Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

INTERCOOLER, AFTERCOOLER,ANDREDUCTION-

GEARKEELCOOLERS Most marine diesels

have either intercoolers or aftercoolers

(sometimes both) to further increase engine

PART THREE: EXHAUST SYSTEMS

Figure 8-9. Hull-
side cooler recess
(Courtesy R.W.
Fernstrum & Co.)

Figure 8-10. Dual
keel cooler: after-
cooler keel-cooler
bypass (Courtesy
Caterpillar, Inc.)

Formula 8-2.

performance. Some intercoolers or aftercool-

ers are heat-exchanger cooled even when the

primary engine cooling is via a keel cooler.

Others are keel cooled. When the intercooler

or aftercooler is keel cooled, their dedicated

keel cooler should be located forward of the

primary engine keel cooler.

The keel cooler for the aftercooler should

be sized per the engine manufacturer’s recom-

mendation. You can also calculate the keel

cooler based on the heat rejection of the after-

cooler in Btu from the engine spec sheets.

Convert Btu to hp or kW using Formula 8-2,

and then use Tables 8-2 and 8-3.

Formula 8-2. Converting Btu to hp or kW

hp× 42. 41 =Btu/min., or, Btu/min. ÷ 42. 41 =hp

kW× 31. 63 =Btu/min., or, Btu/min.

÷ 31. 63 =kW

Reduction gears usually have their own

cooler, which may be keel cooled as de-

scribed. Consult the gear manufacturer for

the heat rejection of the specific gear; how-

ever, you can estimate it based on between

95 and 97 percent efficiency. In other words—

in almost all standard reduction gears—at

least 95 percent of bhp is transmitted to the

propeller shaft. Assuming 95 percent effi-

ciency, on a 200 bhp (149 kW) engine, the heat

rejection in the gear would be 5 percent

(100%−95%=5%). Five percent of 200 bhp

(149 kW) is 19 hp or 7.45 kW. The reduction-

gear keel cooler would be sized for this power.

COOLANT-FLOWVELOCITY ANDTUBELENGTH

SELECTION To avoid erosion of the piping

(particularly at manifold exits and entrances

and at sharp bends), coolant flow velocity

(FV) should generally be less than 8 ft./sec.

(2.45 m/sec.) At the same time—to avoid par-

ticulates and sediment settling out and gradu-

ally clogging the system—flow should be at

least 2 ft./sec. (0.6 m/sec.).

Max. Flow Velocity ≤8 ft./sec.,

or 2.45 m/sec.

Min. Flow Velocity ≥2 ft./sec., or 0.6 m/sec.

Flow rate can be estimated by referring

to the coolant pump specs from the engine

manufacturer. It varies from a high at maxi-

mum rpm to a low at idle. Use the following

procedure to find flow velocity (FV) in the

cooler: