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The four primary systems that offer potential
applications for standard MAXIM Heat Recovery Equipment are:
A. Fluid Heating System Using Exhaust and Jacket Water
Heat
Flow Diagram No. 1 (shown below) illustrates one of the
most frequently used heat recovery systems for reciprocating
engines. In this system, both jacket water and exhaust gas
heat are used to produce hot water. Water is pumped
through the engine jacket and the exhaust gas heat recovery
unit in series where it is heated to the required use
temperature. The system can be unpressurized or pressurized
depending upon the temperature of the water.
Unpressurized systems can be used for water temperatures
below 210° F. For higher temperatures, system pressurization
is required to prevent boiling. Maximum water temperature is
usually limited to 250° F which requires a 20 psig system.
At such high temperatures, the engine oil cooler will
require a separate cooling water circuit. The standard
jacket water pump furnished with the engine will usually not
provide the necessary flowrate with the additional system
flow restrictions; in which case the pump will have to be
replaced with one that develops a higher head. Liquid flow
must be maintained through the heat recovery unit whenever
the engine is running. This requires that the engine
thermostat and the bypass circuit be removed. In their
place, a temperature controlled bypass valve is installed
downstream of the heat recovery unit as shown in Flow
Diagram No. 1. When the available heat exceeds the system
requirements, the excess heat is removed by a heat exchanger
in the fluid return line. Control is maintained by a
temperature controlled valve that regulates the amount of
fluid bypassed to the heat exchanger in order to maintain a
constant return fluid temperature. An expansion tank is
required in the system to absorb liquid expansion and
provide a means of maintaining a pressure on the system in
excess of the saturation pressure. The tank is sized so that
it is about one-fourth full when the system is at ambient
temperature and three-fourths full at operating temperature.
Since there is no flow through the tank, it operates at near
ambient temperature. The MBT, MFT, WVS, or WHS heat recovery
units are all suitable for use in this type of system. All
provide effective silencing of engine exhaust noise.
* or sea water
distillation
B. Fluid Heating System Using Exhaust Heat
A system for heating a single phase fluid with exhaust
gas only is illustrated in Flow Diagram No.2 (shown below).
The heat source can be either a gas turbine or reciprocating
engine and the fluid can be water or one of the organic heat
transfer fluids. The fluid is pumped in a closed loop
through the heat recovery unit where it is heated to the
required temperature by the exhaust gas and then to the heat
user or users where it gives up the heat. Liquid flow must
be maintained in the heat recovery unit to prevent boiling
or degradation of the fluid. Excess heat is removed from the
system by a heat exchanger and temperature controlled mixing
valve in the fluid return line as described previously.
An alternate capacity control method is also shown on the
sketch. The temperature of the fluid leaving the heat
recovery unit is controlled under varying process demands by
a temperature controller and an exhaust gas bypass valve. If
the fluid temperature exceeds the controller set point, the
valve will open a bypass duct and divert part of the exhaust
gas away from the heat transfer surfaces. In using this
method, provisions must be made to prevent gas leakage into
an idle heat recovery unit where the exhaust gas vapors will
condense and corrode the metal surfaces. The expansion tank
acts as an accumulator to absorb liquid expansion, a high
point system vent, and a liquid level check point. If the
tank is installed at the highest point in the system and
connected with double drop legs as shown, it can be used to
vent air and noncondensables from the system. During initial
start-up, valves A and B are opened and valve C is closed
forcing the flow through the expansion tank where entrained
gasses are separated and vented to the atmosphere. During
normal operation, valve A is closed and valves B and C are
opened allowing normal flow return to the pump suction. The
tank is connected to the return line through the open static
line containing valve B. Except during startup, the
tank operates near ambient temperature. To prevent
oxygen from entering the system and to prevent boiling in
the heat recovery
unit, the expansion tank should be pressurized with nitrogen
or another inert gas.
* or sea water
distillation
C. Steam System Using Exhaust and Jacket Water Heat
The system shown in Flow Diagram No. 3 (shown below)
applies to an ebullient cooled reciprocating engine and
recovers both jacket water and exhaust gas heat in the form
of low pressure steam. In an ebullient cooled engine water
is allowed to boil, under controlled conditions, inside the
engine jacket. The resulting steam and water mixture is
piped to an external separator where the steam is removed
for use and the water is returned to the engine. The exhaust
gas heat recovery unit operates in parallel with the engine
jacket and uses a common separator. Steam pressure is
usually limited to 15 psig by the working pressure of the
engine jacket and the maximum operating temperature of the
engine. To maintain a constant pressure in the system and
stabilize the engine temperature, a back pressure valve is
installed in the steam outlet line from the separator.
When steam production exceeds demand, the excess must be
condensed to prevent operation of the safety valves and loss
of treated fluid from the system. This is accomplished by
installing a steam condenser and a pressure controlled valve
in parallel with the heat using process. When steam
pressure exceeds the valve set point, the excess steam is
bypassed to a condenser. The condensate is then
returned to the condensate receiver. The steam separator
must be installed above the heat recovery unit to achieve
adequate fluid circulation. The height is dependent upon the
location of the separator relative to the heat recovery
unit, the size and routing of the interconnecting piping,
and the operating pressure. When a MAXIM® HSS is used,
it should be located adjacent and two feet above the heat
recovery unit. The height should be increased by one
foot for each ten feet of horizontal distance from the heat
recovery unit. The TRP heat recovery unit is designed
specifically for this type system and combines a heat
recovery unit, silencer, and steam separator in a single
package. The MFT in combination with an HSS steam
separator can also be used. For economy in a multiple engine
installation, a MFT exhaust gas heat recovery unit can be
used on each engine with a common separator
and back pressure valve sized to handle the combined flow
from all the units.
* such as sea
water distillation
D. Steam System Using Exhaust Heat
The steam system shown in Flow Diagram No. 4 (shown
below) is one that uses exhaust gas heat recovery only.
This system is applicable to either a reciprocating engine
or gas turbine. Exhaust gas heat is transferred to water in
the heat recovery unit where a portion is vaporized.
The mixture of steam and water flows to the separator where
the steam is removed and piped to the use point. The water
is mixed with feedwater and returned to the heat recovery
unit. With the GTW, additional heat can be recovered to
preheat feedwater in an economizer module installed
downstream of the vaporizer section. An economizer is
considered economical at steam pressures above 100 psig.
Two methods of controlling steam output are shown. In
the preferred method, excess steam, sensed by a rise in
steam main pressure, is bypassed to a condenser and then to
the condensate receiver as described previously. In the
alternate method, a part of the exhaust gas is diverted
around the heat recovery unit to reduce steam production.
Some gas leakage through the diverter valve is to be
anticipated and a small amount of steam may still be
produced with the valve in the full bypass position. If the
steam cannot be used, a condenser sized for about five
percent of the full production will be required. Leakage
into an idle heat recovery unit must be prevented.
Water and acid vapors in the exhaust gas will condense on
and
corrode the metal surfaces. When a separate steam separator
is used it must be installed above the heat recovery unit as
previously described.
* such as sea
water distillation |
