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The Cause and Effect of Various Design Concepts

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1 The Cause and Effect of Various Design Concepts
Process Heat Transfer The Cause and Effect of Various Design Concepts

2 Exchanger Variables Fouled surface area Non-condensible gases
Flooded surface area Variable process inlet and outlet temperatures Variable process flow rates All of these change the BTU demand on the heater, changing the pressure and temperature of the heat transfer media

3 Fouled Surface Area Fouled surface area decreases the heat transfer efficiency of the tube bundle This inherently causes adjustments in the pressure and/or temperature of the heat transfer media being supplied to the exchanger

4 Fouled Surface Area Resulting in more surface exposed to the transfer media in a level control system. This will increase the BTU transfer rate. Higher delivery pressure from the inlet control valve decreases the efficiency of the heat exchanger. Higher pressure lacks the same latent heat content of lower pressure. Energy consumption will increase, while production levels remain unchanged.

5 Non-Condensible Gases
Presence of non-condensibles’ occupies valuable steam space A reduction of viable heat transfer area can result due to the insulating properties Promotion of carbonic acid formation is inherent Excessive amounts can inhibit drainage

6 Flooded Surface Area Promotes corrosion and fouling
Can develop into water hammer Controls process temperature by decreasing available surface area for heat transfer (Level Control) Typically causes process outlet variations

7 Variable Process Inlet & Outlet Temperatures
Changes the BTU exchange rate required or (Delta T) These variable temperatures can increase or decrease exiting pressure based on condensing rate of the heater Will promote flooding on low exchange rate demand

8 Variable Process Flow Rates
Variable flows will change BTU demand on the exchanger Higher flow rates will increase the surface area needed, raising or lowering the outlet pressure based on available surface area Lower flow rates will decrease surface area needed, raising or lowering the outlet pressure based on available surface area

9 Control Options Level Control Steam Control

10 Level Control Level control systems flood exchangers to reduce the amount of useable surface area for BTU transfer Exchangers run flooded due to the control valve on the condensate outlet, modulating to maintain the desired process outlet temperature

11 Steam Control Allows the exchanger to run at the lowest possible steam pressure, which maximizes energy efficiency due to latent heat content Less energy consumed for the same amount of product produced

12 Process Design Summary
Utilize all of the surface area Eliminate corrosion and fouling by keeping the exchanger dry Eliminate non-condensibles Optimize the design by using the lowest pressure steam, to gain more latent heat content per pound

13 Operating Characteristics

14 Filling Steam/Air In - Closed Steam/Air Out - Open Open Check Valve Closed Check Valve Step 1. During filling, the steam or air inlet and check valve on pumping trap outlet are closed. The vent and check valve on the inlet are open.

15 Begin Pumping Steam/Air In - Open Steam/Air Out - Closed Check Valve Closed Open Check Valve Step 2. Float Rises with level of condensate until it passes trip point, and then snap action reverses the positions shown in step one.

16 End Pumping Steam/Air - In Steam/Air - Closed Closed Check Valve Open Check Valve Step 3. Float is lowered as level of condensate falls until snap action again reverses positions.

17 Repeat Filling Steam/Air In - Closed Steam/Air Out - Open Open Check Valve Closed Check Valve Step 4. Steam or air inlet and trap outlet are again closed while vent and condensate inlet are open. Cycle begins anew.

18 Pump Trap Applications

19 Process Heat Exchanger with 100% Turndown Capability

20 Vacuum Reboiler Construction Comparison

21 Hydrocarbon Knockout Drum/Separator

22 Flare Header Drain

23 Flash Vessels

24 Steam Turbine Casing

25 Pump Trap Applications
Process Heat Exchangers Liquid Separators Sumps Vacuum Systems Condensate Drum – Flash Tanks Vented Systems Closed Loop Applications

26 Understanding and Benefiting from Equipment Stall

27 Q = U · A · D T Q = Design Load (BTU/Hr)
U = Manufacturer’s Heat Transfer Value (BTU/ft2/°F/Hr) A = Heat Transfer Surface Area (ft2) DT = (Ts – T2) Approaching Temperature (°F) Ts = Operating Steam Temperature (°F) T2 = Product Outlet Temperature (°F)

28 What is wrong with this application?
Modulating heat exchanger with overhead lift. Flooding problems Water hammer problems What is wrong with this application?

29 Effects of “Stall” Inadequate condensate drainage Water hammer
Frozen coils Corrosion due to Carbonic Acid formation Poor temperature control Control valve hunting (system cycling) Reduction of heat transfer capacity

30 Factors Contributing to “Stall”
Oversized equipment Conservative fouling factors Excessive safety factors Large operating ranges Back pressure at steam trap discharge Changes in system parameters

31 Finding “Stall” Where does Stall occur?? Modulating Control
Air heating coils Shell & tube heat exchangers Plate & frame heat exchangers Absorption chillers Kettles Any type of heat transfer equipment that has Modulating Control

32 Old problem

33 What is the “Stall” Solution?
Use a bigger steam trap? Use a vacuum breaker? Implement a safety drain? Install a Posi-Pressure system? Use an electric pump?

34 Keys to Operation How quick it can fill: This is dictated by head pressure & inlet pipe and check valve size Vent/Equalization: Vent connection must always be in vapor space Pump Out: Motive vs. back pressure and gas used

35 Vocabulary Filling Head: Distance between the top of the pump and the bottom of the receiver or reservoir pipe

36 Vocabulary (Continued)
Receiver/Reservoir Pipe: This is a temporary holding place to store condensate while the pump is in the pump down cycle. The receiver/reservoir pipe is designed and sized to prevent condensate from backing up into the system.

37 Open System Configuration Closed System Configuration

38 Open System Cover the basic points of operation in a vented system.
Be sure to point out the need for: Vacuum breaker Air vent Fill head

39 Open System Advantages: Disadvantages:
Drain multiple pieces of equipment Can use Air or Steam for pump trap operation Easiest to understand Disadvantages: Lose valuable flash steam Must run a potentially expensive atmospheric vent line Size the pump trap based total design load Must compete with electric pumps

40 Closed System One heat exchanger per pump in closed-loop.
F&T is placed downstream of the pump trap with check valve afterwards Alternate vent line configurations If less than ½ psi pressure drop in HX, then vent line can be tied back to HX. Then, there is no need for a vacuum breaker If more than ½ psi pressure drop, usually not known, then tie vent line from pump back to reservoir as shown.

41 Closed System Advantages: Disadvantages: No flash steam loss
No need to run long expensive vent lines Use a smaller pump than in a open system* Return condensate hotter Disadvantages: Dedicated pump for a single piece of equipment More complex Cannot use air as motive force

42 Pump Sizing / Receiver Sizing
Determine head available from equipment (distance from equipment outlet to grade) Select either closed loop or vented design (Note: If multiple sources of condensate, vented system must be used to prevent short circuiting)

43 Pump Sizing / Receiver Sizing
Determine maximum pumping load Calculate maximum back pressure (including lift) Determine motive pressure and gas to be used (use capacity correction factor if using a medium other than steam)

44 Pump Sizing / Receiver Sizing
Check and specify head pressure (distance from bottom of receiver/reservoir to top of selected pump) Make sure to use capacity correction if more or less head is available than standard catalog dimension

45 Pump Sizing / Receiver Sizing
Calculate maximum flash rate & needed vent size – if vented system Determine and size reservoir – if closed loop system Size downstream F&T trap if needed for closed loop system

46 Vented Receiver Sizing
Note: When draining from a single or multiple pieces of equipment in an “open” system, a vented receiver should be installed horizontally above and ahead of the pump trap. In addition to sufficient holding volume of the condensate above the fill head of the pump trap to hold the condensate during the pump trap cycle, the receiver must also be sized to allow enough area for flash steam and condensate separation.

47 Closed Loop Receiver Sizing
Note: When draining from a single piece of equipment in a closed loop system, to achieve maximum energy efficiency a reservoir should be installed horizontally above and ahead of the pump trap. Sufficient reservoir volume is required above the filling head level to hold condensate during the pump trap discharge cycle. The chart above shows the minimum reservoir sizing, based on the condensate load, to prevent equipment flooding during the pump trap discharge cycle.

48 Critical Design Criteria Summary
Maximum condensate flow from exchangers and reboilers Maximum differential pressure across the system Minimum differential pressure across the system (specifically when clean) Minimum tower height needed to achieve maximum condensate flow rate at minimum differential Maximum motive pressure (steam, air, nitrogen, etc.) available to power pumps

49 Critical Design Criteria Summary
Maximum instantaneous discharge rate for downstream pipe sizing & trap sizing Temperature differential of condensate source vs. condensate header design Piping layout to prevent hydraulic shock Total installed cost savings, including construction, on turnkey jobs Integrity of mechanical design due to the critical nature of the service Minimize potential problems with proper designs

50 Maximum Differential Pressure Across the System
Maximum pressure from control valve, including minimal drop Minimum drop across exchanger Maximum pressure – should tube leak occur Elimination of back pressure (bypass to grade) Consider fouled surface area

51 Minimum Differential Pressure Across the System
Consider maximum percentage of turndown on process flow vs. design flow (plus factor) Consider over-surfaced heat transfer area Evaluate downstream relief valve settings on condensate side as traps (etc.) fail and pressurize the return system Undersized return lines are common in facility expansions. Verify effects of additional flow on pipe velocities and back pressures.

52 Minimum Head Pressure Skirt height on reboilers can be minimized by evaluating discharge capacity needed and setting height accordingly. This should be done early in the job scope as it effects tower construction. Additional pump capacity can be achieved by increasing head pressure

53 Maximum Motive Pressure Steam, Air, Nitrogen
Ensure stable source with negligible variations Install drip station to insure dry gas is always present at motive steam valve (pipers often do not realize it is a dead-end steam line)

54 Maximum Design Pressure
Utilization of 2/3 Rule can eliminate relief valves on low pressure side needed for tube rupture cases Use of liquid drain traps can eliminate gas discharge into return header

55 Maximum Instantaneous Discharge Rate
Pump discharge rate must be used when sizing condensate return leads (use bi-phase flow) Pump discharge rate also critical to downstream traps in Pump / Trap combinations

56 Temperature Differential of Source vs. Header
Minimize thermal shock by maintaining DT of 150°F or less When feasible, run separate headers for vacuum temperature condensate Vacuum condensate headers can be sized on single phase flow if dedicated solely for vacuum temperature condensate

57 Piping Layout to Prevent Hydraulic Shock
Discharge lead from pumps should be piped into top of return header Flow patterns should be continual – no opposing flows Check valves should be installed at major elevation changes to disperse hydraulic shock

58 Pipe Sizing Discharge piping should be based on 2-3 times the normal condensing rate due to instantaneous discharge rate of the pump Minimize elevation changes to prevent hydraulic shock Utilize check valves at main header to minimize backflow

59 Pipe Sizing Run separate lines for vacuum temperature condensate to minimize thermal shock potential Always calculate the maximum flash rate in return lines Insure adequate pipe and nozzle diameters to facilitate bidirectional two-phase flow

60 “Expect many enjoyable experiences!”
“Expect many enjoyable experiences!” David M. Armstrong

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