ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 1 Part 3 pps

Heal Transfer Theory 37where qsh = sensible heat duty, Btu/hr W = mass flow rate, Ib/hr C = specific heat of the fluid, Btu/lb-°F Latent Heat The amount of heat energy absorbed or lost b

2-12, oil Inc.)

Heal Transfer Theory 37

where qsh = sensible heat duty, Btu/hr

W = mass flow rate, Ib/hr

C = specific heat of the fluid, Btu/lb-°F

Latent Heat

The amount of heat energy absorbed or lost by a substance whenchanging 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 fromthe steam as it goes through a phase change to water To change water tosteam, heat must be added When a substance changes from a solid to aliquid or from a liquid to a vapor, the heat absorbed is in the form oflatent heat This heat energy is referred to as latent heat because it cannot

be sensed by measuring the temperature

Heat Transfer Theory 39

where ql h = 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 inTable 2-9 The latent heat of vaporization of water is given by hfg in thesteam table (Table 2-6)

Heat Duty for Multiphase Streams

When a process stream consists of more than one phase, the processheat duty can be calculated using the following equation:

where qp = overall process heat duty, Btu/hr

qg = gas heat duty, Btu/hr

q0 = oil heat duty, Btu/hr

qw = water heat duty, Btu/hr

Table 2-9 Latent Heat of Vaporization

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 Qg = gas flow rate, MMscfd

Cg = gas heat capacity, Btu/Mscf °F

Tj = inlet temperature, °F

T2 = outlet temperature, °F

Heat capacity is determined at atmospheric conditions and then rected for temperature and pressure based on reduced pressure and tem-perature

cor-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 ACp is obtained from Figure 2-15 where:

where Pr = gas reduced pressure

P = gas pressure, psia

Pc = gas pseudo critical pressure, psia

Tj = gas reduced temperature

Ta = gas average temperature, °R = 1/2 (Tj + T2)

Tc = gas pseudo critical temperature, °R

The gas pseudo critical pressures and temperatures can be

approximat-ed from Figure 2-16 or they can be calculatapproximat-ed as weightapproximat-ed averages ofthe 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 inour example field For greater precision, a correction for H2S and CO2

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 Q0 = oil flow rate, bpd

SG = oil specific gravity

C0 = oil specific heat, Btu/lb-°F (Figure 2-13)

Tt = initial temperature, °F

T2 = final temperature, °F

Water Sensible Heat Duty

The duty for heating free water may be determined from the followingequation by assuming a water specific heat of 1.0 Btu/lb-°F

where Qw = 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

Sum (Aj x B{) Sum ( Aj)

C Critical Temp °R547.87227.3672.6343.37550.09666.01734.98765.65829,10845.70913.701112.0374.6

Sum (A, x Cj) Sum (Aj)

D Critical psia1071.0493.01036.0667.8707.0616.3

529. 1

550.7490.4488.6436.9304680.5

Sum (Aj xDj)

Sum (A,)

Heat Duty and Phase Changes

If a phase change occurs in the process stream for which heat dutiesare 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 handapproximation it is possible to calculate sensible heat for both the gasand 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 theatmosphere during the process For example, if the process fluid flowsthrough a coil in a water bath, not only is the water bath exchanging heatwith the process fluid, but it is also exchanging heat with the surroundingatmosphere

44 Design of GAS-HANDLING Systems and Facilities

The heat lost to the atmosphere can be calculated in the same manner asany other heat exchange problem using Equation 2-3, The overall heattransfer coefficient may be calculated from a modification of Equation 2-5,

By assuming that the inside film coefficient is very large compared to theoutside film coefficient, by adding a factor for conduction losses throughinsulation, and by eliminating fouling factors to be conservative Equation2-5 becomes;

where h() = outside film coefficient Btu/hr-ft2-°F

= 1 +0.22VW(VW< 16 ft/sec)

= 0.53 VW°-8(VW> 16 ft/sec)

Vw = 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

K2 = 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 sulated equipment and 1-2% for insulated equipment

unin-Heat Transfer from a Fire Tube

A fire tube contains a flame burning inside a piece of pipe which is inturn surrounded by the process fluid In this situation, there is radiant andconvective heat transfer from the flame to the inside surface of the firetube, conductive heat transfer through the wall thickness of the tube, andconvective heat transfer from the outside surface of that tube to the oilbeing treated It would be difficult in such a situation to solve for the heattransfer in terms of an overall heat transfer coefficient Rather, what ismost often done is to size the fire tube by using a heat flux rate The heatflux rate represents the amount of heat that can be transferred from thefire 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

The required fire tube area is thus given by:

For example, if total heat duty (sensible heat, latent heat duty, heatlosses to the atmosphere) was 1 MMBtu/hr and water was being heated,

a heat flux of 10,000 Btu/hr-ft2 would be used and 100 ft2 of fire tubearea 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- 12Sizes 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

*

46 Design of GAS-HANDLING Systems; and Facilities

For natural draft fire tubes, the minimum cross-sectional area of thefire tube is set by limiting the heat release density to 21,000 Btu/hr-irr

At heat release densities above this value, the flame may become ble because of insufficient air Using this limit, a minimum, fire tubediameter is established by:

unsta-where d — minimum fire tube diameter, in

In applying Equation 2-22 note that the burner heat release density will

be somewhat higher than the heat duty, including losses used in Equation2-21, as a standard burner size will be chosen slightly larger than thatrequired Standard burner sizes and minimum fire tube diameters areincluded in Table 2-12

3 Heat Exchangers *

HEAT EXCHANGERS

Heat exchangers used in gas production facilities are shell-and-tube,double-pipe, plate-and-frame, bath-type, forced-air, or direct-fired In thischapter we will discuss the basic concepts for sizing and selecting heatexchangers This is just a brief overview of this complex subject and ismeant to provide the reader with a basis upon which to discuss specificsizing and selection details with heat exchange experts in engineeringcompanies and with vendors

Bath-type heat exchangers can be either direct or indirect In a directbath exchanger, the heating medium exchanges heat directly with thefluid to be heated The heat source for bath heaters can be a coil of a hotheat medium or steam, waste heat exhaust from an engine or turbine, orheat from electric immersion heaters An example of a bath heater is anemulsion heater-treater of the type discussed in Volume 1 In this case, afire tube immersed in the oil transfers heat directly to the oil bath Thecalculation of heat duties and sizing of fire tubes for this type of heatexchanger can be calculated fom Chapter 2

^Reviewed for the 1999 edition by Lei Tan of Paragon Engineering Services, Inc.

47

48 Design of GAS-HANDLING Systems and Facilities

In an indirect bath heat exchanger, the heating medium provides heal

to an intermediary fluid, which then transfers the heat to the fluid beingheated An example of this is the common line heater used on many gaswell streams to keep the temperature above the hydrate formation tem-perature A fire tube heats a water bath, which provides heat to the wellstream flowing through a coil immersed in the bath Details pertaining todesign of indirect bath heaters are presented in Chapter 5,

SHELL-AND-TUBE EXCHANGERS

Shell-and-tube heat exchangers are cylindrical in shape, consisting of abundle of parallel tubes surrounded by an outer casing (shell) Both thetube bundle and the shell are designed as pressure containing elements inaccordance with the pressure and temperature requirements of the fluidsthat flow through each of them The tube-side fluid is isolated from theshell-side fluid either by gasketed joints or by permanent partitions TheStandards of Tubular Exchanger Manufacturers Association (TEMA)define the various types of shell and tube exchangers, as well as designand construction practices

The shell-and-tube exchanger is by far the most common type of heatexchanger used in production operations It can be applied to liquid/liquid,liquid/vapor, or vapor/vapor heat transfer services The TEMA standardsdefine the design requirements for virtually all ranges of temperature andpressure that would be encountered in an oil or gas production facility.The simplest type of shell-and-tube heat exchanger is shown in Figure3-i The essential parts are a shell (1), equipped with two nozzles andhaving tube sheets (2) at both ends, which also serve as flanges for theattachment of the two channels or heads (3) and their respective channelcovers (4) The tubes are expanded into both tube sheets and are equippedwith transverse baffles (5) on the shell side for support The calculation ofthe effective heat transfer surface is based on the distance between theinside faces of the tube sheets instead of the overall tube length

The shell-and-tube exchanger shown in Figure 3-1 is considered tooperate in counter-current flow, since the shell fluid flows across the out-side of the tubes Often, in order to maintain a high enough tube velocity

to avoid laminar flow and to increase heat transfer, the design is modified

so that the tube fluid passes through a fraction of the tubes in two ormore successive "passes" from head to head An example of a two-passfixed-tube exchanger is shown in Figure 3-2

Heat Exchangers 49

Figure 3-2 Fixed head 1 -2 shell-and-tube heat exchanger.

An exchanger in which the shell-side fluid flows in one shell pass andthe tube fluid in two or more passes is called a 1-2 exchanger A singlechannel is employed with a partition to permit the entry and exit of thetube fluid from the same channel At the opposite end of the exchanger abonnet is provided to permit the tube fluid to cross from the first to thesecond pass As with all fixed-tubesheet exchangers, the outsides of thetubes are inaccessible for inspection or mechanical cleaning The insides

of the tubes can be cleaned in place by removing only the channel coverand using a rotary cleaner or wire brush

Baffles

Shell-and-tube exchangers contain several types of baffles to helpdirect the flow of both tube-side and shell-side fluids Pass partition baf-fles force the fluid to flow through several groups of parallel tubes Each

of these groups of tubes is called a "pass," since it passes the fluid fromone head to another By adding pass partition baffles on each end, thetube-side fluid can be forced to take as many passes through theexchanger as desired

Transverse baffles support the tubes that pass through holes in the fle The transverse baffle cannot go all the way across the cross-section

baf-of the shell, because the fluid that is in the shell has to be able to come

50 Design of GAS-HANDLING Systems and Facilities

over the top of the baffle and under the bottom of the next baffle, etc., as

it passes across the tubes that are in the heat exchanger When it isdesired that a fluid pass through the shell with an extremely small pres-sure drop, these will usually be half-circle, 50% plates that provide rigid-ity and prevent the tubes from sagging

Transverse baffles can help maintain greater turbulence for the side fluid, resulting in a higher rate of heat transfer The transverse baf-fles cause the liquid to flow through the shell at right angles to the axis ofthe tubes This can cause considerable turbulence, even when a smallquantity of liquid flows through the shell if the center-to-center distancebetween baffles, called baffle spacing, is sufficiently small The bafflesare held securely by means of baffle spacers, which consist of through-bolts screwed into the tube sheet and a number of smaller lengths of pipethat form shoulders between adjacent baffles

shell-Transverse baffles are drilled plates with heights that are generally75% of the inside diameter of the shell They may be arranged, as shown

in Figure 3-3, for "up-and-down" flow or may be rotated 90° to provide

"side-to-side" flow, the latter being desirable when a mixture of liquidand gas flows through the shell Although other types of transverse baf-fles are sometimes used, such as the orifice baffle shown in Figure 3-4,they are not of general importance

Impingement baffles are placed opposite the shell-side inlet nozzle.The flow into the shell hits the impingement baffle and is dispersedaround the tubes, rather than impinging directly on the top tubes Thiskeeps the full force of the momentum of the flow from impinging on anderoding the top tubes

Longitudinal baffles force the shell-side fluid to make more than onepass through an exchanger With no longitudinal baffle, such as in Figure

Figure 3-3 Transverse baffle detail.