don4331
Senior Airman
The ".... 40 per cent intercooling ...." that Lovesey mentions in his Merlin paper is not an exact number, but merely a rough indication. 35 to 40 % was their objective when they designed the intercooler.
Note also that Lovesey does not call it efficiency of effectiveness.
The efficiency of any heat exchanger is normally defined by the formula that Simon Thomas posted.
The intercooler does not know and does not care what the temperature rise over the supercharger is.
Its performance is only determined by the inlet temperatures of the hot and cold fluids, its heat exchange area A, and its overall heat transfer coefficient U (which depends on fluid velocities and physical properties). When these are all known one can easily calculate the outlet temperatures of the hot and cold fluids.
According to p169 of "The Merlin in Perspective" (RRHT HS2) the Merlin 61 has a full throttle altitude of 23500 ft while delivering +15 psi boost.
So that means that the supercharger must be delivering a pressure ratio of 5.0
At -32 oC ambient and an adiabatic efficiency of about 69 % that would result in a supercharger outlet temperature of 173 oC, which corresponds with a temperature rise of 205 oC.
At an intercooler outlet temperature of 100 oC (according to Hooker) the "percent intercooling" (as defined by Lovesey) is then (173 - 100)/205 = 36 %.
I used an assumed supercharger efficiency of about 69 % to get the 205 oC rise quoted by Lovesey. Probably a few percent too high but the above calculation is just to check whether the numbers quoted by Lovesey and Hooker are plausible.
If the Merlin 61 flies at a different altitude than 23500 ft the ambient temperature will be different, and so will the supercharger outlet temperature, as well as the intercooler outlet temperature, and the newly calculated "percent intercooling" will likely also differ from the 36 % above.
![Daggerr](/forum/data/avatars/s/78/78498.jpg?1686837847)
69% efficiency doesn't seem too high - turbochargers from my misspent youth had sweet spots in the mid-70s
Thanks for point out my error - it was 205*C temperature rise, not temperature of 205*C - duh...
My 'hang up' is still (IMHO) that the denominator in the formula is for the cooling medium - as you say they delta temperature of the supercharger doesn't matter. And while the heat exchanger area and heat transfer coefficient determine the outlet temperature, it is just the outlet temperature which matters.
With updated numbers (173-100)/.4 = 182.5*C as output fluid temperature needs to be <173*C, that would suggest input temp of -9.5*C; that's a plausible number.
The fluid input for the 'radiator' could now be up to 173*C, output -9.5*C; air input -32*C. The delta of only ~22*C suggests that the 'radiator' is more efficient than the aftercooler, but as it has more area, that's not surprising.
And yes, your results will vary if you fly at different altitude/temperature/power level, etc, etc.