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n physics, energy (Ancient Greek: ἐνέργεια energeia "activity, operation"[1]) is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems.[2][3] Since work is defined as a force acting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length.
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n physics, energy (Ancient Greek: ἐνέργεια energeia "activity, operation"[1]) is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems.[2][3] Since work is defined as a force acting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length.
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Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
Module 2.10
Heating With Coils and Jackets
The Steam and Condensate Loop 2.10.1
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
Heating with Coils and Jackets
Vessels can be heated in a number of different ways. This module will deal with indirect heating.
In these systems, the heat is transferred across a heat transfer surface. Options include:
o Submerged steam coils - A widely used form of heat transfer involves the installation inside
a tank of a steam coil immersed in a process fluid.
o Steam jackets - Steam circulates in the annular space between a jacket and the vessel walls,
and heat is transferred through the wall of the vessel.
Submerged steam coils
The use of tank coils is particularly common in marine applications where cargoes of crude oil,
edible oils, tallow and molasses are heated in deep tanks. Many of these liquids are difficult to
handle at ambient temperatures due to their viscosity. Steam heated coils are used to raise the
temperature of these liquids, lowering their viscosity so that they become easier to pump.
Tank coils are also extensively used in electroplating and metal treatment. Electroplating involves
passing articles through several process tanks so that metallic coatings can be deposited on to
their surfaces. One of the first stages in this process is known as pickling, where materials such as
steel and copper are treated by dipping them in tanks of acid or caustic solution to remove any
scale or oxide (e.g. rust) which may have formed.
Steam coil sizing
Having determined the energy required (previous Module), and with knowledge of the
steam pressure / temperature in the coil, the heat transfer surface may be determined using
Equation 2.5.3:
8$∆7 Equation 2.5.3
The heat transfer area calculated is equivalent to the surface area of the coil, and will enable an
appropriate size and layout to be specified.
Determining the 'U' value
To calculate the heat transfer area, a value for the overall heat transfer coefficient, U, must be
chosen. This will vary considerably with the thermal and transport properties of both fluids and
a range of other conditions.
On the product side of the coil a thermal boundary layer will exist in which there is a temperature
gradient between the surface and the bulk fluid. If this temperature difference is relatively large,
then the natural convective currents will be significant and the heat transfer coefficient will be
high.
Assisted circulation (such as stirring) that will induce forced convection, will also result in higher
coefficients. As convection is partially dependent on the bulk motion of the fluid, the viscosity
(which varies with temperature) also has an important bearing on the thermal boundary layer.
Additional variations can also occur on the steam side of the coil, especially with long lengths of
pipe. The coil inlet may have a high steam velocity and may be relatively free from water. However,
further along the length of the coil the steam velocity may be lower, and the coil may be running
partially full of water. In very long coils, such as those sometimes found in seagoing tankers or in
large bulk storage tanks, a significant pressure drop occurs along the length of the coil. To acheive
the mean coil temperature, an average steam pressure of approximately 75% of the inlet pressure
may be used. In extreme cases the average pressure used may be as low as 40% of the inlet
pressure.
2.10.2 The Steam and Condensate Loop
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
Another variable is the coil material itself. The thermal conductivity of the coil material may vary
considerably. However, overall heat transfer is governed to a large extent by the heat resistant
films, and the thermal conductivity of the coil material is not as significant as their combined
effect. Table 2.10.1 provides typical overall heat transfer coefficients for various conditions of
submerged steam coil application. U values for steam pressures between 2 bar g and 6 bar g
should be found by interpolation of the data in the table.
Table 2.10.1 Heat emission rates for steam coils submerged in water
Customary overall heat transfer coefficients U (W /m²°C)
Mean steam /water temperature difference around 30°C 550 - 1 300
Mean steam /water temperature difference around 60°C 1 000 - 1 700
Mean steam /water temperature difference around 110°C 1 300 - 2 700
Recommended rates U (W /m²°C)
Lower pressure coils (<2 bar g) with natural circulation of water 550
Higher pressure coils (>6 bar g) with natural circulation of water 1 100
Lower pressure coils (<2 bar g) with assisted circulation of water 1 100
Higher pressure coils (>6 bar g) with assisted circulation of water 1 700
The range of figures shown in Table 2.10.1 demonstrates the difficulty in providing definitive
'U' values. Customary figures at the higher end of the scale will apply to installations that are
supplied with clean dry steam, small coils and good condensate drainage. The lower end is more
applicable to poor quality steam, long coils and poor condensate drainage.
The recommended overall heat transfer coefficients will apply to typical conditions and
installations. These recommended rates are empirically derived, and will generally ensure that a
generous safety margin applies to the coil sizing.
In the case of fluids other than water, the heat transfer coefficient will vary even more widely due
to the way in which viscosity varies with temperature. However, the values shown in Table 2.10.2
will serve as a guide for some commonly encountered substances, while Table 2.10.3 gives typical
surface areas of pipes per metre length.
Table 2.10.2 Heat emission rates for steam coils submerged in miscellaneous liquids
Medium pressure steam (2 - 6 bar g) with natural liquid convection U (W/m²°C)
Light oils 170
Heavy oils 80 - 110
* Fats 30 - 60
Medium pressure steam (2 - 6 bar g) with forced liquid convection U (W/m²°C)
Light oils (200 sec Redwood at 38°C) 550
Medium oils (1 000 sec Redwood at 38°C) 340
Heavy oils (3 500 sec Redwood at 38°C) 170
** Molasses (10 000 sec Redwood at 38°C) 85
* Fats (50 000 sec Redwood at 38°C) 55
* Certain materials such as tallow and margarine are solid at normal temperatures but have quite low viscosities in the
molten state.
** Commercial molasses frequently contains water and the viscosity is much lower.
Table 2.10.3 Nominal surface areas of steel pipes per meter length
Nominal bore (mm) 15 20 25 32 40 50 65 80 100
Surface area (m² /m) 0.067 0.085 0.106 0.134 0.152 0.189 0.239 0.279 0.358
The Steam and Condensate Loop 2.10.3
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
Example 2.10.1
Continuing from Example 2.9.1 determine:
Part 1. The average steam mass flowrate during start-up. (Mean heat load = 370 kW)
Part 2. The heat transfer area required.
Part 3. A recommended coil surface area.
Part 4. The maximum steam mass flowrate with the recommended heat transfer area.
Part 5. A recommendation for installation, including coil diameter and layout.
The following additional information has been provided:
o Steam pressure onto the control valve = 2.6 bar g (3.6 bar a).
o A stainless steel steam coil provides heat.
o Heat transfer coefficient from steam /coil /liquid, U = 650 W /m²°C
Part 1 Calculate the average steam mass flowrate during start-up
Steam pressure onto the control valve = 2.6 bar g (3.6 bar a)
Critical pressure drop (CPD) will occur across the control valve during start-up, therefore the
minimum steam pressure in the heating coil should be taken as 58% of upstream absolute pressure.
An explanation of this is given in Block 5.
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Part 2 Calculate the heat transfer area required
Using Equation 2.5.3: Q = UADT
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2.10.4 The Steam and Condensate Loop
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
Part 3 A recommendation for coil surface area
Because of the difficulties in providing accurate U values, and to allow for future fouling of the
heat exchange surface, it is usual to add 10% to the calculated heat transfer area.
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Part 4 The maximum steam mass flowrate with the recommended heat transfer area
Maximum heat transfer (and hence steam demand) will occur when the temperature difference
between the steam and the process fluid is at its maximum, and should take into consideration
the extra pipe area allowed for fouling.
(a) Consider the maximum heating capacity of the coil Q(coil)
Using Equation 2.5.3: Q = UADT
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(b) Heat losses from the sides of the tank
At start-up, as the tank temperature = Ambient temperature there are no heat losses.
Therefore: Q(sides) = 0
(c) Heat losses from the liquid surface of the tank Q(surface)
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The Steam and Condensate Loop 2.10.5
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
Part 5 A recommendation for installation, including coil diameter and layout
(a) Determine coil diameter and length
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From Table 2.10.3, a 100 mm pipe has a surface area of 0.358 m² /m run. This application will
require:
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It may be difficult to accommodate this length of large bore heating pipe to install in a 3 m × 3 m
tank.
One solution would be to run a bank of parallel pipes between steam and condensate manifolds,
set at different heights to encourage condensate to run to the lower (condensate) manifold. The
drain line must fall from the bottom of the condensate manifold down to the steam trap (or
pump-trap). See Figure 2.10.1 for a suggested layout.
Steam in
Steam manifold
Tank
Connecting pipes
Condensate manifold
Fig. 2.10.1 Possible layout of coils in a rectangular tank
2.10.6 The Steam and Condensate Loop
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
Note the steam supply is situated at one end of its manifold, whilst the trap set is at the other
end. This will help steam to flow and push condensate through the coils.
In the application, the steam and condensate headers would each be 2.8 m long. As the
condensate manifold is holding condensate, the heat from it will be small compared to the steam
manifold and this can be ignored in the calculation.
The steam manifold should be 100 mm diameter as determined by the previous velocity
calculation. This will provide a heating area of:
2.8 m x 0.358 m² /m = 1.0 m²
Consequently 7 m² - 1 m² = 6 m² of heat transfer area is still required, and must be provided
by the connecting pipes.
Arbitrarily selecting 32 mm pipe as a good compromise between robustness and workability:
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It is necessary to confirm the steam velocity through the connecting tubes:
On the basis of proportionality of heat transfer area, the steam header will condense:
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This leaves 86% of the 865 kg / h = 744 kg / h of steam which must pass through the
18 connecting pipes and also into the lower (condensate) manifold.
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The Steam and Condensate Loop 2.10.7
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
Other steam coil layouts
The design and layout of the steam coil will depend on the process fluid being heated. When the
process fluid to be heated is a corrosive solution, it is normally recommended that the coil inlet
and outlet connections are taken over the lip of the tank, as it is not normally advisable to drill
through the corrosion resistant linings of the tank side. This will ensure that there are no weak
points in the tank lining, where there is a risk of leakage of corrosive liquids. In these cases the
coil itself may also be made of corrosion resistant material such as lead covered steel or copper,
or alloys such as titanium.
However, where there is no danger of corrosion, lifts over the tank structure should be avoided,
and the steam inlet and outlet connections may be taken through the tank side. The presence of
any lift will result in waterlogging of a proportion of the coil length, and possibly waterhammer,
noise and leaking pipework.
Steam heating coils should generally have a gradual fall from the inlet to the outlet to ensure that
condensate runs toward the outlet and does not collect in the bottom of the coil.
Where a lift is unavoidable, it should be designed to include a seal arrangement at the bottom of
the lift and a small bore dip pipe, as shown in Figure 2.10.2.
Condensate outlet
Steam in
Dip pipe
Fig. 2.10.2 Tank with a rising discharge pipe
The seal arrangement allows a small amount of condensate to collect to act as a water seal, and
prevents the occurrence of steam locking. Without this seal, steam can pass over any condensate
collecting in the bottom of the pipe, and close the steam trap at the top of the riser.
The condensate level would then rise and form a temporary water seal, locking the steam between
the bottom of the riser and the steam trap. The steam trap remains closed until the locked steam
condenses, during which time the coil continues to waterlog.
When the locked steam condenses and the steam trap opens, a slug of water is discharged up
the riser. As soon as the water seal is broken, steam will enter the rising pipe and close the trap,
while the broken column of water falls back to lie at the bottom of the heating coil.
The small bore dip pipe will only allow a very small volume of steam to become locked in the
riser. It enables the water column to be easily maintained without steam bubbling through it,
ensuring there is a steady and continuous condensate flow to the outlet.
When the seal is ultimately broken, a smaller volume of water will return to the heating coil than
with an unrestricted large bore riser, but as the water seal arrangement requires a smaller volume
of condensate to form a water seal, it will immediately re-form.
2.10.8 The Steam and Condensate Loop
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
If the process involves articles being dipped into the liquid, it may not be convenient to install
the coil at the bottom of the tank - it may be damaged by the objects being immersed in the
solution. Also, during certain processes, heavy deposits will settle at the bottom of the tank and
can quickly cover the heating surface, inhibiting heat transfer.
For these reasons side hung coils are often used in the electroplating industry. In such cases
serpentine or plate-type coils are arranged down the side of a tank, as shown in Figure 2.10.3.
These coils should also have a fall to the bottom with a water seal and a small bore dip-pipe. This
arrangement has the advantage that it is often easier to install, and also easier to remove for
periodic cleaning if required.
Steam inlet Condensate outlet
Coil
Dip pipe
Water seal
Fig. 2.10.3 Side hung coils
If articles are to be dipped into the tank, it may not be possible to use any sort of agitator to
induce forced convection and prevent temperature gradients occurring throughout the tank.
Whether bottom or side coils are used, it is essential that they are arranged with adequate
coverage so that the heat is distributed evenly throughout the bulk of the liquid.
The diameter of the coil should provide sufficient length of coil for good distribution. A short
length of coil with a large diameter may not provide adequate temperature distribution. However
a very long continuous length of coil may experience a temperature gradient due to the pressure
drop from end to end, resulting in uneven heating of the liquid.
Whilst the next two headings, Sizing the control valve and The condensate removal
device are included in this Module, the new reader should refer to later Blocks and
Modules in The Steam and Condensate Loop for full and comprehensive information,
before attempting sizing and selection of equipment.
Control valve arrangement
The control valve set may be either one or two valves in parallel. A single control valve, large
enough to cope with the maximum flowrate encountered at start-up, may be unable to control
flow accurately at the minimum expected flowrate. This could cause erratic temperature control.
An alternative is to fit two temperature control valves in parallel with:
o One valve (running valve) sized to control at the lower flowrate.
o A second valve (starting valve) to pass the difference between the capacity of the first valve,
and the maximum flowrate.
The starting valve would have a set-point slightly lower than the running valve, so it would close
first, leaving the running valve to control at low loads.
The Steam and Condensate Loop 2.10.9
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
Sizing the control valve
The control valve set (either one valve or two valves in parallel).
The coil has been sized on mean heat transfer values. However, it may be better to size the
control valve to supply the maximum (start-up) load. With large coils in tanks, this will help to
maintain a degree of steam pressure throughout the length of the coil when the steam is turned
on, helping to push condensate through the coil to the steam trapping device. If the control valve
were sized on mean values, steam pressure in the coil at start-up will tend to be lower and the
coil may flood.
Using one valve
Continuing with Example 2.10.1 the maximum steam load is 865 kg /h and the coil is designed
to deliver this at a pressure of 1.1 bar g. The steam valve sizing chart (see Appendix, Block 14)
shows that a Kv of about 20 is required to pass 865 kg/h of steam with a pressure of 3.6 bar a at
the inlet of the control valve, and Critical Pressure Drop (CPD) across the valve. (Module 6.4 will
show how the valve size can be determined by calculation).
A DN40 control valve with a larger Kvs of 25 would therefore need to be selected for the
application.
If one valve is to be used, this valve must ensure the maximum heat load is catered for, while
maintaining the required steam pressure in the coil to assist the drainage of condensate from it at
start-up. However, for reasons previously explained, two valves may be better.
The running load is 52 kW and with the coil running at 1.1 bar g, the running steam load:
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The steam valve sizing chart shows a Kv of 2 is required to pass 85 kg /h with 3.6 bar upstream,
operating at critical pressure drop.
A DN15 KE type valve (Kvs = 4) and a DN25 piston actuated valve (Kvs = 18.6) operating
together will cater for the start-up load. When up to temperature, the larger valve will shut
down, allowing the smaller valve to give good control.
The condensate removal device
The selection and sizing of the condensate removal device will be very much influenced by the
condensate back pressure. For the purpose of this example, it is assumed the back pressure is
atmospheric pressure. The device should be sized so it is able to satisfy both of the following
conditions:
1. Pass 865 kg /h of condensate with 1.1 bar g in the coil, i.e. the full-load condition.
2. Pass the condensate load when steam pressure in the coil equals the condensate back pressure,
i.e. the stall load condition.
2.10.10 The Steam and Condensate Loop
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
If the steam trap is only sized on the first condition, it is possible that it may not pass the stall load
(the condition where the product approaches its required temperature and the control valve
modulates to reduce steam pressure). The stall load may be considerable. With respect to non-
flow type applications such as tanks, this may not be too serious from a thermal viewpoint
because the contents of the tank will almost be at the required temperature, and have a huge
reservoir of heat.
Any reduction in heat transfer at this part of the heating process may therefore have little immediate
effect on the tank contents.
However, condensate will back up into the coil and waterhammer will occur, along with its
associated symptoms and mechanical stresses. Tank coils in large circular tanks tend to be of
robust construction, and are often able to withstand such stresses. Problems can however occur
in rectangular tanks (which tend to be smaller), where vibration in the coil will have more of an
effect on the tank structure. Here, the energy dissipated by the waterhammer causes vibration,
which can be detrimental to the life of the coil, the tank, and the steam trap, as well as creating
unpleasant noise.
With respect to flow-type applications such as plate heat exchangers, a failure to consider the
stall condition will usually have serious implications. This is mainly due to the small volume in
the heat exchanger.
For heat exchangers, any unwanted reduction in the heating surface area, such as that caused by
condensate backing up into the steam space, can affect the flow of heat through the heating
surface. This can cause the control system to become erratic and unstable, and processes requiring
stable or accurate control can suffer with poor performance.
If heat exchangers are oversized, sufficient heating surface may remain when condensate backs
up into the steam space, and reduction of thermal performance may not always occur. However,
with heat exchangers not designed to cope with the effects of waterlogging, this can lead to
corrosion of the heating surface, inevitably reducing the service life of the exchanger. Waterlogging
can, in some applications, be costly. Consider a waterlogging air heater frost coil. Cold air at 4°C
flowing at 3 m /s can soon freeze condensate locked in the coils, resulting in premature and
unwarranted failure. Proper drainage of condensate is essential to maintain the service life of any
heat exchanger and air heater.
Steam traps are devices which modulate to allow varying amounts of condensate to drain from
applications under varying conditions. Float traps are steam traps designed to modulate and
release condensate close to steam temperature, offering maximum plant performance, maximum
plant life, and maximum return on plant investment.
When stall conditions occur, and a steam trap cannot be used, an automatic pump-trap or
pump and trap in combination will ensure correct condensate drainage at all times, thus
maximising the thermal capability and lifetime costs of the plant.
The Steam and Condensate Loop 2.10.11
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
Steam jackets
The most commonly used type of steam jacket consists simply of an outer cylinder surrounding
the vessel, as shown in Figure 2.10.4. Steam circulates in the outer jacket, and condenses on the
wall of the vessel. Jacketed vessels may also be lagged, or may contain an internal air space
surrounding the jacket. This is to ensure that as little steam as possible condenses on the outer
jacket wall, and that the heat is transferred inwards to the vessel.
Automatic
air vent
Steam
Strainer Steam heated
cooking vessel
Strainer
Condensate
Fig. 2.10.4 A conventional jacketed vessel
The heat transfer area (the vessel wall surface area), can be calculated in the same manner as
with a steam coil, using Equation 2.5.3 and the overall heat transfer coefficients provided in
Table 2.10.4.
Although steam jackets may generally be less thermally efficient than submerged coils, due to
radiation losses to the surroundings, they do allow space for the vessels to be agitated so that
heat transfer is promoted. The U values listed in Table 2.10.4. are for moderate non-proximity
agitation.
Commonly the vessel walls are made from stainless steel or glass lined carbon steel. The glass
lining will offer an additional corrosion resistant layer. The size of the steam jacket space will
depend on the size of the vessel, but typically the width may be between 50 mm and 300 mm.
Table 2.10.4 Overall heat transfer coefficients for steam jackets
Process fluid or product Wall material U (W /m²°C)
Stainless steel 850 - 1 700
Water
Glass-lined Carbon steel 400 - 570
Stainless steel 450 - 1 140
Aqueous solution
Glass-lined carbon steel 285 - 480
Stainless steel 285 - 850
Organics
Glass-lined carbon steel 170 - 400
Stainless steel 340 - 910
Light oil
Glass-lined carbon steel 230 - 425
Stainless steel 57 - 285
Heavy oil
Glass-lined carbon steel 57 - 230
2.10.12 The Steam and Condensate Loop
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
Questions
1. A tank of water is to be heated by a mild steel coil from 20°C to 80°C in 4 hours.
The control valve is supplied with steam at 4 bar g. The mean heat- up steam demand
is 98 kg /h and the running demand is 27 kg /h. (Take the U value of the coil to be
550 W/m2°C).
Approximately what length of 25 mm coil will be required ?
a| 12.5 m ¨
b| 7.6 m ¨
c| 10.4 m ¨
d| 12.2 m ¨
2. What is the disadvantage of heating a tank by direct steam injection ?
a| It agitates the solution ¨
b| Some of the enthalpy of water is used ¨
c| Steam traps are not required ¨
d| It dilutes the tank content ¨
3. A published U value from a steam coil to a water based solution is given as
550 - 1 300 W/m²°C.
When would a figure near the lower end of the range be used ?
a| When the steam is known to be of good quality ¨
b| For short coils ¨
c| For small diameter coils ¨
d| When scaling or fouling of the coil takes place ¨
4. Steam coils should enter and leave the top of a tank when:
a| The tank contains a corrosive solution ¨
b| When agitation of the tank solution is required ¨
c| When steam locking of the trap draining a base coil could occur ¨
d| When good heat distribution is required ¨
5. What range of U values would you apply for a mild steel jacket around a stainless steel
tank containing a water and detergent solution ?
a| 285 - 480 ¨
b| 450 - 1 140 ¨
c| 850 - 1 700 ¨
d| 285 - 850 ¨
The Steam and Condensate Loop 2.10.13
Block 2 Steam Engineering Principles and Heat Transfer Heating with Coils and Jackets Module 2.10
6. 20 m of 25 mm stainless steel coil maintains a tank of water based solution at 65°C.
Steam pressure is 3 bar g and there is natural circulation in the tank. What will be the
approximate steam consumption under this condition (Take the U value of the coil to
be 700 W/m2°C) ?
a| 256 kg /h ¨
b| 382 kg /h ¨
c| 287 kg /h ¨
d| 195 kg /h ¨
Answers
1: a, 2: d, 3: d, 4: a, 5: b, 6: d
2.10.14 The Steam and Condensate Loop
