# Thermodynamics question help? is this right?

1. Jun 21, 2012

### Aaron9890

Thermodynamics question help?? is this right??

1. The problem statement, all variables and given/known data

A boiler is supplied with water at a rate of 0.3 kg/s through a pipe of diameter 50mm

a) calculate the energy per second that must be supplied to the boiler,raising the water temperature to 100C, when the water is supplied at 15C

b) If the input power to the boiler is 3.1GJ/h, Calculate the efficiency of the boiler and heat transfer process

The density of water - 1000kg/m^3
specific heat capacity for water - 4.7kj/kgK
latent heat evaporation for water - 2.26MJj /kg
steam pipe outside diameter - 55mm
steam pipe wall thickness - 2.5mm
modulus of elasticity for steel - 210GPA
coefficient of linear expansion of steel - 18 x 10^-6/C

2. Relevant equations

Q = MCAO + ML (mass flow rate x heat capacity x change in temp) + (mass flow rate x latent heat for evaporation of water)

Thermal efficiency = Q out / Q in

3. The attempt at a solution

M = 0.3 kg/s
c = 4.7 KJ/kgK in J = 4700J/kgK
AO = 100C - 15C = 85C
L -= 2.26MJ/KgK in J = 2260000J/kgK

so Q = MCAO + ML
Q = (0.3)x(4700)x(85) + (0.3) x (2260000)
Q = 797 850 J/s or 0.79785MJ/s

Thermal Efficency = Qout / Qin
Thermal Efficency = 797.850J / 3100000000J

2. Jun 21, 2012

### LawrenceC

Re: Thermodynamics question help?? is this right??

3. Jun 21, 2012

### Aaron9890

Re: Thermodynamics question help?? is this right??

for the efficiency??

4. Jun 21, 2012

### LawrenceC

Re: Thermodynamics question help?? is this right??

Hint: Look at the numeric information given in the problem statement more closely.

5. Jun 21, 2012

### rude man

Re: Thermodynamics question help?? is this right??

Hint: how does the 50mm figure in?

6. Jun 21, 2012

### Aaron9890

Re: Thermodynamics question help?? is this right??

all the values in the list there are at the bottom of the page i have, they are for a number of questions, i think there are 6 but only some will apply for this question. So i don't have to use them all!

7. Jun 21, 2012

### rude man

Re: Thermodynamics question help?? is this right??

Good, 'cause you can toss that one out.

8. Jun 21, 2012

### Aaron9890

Re: Thermodynamics question help?? is this right??

c = 4.7 KJ/kgK in J = 4700J/kgK
L -= 2.26MJ/KgK in J = 2260000J/kgK

did i transpose on of these wrong

9. Jun 21, 2012

### rude man

Re: Thermodynamics question help?? is this right??

Well, there's a question here. Is the water just raised to 100oC or is it then changed to steam at 100oC? The question doesn't say. If the former, do you need L? (I suppose 'boiler' does imply steam but who knows ...)

Anyway, like Lawrence told you - watch your units. What should the units of L be, assuming you need L in the first place?