# Homework Help: Problem based on the Energy equation (thermodynamics)

1. Jun 7, 2013

### Urmi Roy

1. The problem statement, all variables and given/known data
We have compressed air @ 6 bar and 298 K. This is input to a pneumatic motor (in which pressure energy is converted into kinetic energy) flow rate is 22 L/sec and we get shaft power (1.06 KW) from pneumatic motor as output. And air leaves pneumatic motor @1 bar. Density of air @ 6 bar is 7 Kg/m^3. And we need to find the efficiency of motor.

2. Relevant equations

I'm using the Steady Flow Energy Equation (SFEE).
So h1 +KE1 +PE1 +Q=h2 +KE2+PE2+W (shaft work)

and h= u +pv

3. The attempt at a solution

Consider the motor as the control volume
Inlet is state 1 and outlet is state 2
State 1-
P1=6 bar =600 KPa
T1= 298K
density= 7 kg/m^3
Q=0.022m^3/s
(all above are given)
From calculation, mass= Qxdensity=0.154kg

State 2-
P2=1 bar=100Kpa
V2= 0.08 m^3 (for the 0.154kg at the outlet)- after using the adiabatic law i.e. P1(V1^γ)=P2(V2^γ)
T2= 178.6 K (from ideal gas equation, after putting (P1xV1)/T1 = (P2xV2)/T2

We neglect the change in KE and PE

But in a pneumatic motor, Q=0 and W is given....so how do we find the efficiency???