Page 200 - Sustainable On-Site CHP Systems Design, Construction, and Operations

The Engineering Pr ocess 173

cooling (CTIC). CTIC, for example, using either a cooling coil or evaporative cooling,
counteracts the negative effects of increased heat rate and reduced capacity caused by
inlet-air temperatures above the nominal rating temperature of 59°F. Typically, the CTG
inlet-pressure drop is limited to no more than 3 in of water column (wc), so the design
engineer must layout the proposed CHP plant and combustion air-inlet ductwork sys-
tem carefully to avoid unnecessary changes in direction which will increase the air-inlet
pressure drop. Smaller angle changes in ductwork result in lower pressure drops than
do larger angle changes (e.g., all else being equal, a 90° elbow has more pressure drop
than a 45° elbow). Ductwork velocity, which is a function of ductwork size for a given
combustion air flow rate, is the controlling variable with respect to pressure drop through
the combustion air-inlet system, as for a given system, the pressure drop varies with the
square of the air flow velocity (double the velocity equals 4 times the pressure drop).
Combustion air systems are not as critical with IC reciprocating engines as they are
with CTGs; however, engines still require cool, clean air for combustion. Like a CTG, an
IC reciprocating engine combustion air system can also include an OSA louver (if the
CHP system is located indoors), inlet-air filters, ductwork and an inlet-air duct silencer
to prevent engine noise from translating out the ductwork to the outside.

Exhaust Systems
On a CTG CHP system with relatively high mass flow rates, similarly to combustion
air-inlet systems, the exhaust gas system pressure drop must be kept as low as possible,
within the manufacturer’s maximum allowable backpressure limit, in order to mini-
mize the reduction of combustion turbine capacity. The maximum allowable CTG back-
pressure is about 8 in of water column, and similarly to the combustion air system,
pressure drop can be minimized by proper duct sizing, minimizing unnecessary duct
twists and turns, and by selecting the heat recovery device for a relatively low pressure
drop. A CTG exhaust system typically includes the exhaust ductwork, emissions con-
trol equipment, the HRSG or heat recovery heat exchanger, continuous emissions mon-
itoring system (CEMS), exhaust bypass valve (if allowed), exhaust silencer (if required)
exhaust stack, and expansion joints to help accommodate thermal expansion. The duc-
twork must be insulated to keep in the heat to be recovered (i.e., minimize heat loss),
and to protect operating personnel.
For a turbocharged IC reciprocating higher engine backpressures up to almost 30 in
of water column are possible without as significant of performance degradation as
compared to a CTG. However, sustainable design still entails designing for the mini-
mum economical pressure drop. Typically, black steel pipe is used for IC reciprocating
engine exhaust as compared to sheet metal ducting for CTG exhaust. Dependent upon
emissions treatment methods employed, some sections of stainless steel pipe may be
required. In addition to the exhaust pipe itself, which ultimately conveys products of
combustion to the outside, typical IC engine exhaust components include: emissions
control equipment; the heat recovery heat exchanger; continuous emissions monitoring
system; exhaust muffler, exhaust stack, and expansion joints to help accommodate
thermal expansion.
An expansion joint should be provided at the IC engine itself to allow for thermal
expansion and to prevent forces and stresses from acting on the engine exhaust flange.
The hanger and seismic supports should be determined, as well as the amount and direc-
tion of thermal expansion, in order to accommodate that expansion with, for example,
expansion joints.