|
Selection of the proper model and size heat
recovery unit involves consideration of the characteristics
of the heat source, the requirements of the heat using
process, the cleanliness and corrosiveness of the exhaust
gas, the amount of noise reduction required, and any
physical size or weight limitations. A.
Heat Source and Working Fluid The heat
source is typically a reciprocating engine or gas turbine
and the recovered heat is most frequently used to produce
hot water, steam, or to heat one of the organic heat
transfer fluids. The requirements of the heat using
process determine the type of working fluid. For
temperatures up to about 350°F, the most common heat
transfer fluids are water and steam. Above 350°F, organic
high temperature heat-transfer fluids are used. Hot water
systems are often used when heat loads are extremely
variable and when reduced pipe sizes and simplified piping
arrangements afford cost savings. When heat recovery
units are used to supplement existing boilers or operate
steam powered equipment, a steam system would be
appropriate.
Heat recovery units designed for steam or
hot water service are not necessarily suitable for use with
organic heat transfer fluids. Since organic fluids degrade
when exposed to excessive temperatures, a more detailed
thermal analysis is required to assure that the maximum bulk
and film temperatures specified for the fluid are not
exceeded. These calculations require a knowledge of
the equipment details. MAXIM engineering assistance
should be requested when selecting heat recovery units using
organic fluids.
The following table lists the primary
applications of MAXIM® Heat Recovery Units as a function of
the heat source and working fluid.
|
MODEL |
TABLE 1 HEAT RECOVERY MODELS AND APPLICATIONS |
|
HEAT
SOURCE |
WORKING FLUID |
|
RECIPROCATING ENGINE |
GAS
TURBINE |
STEAM |
HOT
WATER |
HEAT
TRANS-FER FLUID |
|
MFT
|
X
|
|
X(1)(3) |
X
|
X(4)
|
|
BVS
|
X
|
|
X
|
|
|
|
WVS/WHS
|
X
|
|
|
X
|
|
|
GTW
|
X(4)
|
X
|
X(1)
|
X
|
X(4)
|
|
TRP
|
X
|
|
X(2)
|
|
|
|
SOH
|
X(4)
|
X
|
|
X |
X
|
|
MBT
|
X |
X |
X
|
X
|
|
|
Notes: |
|
|
1. Requires separate steam
separator.
2. For ebullient cooled engines only.
|
3.May be used with ebullient cooled engines.
4. Requires engineering approval before using. |
|
Models MFT, BVS, WVS,
WHS, GTW and TRP are available in pre-engineered sizes that
allow simplified selection and performance rating. The GTW
is modular in construction and its performance depends upon
the number of modules used. SOH and MBT units are designed
to match particular fluid and process requirements and are
not available in standard sizes.
The BVS and TRP series incorporate an
integral separation device and produce saturated steam. The
MFT and GTW series are designed for either steam or water
service and require a separate steam separator when used in
steam service. MAXIM® Model HSS Steam Separators have
been designed specifically for use with MAXIM Heat Recovery
Units and provide the necessary water and steam separation.
In addition they serve as a liquid level control point for
the system and contain necessary gauges, alarm switches, and
safety devices. B. Pressure Drop
The size of the heat recovery unit is a function of pressure
drop (both gas and liquid side) and heat recovery
requirements. The pressure drop of the exhaust gas system
which includes the exhaust piping, heat recovery unit,
auxiliary silencer, and tailpipe must not exceed that
allowed by the engine manufacturer. Loss of power, poor fuel
economy, and overheating are adverse effects of excessive
back pressure. Since the heat recovery unit is the most
expensive component in the exhaust system, as much of the
allowable pressure drop as possible should be reserved for
it in order to minimize its size and cost. It is not
good practice, however, to size the unit with connections
smaller than the exhaust piping. C.
Corrosion Serious corrosion problems can
result from operating the equipment under conditions in
which gas side condensation can occur. Water vapor is
a normal product of combustion of hydrocarbon fuels and air.
If the fuel contains sulfur or hydrogen sulfide, some of the
sulfur oxides resulting from combustion with excess air will
combine with the water vapor to form sulfuric acid.
Water or acid will condense on surfaces that are at a
temperature below the water or acid dewpoint. For the
exhaust gas of a reciprocating engine or gas turbine, the
water dewpoint is typically in the 100-150° F range while
the acid dewpoint is in the 200-300° F range. To
prevent condensation and resultant corrosion, the metal
surfaces of the heat recovery unit must be kept above the
applicable dewpoint. This requires that a limit be placed on
the temperature of the fluid entering the unit and the
amount of heat that can be recovered from the exhaust gas.
As a general guide, it is recommended that the inlet fluid
temperature be above 160° F (200° F if the fuel contains
sulfur or hydrogen sulfide) and that the heat recovery unit
be sized to keep the outlet gas temperature above 350° F at
rated load.
For special corrosion-resistant alloy
construction, MAXIM engineering needs to be contacted.
D. Silencing Some of the MAXIM Heat
Recovery Units are designed to be effective silencers while
others may require the addition of a silencer for effective
noise control. The MFT, TRP, MBT, WHS, WVS, and BVS incorporate
silencing features and rarely require supplementary
silencers. The GTW and SOH provide limited noise
control by themselves and often require the addition of a
silencer for effective control of exhaust noise. |
