Steam Systems
General
Heat transfer units that use steam to produce hot water are known as indirect heaters.
They are often shell and tube type heat exchangers and are generally referred to as
converters, hot water generators, and instanta-neous heaters. The ASME Code for Unfired
Pressure Vessels is the nationally recognized authority prescribing their construction for
given temperatures and pressures. The term used varies with the heating medium and the
manner of application. When these heaters use steam as the heat source they are usually
called steam to water converters.
In steam heated converters, the water to be heated circulates through the tubes and steam
circulates in the shell surrounding the outside of the tubes. This results in condensate
draining to the bottom of the heat exchanger shell as the steam gives up its latent heat.
Steam to Water Heat Exchangers
The operation of the shell and tube heat exchanger is as follows. Steam enters the heat
exchanger shell through the top vapor opening and surrounds the outside of the tubes. As
energy is transferred through the tubes it heats the water inside the tubes. The heat
transfer condenses steam inside the shell forming condensate that drops to the bottom of
the heat exchanger shell. The condensate flows through the bottom condensate outlet and
into a steam trap.
The steam pressure in the heat exchanger shell has a direct correlation to the temperature
of the condensate formed in the shell. The properties of saturated steam are such that the
steam temperature varies with steam pressure (See Table 1). When the latent heat of
vaporization is removed, the resulting condensate will be close to the saturation
temperature. Depending on the system load, slight sub-cooling may occur from the bottom of
the heat exchanger and the inlet piping to the steam trap.
The heat exchanger should be selected to operate at the minimum possible steam pressure.
This allows the lowest possible condensate temperature to discharge from the steam trap
and reduces the amount of flash steam in the return system. When heating fluids up to
200°F, the heat exchanger should be selected based on 2 psig steam pressure in the shell
for the most efficient system operation.
This may require a slightly larger heat exchanger than one operating at higher pressure,
however it will result in a smaller less expensive low pressure steam trap and a smaller
steam regulating valve. The low pressure selection will also limit the maximum temperature
that can occur inside the tubes, should the temperature controller fail in an open
position.
It is standard practice to add a fouling factor in the heat exchanger selection. This
fouling factor adds additional tube surface area to assure adequate heating after normal
scale and corrosion deposits on the tube surfaces. A standard .0005 fouling factor will
add 20 to 25% additional tube surface area. When the heat exchanger is new and the tubes
are clean and shiny, the heat exchanger will operate at lower than design pressure even at
full system load. For example, a new heat exchanger designed for 15 psi steam to heat
water to 160 degrees will generally heat the full system load with 0 psi steam in the heat
exchanger shell.
Heat Exchanger Selection
The heat exchanger should be selected for operation at the minimum pressure to provide the
most efficient operation. The properties of saturated steam tables show a larger amount of
the latent heat is available at low pressure. Less energy remains in the condensate
reducing the flash steam losses. A reasonable guide would be to select a steam pressure
that has a saturation temperature approximately 30°F higher than the required outlet
temperature of the fluid being heated in the tubes. For fluid temperatures up to 200°F, 2
psig steam is recommended.
When a high steam pressure source is used,
the pressure should be reduced by installing a steam pressure regulating valve or by using
a combi-nation temperature pressure regulator.
After selecting the heat exchanger, the next step should be planning the instal-lation.
The heat exchanger should be mounted high enough to allow gravity drainage of the
condensate from the steam trap into a vented gravity return line. If a gravity return line
is not available, a condensate pump should be installed. The heat exchanger should be
mounted with a pitch toward the condensate outlet. A minimum 1/2 inch pitch per 10 foot
length should be provided. The heat exchanger should also be located such that removal of
the tube bundle is possible.
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| Typical Steam to Water Heat Exchanger
Installation |
Steam Traps
The steam trap must be capable of completely draining the condensate from the heat
exchanger shell under all operating conditions. On a heat exchanger using a modulating
temper-ature regulator to heat fluids under 212°F, the steam pressure in the shell can be
0 psig. To assure condensate drainage, the steam trap must be mounted below the heat
exchanger outlet tapping and it must drain by gravity into a vented condensate return
unit. When possible, the trap should be located 15 inches below the heat exchanger outlet.
The 15 inches static head to the trap inlet will provide 1/2 psig static inlet pressure to
the trap when the shell steam pressure is at 0 psig.
The trap should be sized based on this 1/2 psig differential pressure. A safety factor of
1.5 times the calculated full load capacity should be used to handle unusual start up
loads. A float and thermostatic trap is normally the best selection for a heat exchanger.
The thermostatic element quickly vents the air from the heat exchanger shell. The
modulating float element provides continuous condensate drainage equal to the system
condensing rate.
Failure to provide complete condensate drainage will lead to poor temperature control and
possible water hammer. Any lift in the condensate return piping after the trap discharge
requires a positive pressure to develop in the heat exchanger shell to provide condensate
drainage. For this to occur, condensate must back up in the heat exchanger shell until
enough tube surface is covered by condensate to build a positive steam pressure. When the
positive steam pressure develops to move the condensate through the steam trap and up the
vertical return line, over heating can occur on the tube side of the heat exchanger due to
the positive steam pressure remaining in the shell. This results in a wide range of outlet
fluid temperatures from the heat exchanger. A lift in the return line as shown above
should be avoided on heat exchangers using a modulating control valve.
A lift or back pressure in the steam trap return piping can flood the heat exchanger shell
and cause severe water hammer as steam enters the flooded shell. The resulting water
hammer can damage the steam trap, the steam regulating valve, the heat exchanger tubes and
cause the gasket in the heat exchanger and trap to fail.
Trap Installation
The trap should be located below the heat exchanger shell to allow free flow of condensate
into the trap. A strainer complete with a screen blow down valve should be installed ahead
of the steam trap. A shut off valve should be provided in the trap discharge return line
to isolate the unit for service. Unions should be provided to allow trap service or
replacement. The return line from the trap discharge should be pitched into a vented
condensate return unit.
Vacuum Breakers
Most steam to water heat exchangers provide a tapping in the shell to allow installation
of a vacuum breaker. The vacuum breaker allows air to enter the shell when an induced
vacuum occurs. Failure to install a vacuum breaker will allow the heat exchanger shell to
operate at a negative pressure which may cause condensate to be held up in the shell.
During light load, the heat exchanger will have a layer of steam at the top and air under
the steam to provide just the right amount of heat. The vacuum breaker should be mounted
on a vertical pipe 6” to 10” above the topping to provide a cooling leg. This
protects the vacuum breaker from dirt and extreme temperatures.
Steam Regulator
The choice of the temperature regulating valve includes self contained temperature
regulators, pilot operated regulators and pneumatic regulators. The steam inlet pressure
to the regulator must be higher than the required heat exchanger operating pressure to
allow flow. The available steam pressure should be at least two times the heat exchanger
operating pressure to provide modulation of the regulator for good temperature control.
This will also provide the smallest size steam regulator. The steam regulator should be
sized based on the maximum lb./hr. of steam required by the heat exchanger. To properly
size the regulator, the available inlet steam pressure and the heat exchanger design
operating pressure must be known. The steam regulator should not be oversized. Oversizing
the regulator may cause the temperature to overshoot and the regulator will hunt more than
a properly sized regulator. The steam regulator is normally smaller than the connecting
inlet and outlet steam piping.
Regulator Installation
A steam drip trap should be installed in the steam piping ahead of all steam regulating
valves. Failure to install a drip trap will allow condensate to collect in the steam
piping ahead of the regulator. As the regulator opens, the mix of condensate and steam
passing through the regulator may cause water hammer that can destroy the diaphragms or
bellows used to operate the regulator. A steam strainer should also be installed ahead of
the regulators to prevent dirt from entering the valve. Dirt can deposit on the valve seat
and not allow it to close tight. The steam strainer should be installed with the screen
pocket horizontally. Installation with the screen down, as commonly piped for water
service, will allow a condensate pocket to form in the steam line. This condensate pocket
can carry into the main valve and cause water hammer or sluggish operation. Shut off
valves, pressure gauges, a manual bypass and unions should be installed to allow proper
servicing of the valves and strainers. When possible refer to the manufacturer’s
installation manual for proper installation. The temperature sensing bulb should be
installed as close as possible to the heat exchanger outlet. It is important that the full
length of the temperature sensing bulb be inserted in the system piping. Any portion of
the bulb installed in a no flow area will reduce the accuracy of temperature control. When
the sensing bulb is installed in a separable well, heat transfer compound must be
installed between the well and the sensing bulb to aid heat transfer. The tube side of the
heat exchanger should have a continuous running recir-culation pump to provide continuous
flow past the sensing bulb. A minimum 20% recirculation should be provided. Pilot operated
regulators with a pressure pilot require a downstream pressure sensing line. The pressure
sensing line connection should be connected in a non-turbulent area downstream of the main
valve; a min-imum 10 pipe diameters downstream of the main valve is recommended. The steam
pressure sensing connection can also be connected directly to the heat exchanger shell.
Condensate Coolers
When heat exchangers operate at high pressure, consideration should be given to the
addition of a condensate cooler. The justification will depend on the size of the heat
exchanger and the actual number of hours per day the unit will be in operation. With a
condensate cooler, the discharge from the steam trap on the steam heat exchanger outlet is
piped through a water-to-water heat exchanger. A second trap is then installed on the
discharge of the water-to-water heat exchanger to maintain saturation pressure and prevent
flashing and water hammer from occurring in the condensate cooler. A separate thermostatic
trap is installed to allow direct air venting of the steam heat exchanger into the vented
return line downstream of the condensate cooler.
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| Installation with Condensate Cooler |
The water-to-water heat exchanger design
differs from a steam heat exchanger. The water-to-water heat exchanger has internal
baffles to direct the water flow across the tubes to improve heat transfer. Water-to-water
heat exchangers are externally distin-guishable as the shell inlet and outlet tappings are
the same size; steam heat exchangers have a large vapor opening in the top of the shell
and a smaller condensate outlet in the bottom.
The fluid in the condensate cooler tubes may be the inlet water to the steam heat
exchanger tubes. When the initial temperature of the fluid is too high to cool the
condensate below 212°F, a separate fluid may be heated. Preheating domestic hot water or
preheating boiler make up water are two possibilities.
This article is reprinted from Fluid Handling’s Steam Team Bulletin FHD-206.
Reprinted from TechTalk June 1999, Volume 14, Issue 2 |
Article
Morton Grove
Tel 847 966-3700
Steam Control and Condensate Drainage for Heat Exchangers
