Determining the Best Heat Recovery System to Maximize Boiler Efficiency

Transcription

1 Determining the Best Heat Recovery System to Maximize Boiler Efficiency

2 Overview Background Reduce facility operating costs Waste heat recovery Boiler Economizer Benefits Flow types Applications Designs Blowdown Heat Recovery Benefits Types Flash tank economizer Blowdown heat recovery unit 2

3 Thermodynamic Laws First Law The energy of the universe is constant it can be neither created or destroyed, but only transferred and transformed Heat and mechanical energy can be converted one to another 3

4 Thermodynamic Laws Second Law Heat travels from hot to cold THIS IS TODAY S TOPIC 4

5 Steam Basic Concepts Temperature-Heat Diagram For 1 lb Of Water At Atmospheric Pressure (14.7 psia) 1 lb steam at 212 O F Sensible Latent Sensible Latent 212 º F 200 º F 1 lb water at 212 O F 100 º F 32 º F 0 º F lb water at 32 O F Btu per pound of water 5

6 Design Considerations For Heat Transfer Heat Transfer Coefficient Proportionality between heat flux/unit area and Delta T LMTD Logarithmic average Delta T between hot and cold streams at each end of Hx Reynolds # Ratio of inertia forces to viscous forces: <2000 = Laminar, >4000 = Turbulent U Factor Rate of heat transfer over an area Fouling Factor Depends on the foulent and temperature 6

7 Heat Transfer Typical Boiler Tube 7

8 Heat Transfer Cleaver-Brooks Advanced Heat Transfer Tube 8

9 Economizer Flow Types Parallel Fluids flow in the same direction Hot side inlet temperature is in contact with the cold side inlet temperature Delta T narrows Used in applications where minimum temperature is adequate 9

10 Economizer Flow Types Counter Fluids flow opposite direction and enter heat exchanger at opposite ends Most efficient when comparing heat transfer rate per unit surface area Average T (difference in temperature) between the two fluids over the length of the heat exchanger is maximized 10

11 Economizer Flow Types Cross 11

12 Economizer Flow Types Cross Fluids flow perpendicular to each other Can be most practical design for high mass flows and space restrictions Still maintains high Delta T throughout the exchanger 12

13 HT Coefficient And High Reynolds 13

14 14 Boiler Economizers

15 Economizer Design Non-Condensing Most common design Recover sensible heat only Single stage Cross or counter flow Applications Steam boiler feedwater preheat Proportional feedwater control On/off feedwater control Process water (higher temperatures) 15

16 Economizer Design Condensing Recover sensible and latent Saturation temperature Natural gas ~135ºF Single stage Condense on natural gas only Applications water temperature less than ~120ºF Cold make-up water Process flow (lower temperature) Hot water return 16

17 Economizer Design 2 Stage Condensing 2 Stages 1st stage sensible heat recovery Standard (non-condensing) economizer 2nd stage sensible and latent heat Condensing economizer Condense on natural gas only Applications 1st stage Preheat boiler feedwater (MFWV) On/off feedwater control 2nd stage Any cool liquid stream (50-120ºF) Make-up water Process water 17

18 Economizer Applications Steam Boiler Feedwater Preheat Run feedwater from a boiler feed system or deaerator through the economizer and into the boiler Modulating boiler feedwater control On/off feedwater control Make-up water Process Water 18

19 Economizer Applications: Steam Boiler Feedwater Preheat With MFWV Non-condensing Outlet Inlet 19

20 Economizer Applications: On/Off Feedwater Control Non-condensing 20

21 Economizer Applications: Make-Up Water Condensing or Non-condensing 21

22 Economizer Applications: Process Flow Condensing or Non-condensing 22

23 Economizer Benefits Increase Boiler Room Efficiency By 3-8% Dependent on product design and application Reduce NOx Emissions Save Operating Costs Average total payback: months 23

24 Blowdown Heat Recovery Types: Blowdown Heat Recovery Unit Proportional blowdown heat recovery system Atmospheric feed or deaerator Automatically controls TDS Single or multiple boilers Blowdown is cooled before discharge (code compliance) 24

25 Blowdown Heat Recovery Types: Flash Tank Economizer Recover blowdown energy (latent and sensible heat) Use flash steam for LP use Blowdown is cooled before discharge (per code) Used for single or multiple boilers 25

26 Calculating Flash Steam % HF1 HF2 / HFG2 = % Flash HF1: HF2: HFG2: Sensible BTUs in Sensible BTUs out Latent BTUs out For example, a boiler operating at 200 psig with 5 psig: = / 961 = 17% Flash BTUs SAVED Water and sewer charges SAVED Chemicals SAVED 26

27 Integration Reclaim heat loss and reduce facility operating costs Economizer BDHR TDS control Include O 2 trim HTDB Parallel positioning VSD Integrated lead lag DA control Draft control 27

28 Integrating The System Bypass Damper Deaerator Water Treatment 28

29 Questions?