Basic Data & Course Rules of Thumb
Additional information is in chapter 9 of the text.
Basic Data
Utility Availability (See also Turton Table 6.3 page 197)
Utility
Pressure
Temperature
Cooling Tower Water
5 bar(g)
30 °C
Cooling Water Return
3.5 bar(g)
< 45 °C
Boiler Feed Water
6 bar(g)
90 °C
High Pressure Steam
41 bar(g)
Sat'd = 251 °C
Medium Pressure Steam
11 bar(g)
184 °C
Low Pressure Steam
6 bar(g)
159 °C
Natural Gas
4 bar(g)
25 °C
Hydraulics and Simulations:
1. All equipment has a pressure differential across it.
Pumps, blowers, fans, compressors, ejectors etc. all have pressure increases across them.
Piping, control valves, heat exchangers, tanks etc. all have pressure DROP across them when
material is flowing through them. The exception is a heat exchanger that is acting as a condenser.
A condenser can have flow, but no significant pressure change.
Simulators usually do not show the control valves or piping pressure drop.
2. By Itself, Water will only flows downhill
To move any fluid around a plant you need pressure differential. The fluids will flow in the
direction of decreasing pressure. Gravity also comes into play when we are dealing with liquids
(higher density fluids). When a fluid is moved vertically upwards (i.e to the top of a column, or
the feed stage), it takes energy to get it there. There must be either some pressure available to
move it upwards or we need a pump to give us that pressure. As the fluid move upwards the
pressure in the pipe drops. Similarly, when a fluid moves vertically downwards, it's pressure
increases due to "static head", but since there's frictional loss from the flow the pressure increase
isn't quite as much as the pure static head.
Static head is usually measured in feet or meters of fluid. It's an often confusing concept to
master, but the static heat at the bottom of a 1 meter of column of mercury is exactly the same
static head as if the column contained water. However, the pressure (in kPa, or bar, or psig) at
the bottom of the column will be completely different. When we get to pump sizing you see
why we often talk of static head (it's because pumps produce a constant static head at any given
flow - why do you think that fact would be useful?).
Simulators rarely, include the effects of "static head"
3. Simulators usually ignore items 1 & 2 above. But the design of the process can not.
4. It requires energy to move a fluid horizontally through piping and valves because of the
'friction' that's imposed by the piping. This energy can be supplied by gravity, upstream pressure,
fluid flashing, or a fluid mover (pump, fan, compressor ...).
5. Pressure drop through most items listed above is related to the square of the fluid velocity or
flow. So, if you know the pressure drop at one flow rate, you can quickly estimate the pressure
drop at other flows by squaring the ratio of the flow rates and multiplying this by the first
pressure drop.
Maximum Operating Pressure:
All equipment in CapCost requires the field "Maximum Operating Pressure" be provided. First,
what do they mean by Max. Operating Pressure? If you think of a Aluminum Can you can
probably imagine that if you connected up a source of high pressure gas (like a nitrogen bottle at
2000 psig) you would blow the aluminum can to pieces. Similarly, if you drew enough vacuum
on the tin can it would implode to a crumpled heap. The thickness of the material in the can is
carefully chosen (using calculations usually done by mechanical engineers) to meet certain
pressure and vacuum requirements. If you were to believe me that the cost of the can is mostly
related to the thickness (i.e. weight of aluminum) then you can imagine that the pressure the can
is required to withstand will also be correlated to the cost (the thickness, and thus the weight).
For this reason we can use the Maximum Operating Pressure as a key indicator of price.
How do we determine the Maximum Operating Pressure? We use our pressure profile to assist us
in this exercise. You need to know what the normal operating pressure is (that's easy, it's the
pressure in the simulation), then you need to understand what the possible excursions from
normal conditions could be.
For example, if you have a distillation column that the simulator says normally operates at 50
psig (3.4 barg) you might consider if there are special start-up (start-up with water or start-up
under full reflux), shut down, or maintenance requirements (steam out cleaning) which might
require the column be operated at a higher pressure for short periods of time. Once you
understand this, the next step is to add a little safety margin to your design pressure.
Your text (pg 342) defines a couple of different pressure levels. They are:
Normal Operating Pressure (the value which is used in the simulation)
Maximum Operating Pressure = Normal Operating Pressure + 25 psi (but in real life you
should be including all the potential odd ball special pressure conditions (i.e. start-up,
shut down etc.) + 25 psi)
Design Pressure = 10% or 10 to 25 psi + Maximum Operating Pressure
We should take the simulation pressures and add 25 psig to determine the maximum operating
pressures. It's best to make the column and the process wetted side of the condenser, reboiler,
and reflux tank all the same maximum operating pressure, it will save you providing a whole
bunch of relief valves.
Although CAPCOST does not include the following in its calculations other factored estimating
programs do. The operating temperature of the equipment also affects the required metal
thickness. Perhaps you can imagine the same aluminum can, heated up to near it's melting point.
As the metal temperature increases, the strength of the metal decreases. The same metal
thickness at 700 °F can sustain much less pressure than when it's at 150 °F so thicker, more
expensive, metal is required at higher temperatures. The mechanical engineers use charts of
metal type versus temperature in the calculation of metal thickness.
Not only do metals lose strength at high temperatures, but also they become brittle and/or lose
strength at low temperatures. So, for equipment operated at -40 deg C or less you will have to
use alloys of steel which are more expensive then plain vanilla carbon steel. The Canadian
weather creates special circumstances. If you were planning to operate equipment at standard
temperatures (20 to 650 deg F) but the equipment is being built in the far north where
temperatures can drop below -40 deg F, you may still need to provide the high alloy steel to
prevent the equipment from being damaged during construction. There are stories of construction
workers dropping a hammer on a pump, the casing cracked, and a new pump had to be flown in.
Heuristics to help you along the way
Columns:
1. Use a 24" tray spacing.
2. Assume that the pressure drop of every tray is 3" of liquid.
3. Some suggested tray efficiencies are: 60-90% for light hydrocarbons and aqueous
solutions, 10-20% for gas absorption and stripping. If you do not have any plant data,
take the conservative number. Suppose the tray efficiency is 60%. You will need x/0.60
real trays in a column, where x is the number of trays in the simulation. Per the textbook,
add 10% of this number for a total of 1.10 * x / 0.60 trays and round up to the nearest
integer.
4. Columns should have a double tray space at feed locations.
5. Columns require hold-up in their base to smooth any operating variability, and at the top
provide vapour disengagement, etc. For towers about 3 ft. in diameter add 4 ft. at the top
and 6 ft. at the bottom.
6. Tray efficiencies for light hydrocarbons and aqueous solutions are 60-90%, for gas
absorption and stripping,
7. Operating pressure is often determined by the temperature of available heating/cooling
utilities. Another factor can be maximum allowable reboiler temperature to avoid
chemical decomposition/degradation.
8. Economical optimum reflux ratio is 1.2 to 1.5 times the minimum reflux ratio, or use
twice the minimum number of trays
9. Reflux pumps should be 10% oversize.
10. For towers with a diameter of about 3 ft, add 1.2m (4 ft) at the top for vapour
disengagement, and 1.8 m (6 ft) at the bottom for liquid holdup and reboiler return
11. L/D should be less than 30, height less than 175 ft
Vessels & Drums (i.e. reflux drums) with design pressures greater than 15 psig (about 1
barg):
1. Flash tanks should be sized to ensure most of the liquid is separated from the vapour.
You can follow the instructions I give in class or Table 9.6 Item 9. Or see the sample
calculations.
2. K values for vertical vapour separators are 0.16 for a vessel with no demister mesh and
0.35 for a vessel with a demister mesh. Minimum velocity through demister should
normally be 30% of design velocity and always greater than 10% (otherwise particles
will drift through the mesh and not be intercepted).
3. Assume the Length to Diameter ratio ( L/D) is 3 unless circumstances require otherwise
(i.e those in 2. above) . Round diameters and lengths off to the nearest 6" (yes, use
imperial).
4. Add ASME 2:1 elliptical heads when you need the true physical length of a vessel. A 2:1
elliptical head has a depth of 1/4 the diameter. For our rough costing/sizing calculations
you can ignore their volume in your calculations. Most programs for costing require the
length of a vessel. Usually the length the programs require is the "tan to tan" length,
which means the heads are not included in the length.
5. Reflux drums require a flow rate and a hold-up time to be sized. See table 9.6 of the text
for guidance in hold up volumes.
Tanks (storage)
1. Use a Cone roof tank for design pressures (0 to 2 psig).
2. Floating roof tanks can be used for liquids with higher vapour pressures (up to about 8
psig, but I need to confirm this).
3. You can use a Sphere for design pressures ( 2 to 15 psig).
4. Assume that raw materials that come in trucks or trains require 2 weeks of storage
(catalyst may be more), or1.5 times the volume of the train or truck, whichever is greater.
5. Assume products that will be shipped by truck or train require 1 month of storage.
6. Large Cone Roof storage tanks typically have a design pressure of 2.5 inches water
(gauge pressure). The implications of this are that fluid going into the tank has to have a
vapour pressure of no more than 1/2 this amount pressure. If the liquid vapour pressure
was higher, then the fluid would flash upon entering the tank and you would have either a
vapour being released out the tank vent, or the tank would explode.

Simulators are super tools for checking the vapour pressure of a stream. The quick
way to do this is to create a stream and specify it's properties as another stream
(the one you want to check the vapour pressure of). Then, you should ensure the
streams temperature is specified, and then clear the stream pressure and specify
the stream vapour fraction as 1.0 (that's equivalent to the stream bubble point).
HYSYS then calculates the resulting pressure, which is its vapour pressure.
If you find the stream vapour pressure exceeds 1/2 the tank design pressure you need to
make adjustments to the temperature of the incoming stream (a.k.a. you need to add a
cooler to the stream), or use a tank with a higher design pressure.
Heat Exchangers:
1. HYSYS by itself does not design heat exchangers to the extent required by CAPCOST (it
does not provide the surface area required). Some exchangers can be sized in HYSYS
sufficient for use by CAPCOST.
2. However, you first need ask yourself the question, "do I have all the heat exchangers?" For
instance, do you need a heat exchanger to start-up or shut down the process that isn't
necessary for steady state. Do you need an exchanger to cool or heat the final product before
it’s sent off site or put into storage tanks? Do you need an exchanger in the storage tank(s) to
keep the material from freezing?
3. Refer to Turton Table 9.11 for Heat exchanger Heuristics.
4. For a conservative estimate set F=0.9 for shell and tube exchangers with no phase changes.
Where F is the LMTD correction factor for non-countercurrent flow. Q=U*A*F*LMTD
5. Standard tubes are 3/4 in OD, on a 1 in triangle spacing, 16 ft long
6. Tube side is for corrosive, fouling, scaling, and high pressure fluids Shell side is for viscous
and condensing fluids
7. Pressure drops are 0.1bar (1.5 psi) for boiling and 0.2 - 0.62 (3-9 psi) for other services.
Typical liquid exchangers allow 10 psi pressure differential. The condensing side of an
exchanger can be considered to have no pressure drop.
8. Minimum temperature approach is 5C for refrigerants, 10C for others .
9. The selection of utility stream should consider upset conditions such that cooling water or
steam doesn’t freeze when used in a process operating at low temperatures. Use glycol or
similar heat transfer media to avoid this problem.
10. Conditions which involve high temperatures may exceed reasonable steam pressures (> 41
bar ~ 250 deg C) the use of Dowtherm A (or equivalent) heat transfer fluid or a fired heater
may be required or more economical to achieve these process conditions.
11. Approximate overall heat transfer coefficients
Hot Fluid
Cold Fluid
Overall Heat Transfer Coefficient (including
fouling factor), "U value"
BTU/hr ft2 F
kW / m2 C
Coolers / Condensers
Water
Water
500
2.84
Methanol
Water
500
2.84
Light Organics
(viscosities < 0.5 cp ,
i.e. benzene, toluene,
acetone, ethanol,
gasoline, light
kerosene, and naptha)
Water
150
0.85
Gases
Water
30
0.17
Heaters / Reboilers
Steam
Water
700
3.97
Steam
Methanol
700
3.97
Steam
Light Organics
200
1.14
Steam
Medium Organics
100
0.568
Steam
Gases
40
0.227
Process Exchangers (No Phase Change)
Water
Water
200
1.14
Light Organics
Light Organics
75
0.43
Water
Gas (100 psi)
40
0.23
Water
Gas (1000 psi)
90
0.51
Adapted from Peters and Timmerhaus "Plant Design and Economics for Chemical
Engineers" , and Kern "Heat Transfer", and the Gas Processors Suppliers Association
Handbook (GPSA)
Pumps:
1. For the purposes of equipment layout (plan and elevation views) assume pumps are 5 ft
long, 3 ft wide, and 3 ft tall.
2. Pump Rate Flow rate = 1.20 * Normal Flow
3. Consider the need for two pumps (a hot spare, as we call it) in applications where a single
failure of a pump might cause a shut down of the entire process. You're the designer you
make the call. If you want to approach it from a business point of view, look at the cost of
the pump versus say 4 hours of downtime (lost $ of production) which would be required
to replace the pump assuming there is a spare one on site.
4. Do show spare pumps on the P&ID’s, not as the book suggests, label them A and B.
Spare pumps should be shown on the layout diagrams too.
5. Single Stage Centrifugal Pumps for 0.057-18.9 m3/min, 152 m maximum head
6. Multiple Stage Centrifugal Pumps for 0.076-41.6 m3/min, 1675 m maximum head
7. Centrifugal pump efficiency: 45% at 0.378 m3/min, 70% at 1.89 m3/min, 80% at 37.8
m3/min
8. Axial Pumps for 0.076-0.378 m3/min, 12 m maximum head, 65-85% efficiency
9. Rotary Pumps for 0.00378-18.9 m3/min, 15,200 m maximum head, 50-80% efficiency
10. Reciprocating Pumps for 0.0378-37.8 m3/min, 300 km maximum head, 70% efficiency at
7.46 kW - 85% at 37.3 kW - 90% at 373 kW
Compressors:
1. Assume for this process that the compressor, it's intercooler, and it's aftercooler, will be
housed in a "compressor building" with the following dimensions: 15 ft x 15 ft x 15 ft.
2. Compression ratio should be about the same in each stage of a multistage unit
3. Efficiency of reciprocating compressors: 65% at 1.5 compression ratio, 75% at 2.0, 80-85%
at 3-6
4. Efficiency of large centrifugal compressors: 77% at 2.83-47.2 m3/s at suction
Reactor:
1. The reactor is a vessel. It has specific volume requirements, as you already know. In
addition, there is a requirement that it be well mixed. You may choose to do the mixing
as you wish. If you use a vertical mixer in the reactor I suggest you keep the overall
length of the agitator less than 10 ft (this has implications that supersede the 3:1 L/D ratio
guideline).
Fired Heaters:
1. There is no requirement for a fired heater.
Pipe Racks:
1. We'll need to run a pipe rack about 20 ft wide and 20 ft high to supply utilities to the heat
exchangers and other equipment, provide electrical distribution to motors and, provide
space for the wiring for the instruments.
Flare:
1. Should the reactor or any other piece of equipment ever begin to overpressure (due to a
process upset or a fire) the relief valves which vent the contents of the vessel(s) will
either vent to atmosphere or to a disposal system. You should decide on whether the
contents are hazardous enough that they require a disposal system (flare, condenser, etc.)
or simply let it blow to atmosphere. Assume there is sufficient flare capacity on site if
you require it.
Power Equipment:
1. Electric motors are 85-95% efficient, steam turbines 42-78%, gas engines and turbines
28-38%