2-12, oil Inc.) Heal Transfer Theory 37 where q sh = sensible heat duty, Btu/hr W = mass flow rate, Ib/hr C = specific heat of the fluid, Btu/lb-°F T] = initial temperature, °F T 2 = final temperature, °F The specific heat of hydrocarbon vapors and liquids is given by Fig- ures 2-13 and 2-14. In Chapter 6 of Volume 1, it was assumed that C = 0.5 Btu/lb- 0 F for crude oil. It can be seen from Figure 2-13 that this is true for the range of treating temperatures and crude gravities normally encountered in oil treating. Latent Heat The amount of heat energy absorbed or lost by a substance when changing phases is called "latent heat." When steam is condensed to Figure 2-13. Specific heats of hydrocarbon liquids. (From Hoicomb and Brown, /no*. Ehg. Chem., 34, 595, 1942; reprinted from Process Heat Transfer, Kern, McGraw- HiflCo.,01950.} 38 Design of GAS-HANDLING Systems and Facilities Figure 2-14. Specific heals of hydrocarbon vapors. (From Holcomb ami Brown, /no. £n§. Chem., 34, 595, 1942; reprinted from Process Heat Transfer, Kern, McGraw-Hill Co., ©1950.) water, the temperature doesn't change, but heat must be extracted from the steam as it goes through a phase change to water. To change water to steam, heat must be added. When a substance changes from a solid to a liquid or from a liquid to a vapor, the heat absorbed is in the form of latent heat. This heat energy is referred to as latent heat because it cannot be sensed by measuring the temperature. Heat Transfer Theory 39 where q lh = latent heat duty, Btu/hr W = mass flow rate, Ib/hr K = latent heat, Btu/lb The latent heat of vaporization for hydrocarbon compounds is given in Table 2-9. The latent heat of vaporization of water is given by h fg in the steam table (Table 2-6). Heat Duty for Multiphase Streams When a process stream consists of more than one phase, the process heat duty can be calculated using the following equation: q p = q g + qo + qw (2-13) where q p = overall process heat duty, Btu/hr q g = gas heat duty, Btu/hr q 0 = oil heat duty, Btu/hr q w = water heat duty, Btu/hr Table 2-9 Latent Heat of Vaporization Compound Methane Ethane Propane n-Butane Isobutane n-Pentane Isopentane Hexane Heptane Octane Nonane Decane Heat of Vaporization, 1 4.696 psia at Boiling Point, Btu/lb 219.22 210.41 183.05 165.65 157.53 153.59 147.13 143.95 136.01 129.53 123.76 118.68 40 Design of GAS-HANDLING Systems and Facilities Natural Gas Sensible Heat Duty at Constant Pressure The sensible heat duty for natural gas at constant pressure is: where Q g = gas flow rate, MMscfd C g = gas heat capacity, Btu/Mscf °F Tj = inlet temperature, °F T 2 = outlet temperature, °F Heat capacity is determined at atmospheric conditions and then cor- rected for temperature and pressure based on reduced pressure and tem- perature. where C = gas specific heat at one atmosphere pressure, Btu/lb-°F (Figure 2-14) ACp = correction factor S = gas specific gravity The correction factor AC p is obtained from Figure 2-15 where: where P r = gas reduced pressure P = gas pressure, psia P c = gas pseudo critical pressure, psia Tj. = gas reduced temperature T a = gas average temperature, °R = 1/2 (Tj + T 2 ) T c = gas pseudo critical temperature, °R The gas pseudo critical pressures and temperatures can be approximat- ed from Figure 2-16 or they can be calculated as weighted averages of the critical temperatures and pressures of the various components on a Heat Transfer Theory 41 Figure 2-15. Heat capacity correction factor. (From Chemical Engineer's Handbook, 5m Edition, R. Perry and C. Chilton, McGraw-Hill Co., © 1973.) mole fraction basis. Table 2-10 shows a calculation for the gas stream in our example field. For greater precision, a correction for H 2 S and CO 2 content may be required. Refer to the Gas Processors Suppliers Associa- tion's Engineering Data Book or other text for a correction procedure. OH Sensible Heat Duty The sensible heat duty for the oil phase is: 42 Design of GAS-HANDLING Systems and Facilities Figure 2-16. Pseudo critical properties of natural gases. (From Gas Processors Suppliers Association, Engineering Data Handbook, 9th Edition.} where Q 0 = oil flow rate, bpd SG = oil specific gravity C 0 = oil specific heat, Btu/lb-°F (Figure 2-13) Tt = initial temperature, °F T 2 = final temperature, °F Water Sensible Heat Duty The duty for heating free water may be determined from the following equation by assuming a water specific heat of 1.0 Btu/lb-°F. where Q w = water flow rate, bpd Heat Transfer Theory 43 Table 2-10 Estimate of Specific Gravity, Pseudo Critical Temperature and Pseudo Critical Pressure for the Example Field CO, N/ H 2 S C, C-, d iC 4 nC 4 iC 5 nC^ C 6 " c- Computed Value Computation Specific Gravity = A Mole % Gas Composition 4.03 1.44 0.0019 85.55 5.74 1.79 0.41 0.41 0.20 0.13 0.15 0.15 100.00 Sum (Aj) 19.48 _ '-^ - o 67 — — \J,\J 1 29 B Molecular Weight 44.010 28.013 34.076 16.043 30.070 44.097 58.124 58.124 72.151 72.151 86.178 147 19.48 Sum (Aj x B { ) Sum ( Aj) C Critical Temp. °R 547.87 227.3 672.6 343.37 550.09 666.01 734.98 765.65 829,10 845.70 913.70 1112.0 374.6 Sum (A, x Cj) Sum (Aj) D Critical psia 1071.0 493.0 1036.0 667.8 707.0 616.3 529. 1 550.7 490.4 488.6 436.9 304 680.5 Sum (Aj xDj) Sum (A,) Heat Duty and Phase Changes If a phase change occurs in the process stream for which heat duties are being calculated, it is best to perform a flash calculation and deter- mine the heat loss or gain by the change in enthalpy. For a quick hand approximation it is possible to calculate sensible heat for both the gas and liquid phases of each component. The sum of all the latent and sensi- ble heats is the approximate total heat duty. Heat Lost to Atmosphere The total heat duty required to raise a substance from one temperature to another temperature must include an allowance for heat lost to the atmosphere during the process. For example, if the process fluid flows through a coil in a water bath, not only is the water bath exchanging heat with the process fluid, but it is also exchanging heat with the surrounding atmosphere. 44 Design of GAS-HANDLING Systems and Facilities The heat lost to the atmosphere can be calculated in the same manner as any other heat exchange problem using Equation 2-3, The overall heat transfer coefficient may be calculated from a modification of Equation 2-5, By assuming that the inside film coefficient is very large compared to the outside film coefficient, by adding a factor for conduction losses through insulation, and by eliminating fouling factors to be conservative. Equation 2-5 becomes; where h () = outside film coefficient Btu/hr-ft 2 -°F = 1 +0.22V W (V W < 16 ft/sec) = 0.53 V W °- 8 (V W > 16 ft/sec) V w = wind velocity (ft/sec) = 1.47x(mph) AX} = shell thickness, ft K { = shell thermal conductivity Btu/hr-ft-°F = 30 for carbon steel (Table 2-3) AX2 = insulation thickness, ft K 2 = insulation thermal conductivity, Btu/hr-ft-°F = 0.03 for mineral wool For preliminary calculations it is sometimes assumed that the heat lost to atmosphere is approximately 5-10 % of the process heat duty for unin- sulated equipment and 1-2% for insulated equipment. Heat Transfer from a Fire Tube A fire tube contains a flame burning inside a piece of pipe which is in turn surrounded by the process fluid. In this situation, there is radiant and convective heat transfer from the flame to the inside surface of the fire tube, conductive heat transfer through the wall thickness of the tube, and convective heat transfer from the outside surface of that tube to the oil being treated. It would be difficult in such a situation to solve for the heat transfer in terms of an overall heat transfer coefficient. Rather, what is most often done is to size the fire tube by using a heat flux rate. The heat flux rate represents the amount of heat that can be transferred from the fire tube to the process per unit area of outside surface of the fire tube. Common heat flux rates are given in Table 2-11. Hear Transfer Theory 45 Table 2- II Common Heat Flux Rates Medium Being Heated Water Boiling water Crude oil Heat medium oils Glycol Amine Design Flux Rate Btu/hr-ft 2 10,000 10,000 8,000 8,000 7,500 7,500 The required fire tube area is thus given by: For example, if total heat duty (sensible heat, latent heat duty, heat losses to the atmosphere) was 1 MMBtu/hr and water was being heated, a heat flux of 10,000 Btu/hr-ft 2 would be used and .100 ft 2 of fire tube area would be required. Standard Burner Btu/hr 100,000 250,000 500,000 750,000 1,000,000 1,500,000 2,000,000 2,500,000 3,000,000 3,500,000 4,000,000 5,000,000 Table 2- 12 Sizes and Minimum Diameter Minimum Diameter-in. 2.5 3.9 5.5 6.7 7.8 9.5 11.0 12,3 13.5 14.6 15.6 17.4 * [...]... be unbolted to perform maintenance and the channel can be unbolted without pulling the tube sheet The "E" designates a one-pass shell The shell fluid comes in one end and goes out the other The rear of the heat exchanger is an internal floating head The head can move back and forth as the tubesheet expands and contracts L 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Stationary Head—Channel Stationary... arrangements of the shells, tubes and baffles in heat exchangers Figure 3- 6 is a list of TEMA standard classifications for heat exchangers, which helps to describe the various options These Figure 3- 5 Common tube layouts for shell -and- tube heat exchangers Heat Exchangers 53 Figure 3- 6 Heat exchanger nomenclature (From Tubular Exchanger Manufacturers Association,© 19 78.) 54 Design of GAS-HANDLING Systems and Facilities. .. Head Flange Floating Head Backing Device Split Shear Ring 20 21 22 23 24 25 26 27, 28 29 30 31 32 33 34 35 36 37 38 39 Slip-on Backing Flange Floating Head Cover—External Floating Tubesheet Skirt Packing Box Packing Packing Gland Lantern Ring Tierods and Spacers Transverse Baffles or Support Plates Impingement Ptate Longitudinal Baffle Pass Partition Vent Connection Drain Connection Instrument Connection... tubesheet, 23- in port diameter and 37 -in inside shell diameter with tubes 16 ft long is denoted as SIZE 23/ 37 -19 2 TYPE CKT Selection of Types In selecting an exchanger, one must know the advantages and disadvantages of each type The three basic types of shell -and- tube exchangers are fixed tube sheet, floating head, and U-tube Table 3 -1 summarizes the comparison between these three exchangers Table 3 -1 Heat... otherwise, it would plug the fins 60 Design of GAS-HANDLING Systems and Facilities TEMA Glasses and Tube Materials TEMA standards provide for two classes of shell and tube exchanger qualities Class C is the less stringent and is typically used in onshore applications and where the temperature is above ~20°F Class R is normally used offshore and in cold temperature service Table 3- 2 shows the most important... cleaning and cause a lower pressure drop when shell-side fluid flows perpendicularly to the tube axis The tube pitch is the shortest center-to-center distance between adjacent tubes The common pitches for square patterns are %-in OD on 52 Design of GAS-HANDLING Systems and Facilities I -in and 1- in OD on l!4-in For triangular patterns these are %-in OD on %-in., M-in OD on 1- in., and 1- in OD on 1/ 4-in,... longitudinal baffles C R Major Features %-inch A, 1, 11 4, I 1 /? 54-inch 3 Mfi-inch 14 , -M, 14 , -X %, I, 1J4, I!/; M&~inch R greaiter than C In some cases R greater than C 1^ -inch 14 -inch 22 Impingement protection required v ~> Cross-over area for multi-pass floating heads or channels 1. 3 times flow area through tubes of one pass Required Stress relieving of fabricated floating covers, or channels Gaskets... the design requirements for virtually all ranges of temperature and pressure that would be encountered in an oil or gas production facility The simplest type of shell -and- tube heat exchanger is shown in Figure 3- i The essential parts are a shell (1) , equipped with two nozzles and having tube sheets (2) at both ends, which also serve as flanges for the attachment of the two channels or heads (3) and. .. design of indirect bath heaters are presented in Chapter 5, SHELL -AND- TUBE EXCHANGERS Shell -and- tube heat exchangers are cylindrical in shape, consisting of a bundle of parallel tubes surrounded by an outer casing (shell) Both the tube bundle and the shell are designed as pressure containing elements in accordance with the pressure and temperature requirements of the fluids that flow through each of. .. the shell type Types A and B bolt onto the shell In type C, the head cannot be unbolted for maintenance The shell types are E, F, G, H, J, and K E is a one-pass shell The fluid comes in on one side and goes out the other side F is a two-pass shell with a longitudinal plate in it The fluid in the shell makes two passes 56 Design of GAS-HANDLING Systems and Facilities Figure 3- 9 Heat exchanger components . psia at Boiling Point, Btu/lb 219 .22 210 . 41 1 83. 05 16 5.65 15 7. 53 15 3. 59 14 7 . 13 14 3. 95 13 6. 01 129. 53 12 3. 76 11 8.68 40 Design of GAS-HANDLING Systems and Facilities Natural Gas Sensible . 1 29 B Molecular Weight 44. 010 28. 0 13 34 .076 16 .0 43 30.070 44.097 58 .12 4 58 .12 4 72 .15 1 72 .15 1 86 .17 8 14 7 19 .48 Sum (Aj x B { ) Sum ( Aj) C Critical Temp. °R 547.87 227 .3 672.6 34 3 .37 550.09 666. 01 734 .98 765.65 829 ,10 845.70 9 13 .70 11 12.0 37 4.6 Sum (A, x Cj) Sum . Composition 4. 03 1. 44 0.0 019 85.55 5.74 1. 79 0. 41 0. 41 0.20 0 . 13 0 .15 0 .15 10 0.00 Sum (Aj) 19 .48 _ '-^ - o 67 — — J,J 1 29 B Molecular Weight 44. 010 28. 0 13 34 .076 16 .0 43 30.070 44.097 58 .12 4 58 .12 4 72 .15 1 72 .15 1 86 .17 8 14 7 19 .48 Sum