Positive Condensate Drainage from Heat Transfer Equipment Under Modulating Steam Conditions
Total Page：16
File Type：pdf, Size：1020Kb
Understand how a good heating system design on paper can become a big problem once installed. Positive Condensate Drainage from Heat Transfer Equipment under Modulating Steam Conditions November 13, 2017 CNY Expo - Looked Good on Paper… Types of Heat Transfer Direct The heating medium is directly mixed (convection) with the substance being heated i.e. “Direct injection”. Indirect (Heat Exchange Equipment) Heat energy from the heating medium is passed to the substance being heated through a physical barrier (conduction). Steam Heat Transfer 101 1. Steam Supply 2. Heat Transfer 3. Condensate Removal Heat Exchanger Flow Heat Exchanger Sizing Q = U x A x ∆T, where U = K /(dx * Fouling Factors) Type & Thickness of Materials of Construction. Standard HX sizes: 16.3-, 25.8-, 35.2-, 44.6-, 54.0-, 63.4-, 72.9-, 82.3-, 91.7-SQFT Copper Example: 887,760 = 324 x 27.4 x 100 Copper Application requires 27.4 SQFT but the Stainless Steel closest suitable size is 35.2 SQFT. dX A Double Wall Therefore, the HX starts over-sized by 28.5%. Normal Operation Product Temperature Input P1 Heat Exchanger P1 > P2 = Heat Exchanger Dry P2 Vacuum = Negative Differential Pressure Steam occupies 1,675 times the amount of space than water. 3 ft. When steam condenses in a “Closed-System”, a vacuum is created. 3 ft. 3 ft. Conventional Condensate Removal System Pressure: Modulating vs. Constant Pressure Time Pressure Time Modulating Steam Traps Valve Head & Seat Air Vent Float Mechanism Inverted Bucket Float & Thermostatic • Continuous Steam – Good • Continuous Steam – Excellent • Intermittent (On/Off) Steam – Poor • Intermittent (On/Off) Steam – Excellent • Economical Choice: Metal-on-Metal • Optimum Choice for Process Downstream Lift = System Back Pressure Stall Condition – No Steam Supply Product Temperature Input P1 Heat Exchanger P1<P2=Heat Exchanger Flooded P2 Stall Chart Domestic Hot Water 15 PSIG Steam Supply @ 250F 40F to 140F Trap Mode @ 100 GPM Stall Pump Mode 0 PSIG Back-Pressure @ 212F (aka Gravity-Drain) What does a stall chart not take into consideration? 65% = Stall of Load Point After HX is selected…now the real world. For real-world conditions: Q = U x A x ∆T, where U = K /(dx) Domestic Water Domestic Water Flow Rate Varies with demand, typically low Incoming water temperature rises in summer to no flow in evening hours. months. Building Heat Building Heat Flow rate varies with VFDs responding to Outside Air Temperature rises is summer demand. months. * Less flow requires less flow of heat (Q). *Both reduce ∆T requiring less flow of heat. Saturated Steam Table Actual Stall Conditions 40F to 140F @ 100 GPM using 15 PSIG Steam Supply Heat Load Summary - Calculating Stall Steam Temp Steam Pressure Latent Heat of Steam & Condensate Trap Flow Rate Heat Load System in HX in HX Steam Flow Differential (GPM) (BTU/hr) Condition (F) (PSIG) (BTUs) (lbs/hr) (ΔP) 100 5,004,000 245.5 12.87 948.6 5,275 12.87 Trap Mode 95 4,753,800 237.7 9.27 953.8 4,984 9.27 Trap Mode 85 4,253,400 222.2 3.25 963.8 4,413 3.25 Trap Mode 75 3,753,000 206.6 -1.49 973.7 3,854 -1.49 Stall - Pump Mode 65 3,252,600 191.1 -5.12 983.2 3,308 -5.12 Stall - Pump Mode 50 2,502,000 167.8 -8.99 997.4 2,509 -8.99 Stall - Pump Mode 25 1,251,000 128.9 -12.53 1020.3 1,226 -12.53 Stall - Pump Mode 15 750,600 113.3 -13.29 1029.7 729 -13.29 Stall - Pump Mode 10 500,400 105.6 -13.57 1033.7 484 -13.57 Stall - Pump Mode 5 250,200 97.8 -13.81 1038.1 241 -13.81 Stall - Pump Mode Reasons for System Stall • Overly conservative fouling factors during HEX design – adds additional surface area • Back pressure at equipment discharge – elevation or static pressure • Modulating Control – Steam pressure • Vacuum • Process demands- Flow or temp changes • Oversized equipment – excess surface area Effects of System Stall • Inadequate condensate drainage • Water hammer (Thermal Shock) • Frozen coils, damaged tube bundles • Poor temperature control • Control valve hunting – control stability • Reduction in heat transfer capacity Pumping Condensate • What’s unique about condensate? • High temperature fluid. • Constant phase change, or flashing. • Intermittent supply, inconsistent suction head. 2 Types of Condensate Pumps 1. Electric Pumps 2. Pressure-Powered Pumps Electric Pump Operation What does a lower pressure due to the boiling point? Lowers saturation point. Generates flash steam. Flash steam causes cavitation. Vent = Energy Loss What is Flash Steam? Flash Capacity Calculation Return & Vent Sizing Standard Vent Size on Electric Pump Receivers Too Small! Not designed for flash steam. Sub-Cooling - Avoid Cavitation • Flash additional energy upstream away from mechanical pumps. • Sub-Cool condensate to 190 F before entering pump receiver. • High & Medium Pressure Condensate provide options for Heat Recovery What do we Gain/Lose from Sub-Cooling? Lose 28 BTUs/lb of Condensate @ 15 PSIG Gain 87 BTUs/lb of Condensate @ 60 PSIG 25% of Potential 135 BTUs/lb of Condensate @ 125 PSIG Dissolved Oxygen How do we Remove the Flash Vent? Does not require Sub- Cooling of Condensate No Energy Loss Use a Pressure Pump Up to 400 F Pressure Pump & Steam Trap Combo Pump-Trap Combo w/ Single Float Mechanism Stall Alleviation Closed-Loop System Off – 0 PSIG Steam Supply Heat Exchanger stays dry 100% of the time! 1. HX reaches temperature or demand stops. 2. Control Valve Shuts Power • 0 PSIG Steam Supply Supply • Lose Positive Differential Motive Pressure Steam/Air, 3. Steam within System Vacuum No Electrical Condenses 1,000+ BTUs/lb. of Steam Power!!! • Draws a vacuum. • More efficient heat exchange. Gravity 4. Pumps Drain • Create Positive Differential Pressure 125 PSIG • Maintains Vacuum 280 ft. + of Lift Conditions!!! Typical Installation Open-Loop, Gravity-Drain Design Roof 125# HPS 10# LPC Closed-Loop, Feed-Forward Design Roof 125#15# HPS HPS 10# LPC Closed-Loop, Pump-Trap Design 125# HPS 10# LPC Condensate Pump Application Considerations 1. Will there be lift after the steam trap? 2. Will there be variable process conditions? 3. Will the leaving process conditions temperature be equal or less than 212°F System Stall Solutions Installation of a vacuum breaker: Objective: To relieve a vacuum within equipment allowing for condensate drainage. Shortcoming: This practice will only help if the condensate is gravity drain to atmosphere, Allows undesirable air into the system. Vacuum breakers often fail due to a poorly chosen location Loss of valuable flash steam System Stall Solutions Installation of a safety drain: Objective: The use of a second steam trap located above the primary trap which discharges condensate to drain when the system goes into a stall condition. Shortcoming: A significant amount of condensate/flash steam and valuable BTU’s are lost down the drain when the system is in stall. Stall load may be as high as 90% or more of the design load, therefore 90% of the condensate coming from the equipment goes down the drain System Stall Solutions Installation of a positive pressure system: Objective: The use of air or other gas to maintain set pressure to ensure a positive pressure differential across the trap allowing for condensate drainage. Shortcomings: Injects a significant amount of undesirable air into the equipment. This large amount of air may cause multiple problems: Air acts as an insulator thereby decreasing the heat transfer capacity of the equipment. A heavy dependence on air vents to evacuate the air from the equipment. Air vents may be open a significant amount of time allowing for loss of valuable BTU’s. System Stall Solutions Closed Loop Condensate System The application of a “closed” system pump trap on your modulating steam equipment can provide the following benefits: Continuous condensate drainage, even in a vacuum Eliminates the need for vacuum breakers Saves valuable flash steam from escaping into the atmosphere No need to run expensive vent lines No rotating seals, cavitation, or NPSH requirements Negligible operating cost Longer equipment life Reduced corrosion Better temperature control Shortcomings: Relative costs versus conventional systems. Spot the issue?? How did this end up this way? If we don’t fix it add on… Done right with forethought Recap • By design heat exchange equipment have excess surface area • Condensate must flow from a higher pressure to a lower pressure – account for this in design • Air and non –condensable gases need to be managed along with flash steam in the condensate system. • Whenever you have modulating steam pressure for temperature control the potential for system stall exists. • Electric condensate pump receivers are not used as flash tanks How well did I do?? 1. Identify two types of heat transfer. 2. In the equation Q = U x A x ∆T, - Define U, Define A 3. Define the general definition of “system stall”. 4. When lowering the pressure of condensate in system we create? 5. Heat exchangers typically have surplus surface area – T/F 6. A flash tank in front of an electric condensate pump is a good idea – T/F 7. Reducing the effects / potential for system stall will improve – List three items 8. List three consideration where you may want to incorporate a condensate pumping system. Selection. Expertise. Solutions..