Course Review 2 Quantitative Overview Chapter 1 (Introduction) - - PowerPoint PPT Presentation

Mechanical Equipment (ENGI-7903) Spring 2013 Course Review

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

2

Quantitative Overview

• Chapter 1 (Introduction)

– 0.5 session, 14 slides

• Chapter 2 (Thermofluids fundamentals)

– 2.5 sessions, 20 slides, 4 examples

• Chapter 3 (Flow Analysis)

– 9 sessions, 51 slides, 12 examples

• Chapter 4 (Turbomachinery)

– 5 sessions, 30 slides, 6 examples

• Chapter 5 (Heat Exchangers)

– 6 sessions, 45 slides, 9 examples

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

3

Chapters 1 and 2

• Chapter 1 (Mechanical Equipment and Systems

Design)

– Mechanical equipment codes – System Identification

• Chapter 2 (Thermodynamics, Fluid Dynamics, and

Heat Transfer)

– Thermodynamics – Fluid dynamics (choose CV wisely, …) – Heat Transfer (Forced convection correlations for Nu) – Dimensionless groups

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

4

Chapter 3 (Flow Analysis)

• Mechanical Energy Balance

– Applications:

• Pump (compressor, fan, blower) pressure rise when flow rate is given
• Elevation (head) required for a given flow rate
• Power generation by turbine
• Pressure (head) loss of a system (when flow velocity is known)
• Velocity (volumetric or mass flow rate) as a result of a given pressure

drop, this requires iteration

• Special cases, for example a fountain height
• etc.

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

5

Chapter 3 (Flow Analysis)

• Head loss calculations:

– Single phase or two phase? – Laminar or turbulent?

• fRe=C (for laminar flows and “C” depends on the geometry)
• Add entrance effects if the channel is not long enough
• Different models (graphs) for “f” in turbulent flow

– How many minor losses and/or equipments? – Other required information:

• Velocity (or volumetric or mass flow rate). If this is to be calculated for a

given pressure drop, we need to iterate!

• Pipe diameter (or hydraulic diameter in case of turbulent flow in non-

circular channels)

• Wall roughness (pipe material)

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

6

Chapter 3 (Flow Analysis)

• How to iterate for flow rate (or velocity) if the flow is

laminar

– Step 1: Guess a value for velocity – Step 2: Calculate Re, L*, and fapp – Step 3: Solve for velocity (flow rate) using the new equation – Step 4: Check for convergence

• Or:

– Substitute all variables as functions of flow rate and then solve the new equation in which there is only one unknown.

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

7

Chapter 3 (Flow Analysis)

• How to iterate for flow rate (or velocity) if the flow is

turbulent

– Step 1: Guess a value for “f”. If we do not have an idea of the possible values, we can use f=0 as initial guess. – Step 2: Solve the equation for the flow rate. – Step 3: Update “Re” using the flow rate calculated in step 2. – Step 4: Update “f” using the calculated “Re” in step 3 and an appropriate model (Blasius, Swammee-Jain, Churchil, etc.) – Step 5: Solve the equation for the flow rate using the “f” from step 4. – Step 6: Compare the flow rates of steps 5 and 2 and iterate again if difference is high.

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

8

Chapter 3 (Flow Analysis)

• Pipes in series

– Same flow rate through all pipes in series – Total pressure drop is sum of pressure drops in all pipes in series

• Pipes in parallel

– Same pressure drop in all pipes in parallel – Total flow rate is sum of flow rates in all pipes in parallel

• Piping networks

– Zero pressure drop in each loop – Mass balance at each junction (node)

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

9

Chapter 4 (Turbomachinery)

• Positive displacement and kinetic pumps
• Performance curve for centrifugal pumps includes

head, efficiency, power, and NPSHR versus flow rate.

• Calculating the system working point:

– Graphically or mathematically – If we know the pump performance function we can solve for working flow rate mathematically e.g. iterative methods. – The iterative procedure is much like what we did in chapter 3 i.e. initial guess for friction factor and solve for flow rate and then update friction factor …

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

10

Chapter 4 (Turbomachinery)

• Pumps in series

– We add heads at the same flow rate for pump curve – Now we can iterate using the new curve

• Pumps in parallel

– We add flow rates at the same head for pump curve – Now we can iterate using the new curve – Special care for different pumps in parallel

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

11

Chapter 4 (Turbomachinery)

• NPSH calculations

– Note that hf,i for NPSHA calculation includes the head losses of the inlet piping i.e. from inlet of the piping system to the inlet of the pump.

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

12

Chapter 5 (Heat Exchangers)

• Different heat exchanger problems:

– Type 1: mc, mh are known. Th,i, Th,o, Tc,i, Tc,o are known. A = ? Appropriate method is LMTD method. ε-NTU may be used as well. – Type 2: U and A are known. Th,i, Tc,i are known. Th,o and Tc,o = ? Appropriate method is ε-NTU method.

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

13

Chapter 5 (Heat Exchangers)

• LMTD method

– Step 1: Energy balance – Step 2: Calculating U if necessary – Step 3: Calculating ΔTLMTD – Step 4: Calculating F (correction factor) if necessary – Step 5: Calculating A, and other dimensions

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

14

Chapter 5 (Heat Exchangers)

• ε-NTU method (for type 1 problems)

– Step 1: Calculating Cc and Ch and determine Cmin and Cmax – Step 2: Calculating Cr – Step 3: Calculating Qmax, Qact – Step 4: Calculating ε – Step 5: Calculating NTU using ε and Cr – Step 6: Calculating A and other dimensions

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

15

Chapter 5 (Heat Exchangers)

• ε-NTU method (for type 2 problems)

– Step 1: Calculating Cc and Ch and determine Cmin and Cmax – Step 2: Calculating Cr – Step 3: Calculating Qmax – Step 4: Calculating NTU – Step 5: Calculating ε using NTU and Cr – Step 6: Calculating Qact – Step 7: Calculating outlet temperatures using energy balance

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

16

Chapter 5 (Heat Exchangers)

• Notes:

– Different methods and formula for pressure drop in different heat exchangers – Fouling leads to higher hydraulic resistance and lower thermal performance – Special attention to index and nomenclature. For example index “f” means finned somewhere and fouled somewhere else.

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

17

Frequent Mistakes

• Midterm exam (2011):

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

18

Frequent Mistakes (cont.)

• Midterm exam (2012):

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

19

Frequent Mistakes (cont.)

• Final exam (2012):

Faculty of Engineering and Applied Science

Memorial University of Newfoundland

• St. John’s, Newfoundland, Canada

20

Frequent Mistakes (cont.)

• Midterm exam (2013):