits the conductances sum. In the transfer of energy from one room to another through a solid wall (Fig. 2-7) assume that both the gas and the other walls of

Ammonia-water Vapor Absorption Refrigeration System Block Diagram

Analyser Rectifiet Heat Exchanger

The COP of the vapor-compression unit is 3.6, and the COPabs of the absorption unit is 0.7. What is (a) the total refrigerating capacity and (b) the COP of the combined system Solution (a) If heat loss from the steam turbine is neglected, the power delivered by the turbine P equals that extracted from the steam as it flows through the turbine P (1.2 kg s) (3080 - 2675 kJ kg) 486 kW Vapor-compression unit Absorption unit Figure 17-16 Combined absorption and vapor-compression system....

Vaporcompressionsystem Analysis

14-1 Balance points and system simulation The performance characteristics of individual components making up the vapor-compression system have been explored in Chap. 11 for compressors, Chap. 12 for condensers and evaporators, and Chap. 13 for expansion devices. These components never work in isolation but are combined into a system, so that their behavior is interdependent. It is the purpose of this chapter to predict the performance of the entire system when the characteristics of the...

E

Figure 8-9 Coil condition curves with same entering wet-bulb temperatures, face velocities, and refrigerant temperatures. frigerant evaporating at 7 C. The coil has an air-side heat-transfer area of 15 m2 per square meter of face area per row of tubes. The ratio of air-side to refrigerant-side area is 14. The values of hr and hc are 2050 and 65 W m2 K, respectively. Calculate (a) the face area, (b) the enthalpy of the outlet air, (c) the wetted-surface temperatures at the air inlet, air outlet,...

Flash Intercooling Diagram

One Compressor Two Evaporators

Figure 16-5 Intercooling with (a) a water-cooled heat exchanger, and (b) liquid refrigerant. Figure 16-5 Intercooling with (a) a water-cooled heat exchanger, and (b) liquid refrigerant. A further benefit of intercooling the ammonia is that the discharge temperature from the high-stage compressor will be reduced from a value of 146 C, the temperature at 3 based on isentropic compression, to 77 C. The lower discharge temperature permits better lubrication and results in longer life of the...

1467 3906

Power for low-temperature system 0.174(1815 - 1423) 68.2 kW Power for high-temperature system 0.186(1625 - 1467) 29.4 The combined power for the compressors serving the evaporators individually is greater than with the combined system in Fig. 16-12. The power required for the high-temperature evaporator is the same in both cases, so all of the saving is attributable to flash-gas removal and intercooling of refrigerant serving the low-temperature evaporator. The intermediate pressure of the...

Fan And Duct Systems

6-1 Conveying air Chapter 5 explained the arrangement of the popular air systems (variable-volume, terminal-reheat, etc.), and this chapter follows up by concentrating on four topics associated with the flow of air in an air system (1) computing pressure drops of air flowing through ducts and fittings, (2) extending the computation of pressure drops to designing a duct system, (3) understanding the characteristics of a fan, both independently and in conjunction with a duct system, and (4)...

0 02 04 06 08 10 12 14 16 18 20 22 24 26 28

Figure 2-2 Skeleton pressure-enthalpy diagram for water. Example 2-4 If 9 kg s of liquid water at 50 C flows into a boiler, is heated, boiled, and superheated to a temperature of 150 C and the entire process takes place at standard atmospheric pressure, what is the rate of heat transfer to the water Solution The process consists of three distinct parts (1) bringing the temperature of the subcooled water up to its saturation temperature, (2) converting liquid'at 100 C into vapor at 100 C, and...

Combine The Condensing Unit Of Prob 14-1

14-1 Either graphically or by using the computer, for an ambient temperature of 30 C develop the performance characteristics of a condensing unit (of the form of Fig. 14-6 or Table 14-3) if the compressor has performance shown by Fig. 14-1 for Eqs. (14-1) and (14-2)1 and the condenser has characteristics shown by Fig. 14-3 or Eq. (14-4)1. Ans. qe, kW 122.8 104.4 87.6 72.5 59.3 te, C 10 5.0 0 -5.0 -10.0 14-2 Combine the condensing unit of Prob. 14-1 (using answers provided) with the evaporator...

TwVtr21M2

The power required at the shaft is the product of the torque and the rotative speed co rotative speed, rad s At least at very low refrigerant flow rates the tip speed of the impeller and the tangential velocity of the refrigerant are nearly identical therefore Another expression for ideal power is the product of the mass rate of flow and the isentropic work of compression, Equating the two expressions for power, Eqs. (11-14) and (11-15), yields Although Eq. (11-16) is based on some...

What Purpose The Heat Exchanger Use In Refrigeration Systems

Liquid Vapour Heat Exchanger

Figure 10-11 (a) The standard vapor-compression cycle on the pressure-enthalpy diagram (b) flow diagram. rather than steady, process 1-2 still represents the action of the compressor. At a short distance in the pipe away from the compressor, the flow has smoothed out and approaches steady flow. Knowledge of the work of compression is important because it may be one of the largest operating costs of the system. The heat rejection in kilojoules per kilogram is the heat transferred from the...

Hi 4837 kJkg

8-5 Calculating the surface area of a coil The foregoing relations can now be applied to compute the surface area of a coil when the entering conditions and flow rate of the air, the temperatures of the chilled water or refrigerant, and the heat-transfer conditions are known.1 Example 8-2 A counterflow chilled-water coil is to cool 2.5 kg s of air from an entering condition of 30 C dry-bulb and 21 C wet-bulb temperature to a final wet-bulb temperature of 13 C. Chilled water enters the coil at 7...

Catalog Data For A Six Cylinder Refrigerant 22 Compressor Operating At

Clearance Volummmetric Efficiency

Figure 11-11 Work of compression and compressor power for an ideal Refrigerant 22 compressor, 4.5 percent clearance, 50 L s displacement rate, and -20 C evaporating temperature. Figure 11-11 Work of compression and compressor power for an ideal Refrigerant 22 compressor, 4.5 percent clearance, 50 L s displacement rate, and -20 C evaporating temperature. compressor power is the product of the work of compression in kilojoules per kilogram and the mass rate of flow. The work of compression in...

Tfr

Where A - apparent solar irradiation, W m2 B atmospheric extinction coefficient, dimensionless . Values of A and B depend on the month of the year and are tabulated in Ref. 1. The value of A is about 1230 W m2 in December and January and 1080 in midsummer. Values of B range from 0.14 in winter to 0.21 in summer. The maximum direct normal radiation at the earth's surface is of the order of 970 W m2. 20-5 Glazing characteristics We can now compute the intensity of direct irradiation, which is the...

Cooling Towers And Evaporative Condensers

19-1 Heat rejection to atmosphere Most refrigeration systems reject heat to the atmosphere. While there are applications where the rejected heat from the cycle is used for another purpose, as in certain heat pumps discussed in Chap. 18, and other applications where heat is rejected to a nearby body of water, most refrigeration systems reject heat to ambient air. One type of equipment for performing this heat exchange is the air-cooled condenser, discussed in Chap. 12, but another concept is to...

Multipressure Systems

16-1 Multipressure systems in industrial refrigeration A multipressure system is a refrigeration system that has two or more low-side pressures. The low-side pressure is the pressure of the refrigerant between the expansion valve and the intake of the compressor. A multipressure system is distinguished from the single-pressure system, which has but one low-side pressure. A multipressure system may be found, for example, in a dairy where one evaporator operates at -35 C to harden ice cream while...

Fuzzy Control For Compressor

Figure 16-16 Intercooling system in Prob. 16-3. 16-4 A refrigerant 22 system has a capacity of 180 kW at an evaporating temperature of -30 C when the condensing pressure is 1500 kPa. (a) Compute the power requirement for a system with a single compressor. (b) Compute the total power required by the two compressors in the system shown in Fig. 16-17 where there is no intercooling but there is flash-gas removal at 600 kPa. Am. 60.7 kW Figure 16-18 Pumping capacity of low- and high-stage...

Ntu And Guaranty Test On Cooling Towers

Which agrees with the hand calculation in Example 19-1. The foregoing method has tacitly assumed that the temperature of the surface of the water droplets prevails through the droplet. Actually the interior of the droplet has a higher temperature than that of the surface, and heat flows by conduction to the surface where the heat- and mass-transfer process occurs. The experimentally determined value of hcA cpm includes the influence of this internal conduction. Cooling-tower designers and...

The Machine Room Housing The Compressor And Condenser Of A Refrigerant

15-1 The machine room housing the compressor and condenser of a refrigerant 12 system has dimensions 5 by 4 by 3 m. Calculate the mass of the refrigerant which would have to escape into the space to cause a toxic concentration for a 2-h exposure. Arts. 60 kg 15-2 Using data from Table 15-4 for the standard vapor-compression cycle operating with an evaporating temperature of -15 C and a condensing temperature of 30 C, calculate the mass flow rate of refrigerant per kilowatt of refrigeration and...

70 60

Figure 19-4 Enthalpy-temperature diagram of air and water. rated air at these temperatures are fy in and hj out, respectively. Designate the enthalpy of entering air as ha in and that of the leaving air as ha out. The saturation line in Fig. 19-4 represents the water temperature and enthalpy of the saturated air at this water temperature. Only the enthalpy coordinate applies to the air-operating line, however. The slope of the air-operating line is 4.19 L G, which can be shown from Eq. (19-1)....

02 04 0608 1 15 2 3 4 6 8 10 15 20 30 50 100

Figure 6-2 Pressure drop in straight, circular, sheet-metal ducts, 20 C air, absolute roughness 0.00015 m. The equivalent diameter calculated by Eq. (6-5) can be used in conjunction with Fig. 6-2 in a special way. Figure 6-2 can be used if the chart is entered with the actual velocity and the equivalent diameter calculated from Eq. (6-5). The airflow rate, however, applies to circular and not rectangular ducts. To be able to use Fig. 6-2 for rectangular ducts when entering the chart with the...

0000003873

Figure 14-2 Heat-rejection rate of a York (Division of Borg-Wamer) hermetic reciprocating compressor, H62SP-22E, refrigerant 22, 1750 r min. Figure 14-2 Heat-rejection rate of a York (Division of Borg-Wamer) hermetic reciprocating compressor, H62SP-22E, refrigerant 22, 1750 r min. has been chosen as the condensing temperature for future convenience, and each curve in the family applies to a different evaporating temperature. 14-3 Condenser performance The precise representation of the...

Fan Duct System Is Designed So That When The Air Temperature Is 20c

6-5 A duct 0.4 m high and 0.8 m wide, suspended from a ceiling in a corridor, makes a right-angle turn in the horizontal plane. The inner radius is 0.2 m, and the outer radius is 1.0 m, measured from the same center. The velocity of air in the duct is 10 m s. To how many meters of straight duct is the pressure loss in this elbow equivalent Ans. 15 m. 6-6 An 0.3- by 0.4-m branch duct leaves an 0.3- by 0.6-m main duct at an angle of 60 . The air temperature is 20 C. The dimensions of the main...