Stephen Lowe ASHRAE Hampton Roads Chapter Past President AECOM Design Mechanical Engineering Discipline Manager, Virginia Beach Division Professional Engineer Commonwealth of Virginia, NCEES BSME University of Virginia, 2002 Sheet metal fabricator, equipment installer, refrigeration service technician during high school and college Consulting Engineer from 2003 Present
Overview Introduction Cooling Tower Basics Principles of Operation Types of Cooling Towers Common Applications Design Considerations Selection Criteria Open Piping and Pumping Systems Water Quality Tower Location Ancillary Systems Construction Issues Constructability, Quality Control and Startup Rating and Certification Lessons Learned Conclusion Resources Questions and Discussion
Cooling Tower Basics Principles of Operation Cooling Via Evaporation depends on Wet Bulb Temperature Design Dry Bulb Temperatures commonly exceed 95 deg F: poor delta T for cooling processes Design Wet bulb much less: ~80 deg F Use a process that depends on WB to take advantage of greater delta T Evaporative effect is a change of state and thus a large transfer of energy occurs relative to the required mass flow
Open Cooling Tower Closed Circuit i Cooling Tower
Cross Flow Arrangement Counter Flow Arrangement
Cooling Tower Basics Common Applications Power Plant Steam Condensation Process Cooling HVAC Heat Rejection Chiller Condenser Heat WSHP Loop Heat *We will be focusing on open, cross flow cooling towers for chiller condenser heat rejection for the remainder of the presentation.
Basic HVAC Cooling Tower Application
Design Considerations Design Considerations Selection Criteria Open Piping and Pumping Systems Water Quality Tower Location Ancillary Systems
Design Considerations Selection Criteria Process (Chiller Condenser) Temperature Requirements Hot Water Temperature (HWT): Temperature of water entering the tower Cold Water Temperature (CWT): Temperature of water leaving the tower Tower Range: HWT CWT Lower CWT generally results in higher Chiller Efficiency Flow Rate or Capacity Ambient Conditions Evaporation (or Entering) Design Wet Bulb (EWB) Approach: CWT EWB Lower approach generally results in a larger tower
Design Conditions
Design Conditions NO YES
Design Conditions Use Evaporation Design WB for selection of evaporative equipment Use of 0.4%, 1%, or 2% condition i is up to the designer based on specific application and design firm practice A 2 degree F margin of safety is recommended to account for recirculation (rule of thumb further analysis is required for the specific application) Condenser water delta T depends on many factors such as the process that is cooled and optimization of the pumping system
Design Considerations Open Piping and Pumping Systems Cooling tower pumping application troubles are generally related to one of the following issues: Incorrect Pump Head Estimation Pump Cavitation Air Introduction to Pump Suction Improper Bypass Configuration/Control
Design Considerations Open Piping and Pumping Systems Pump Head Estimation Determine actual static lf lift height h Determine impact of siphon draw: either break the siphon or design to maintain it. Determine friction head Pipe Equipment (condenser, tower spray nozzle, etc.) Components (valves, strainers, etc.)
Design Considerations Open Piping and Pumping Systems Pump head estimation errors can cause: Unstable pump operation Unstable condenser flow at the chiller Admission of air into the pump pinlet Energy waste due to need for extensive throttling Many wasted hours and headaches finding the problem!
Design Considerations Open Piping and Pumping Systems Pump cavitation is the flashing (boiling) of the working fluid (water) into a vapor due to excessively low suction pressure and is often confused with air introduction ti into the pump suction. Net Positive Suction Head (NPSH) is the measure of suction pressure. The required suction pressure for a particular pump is indicated NPSHR and is published as a curve on the pump selection chart. The suction pressure available to the pump in a particular system is indicated NPSHA and must be calculated by the engineer. If NPSHA < NPSHR, cavitation ti will occur.
NPSHA Example Calculation
Design Considerations Open Piping and Pumping Systems B & G Cooling Tower Pumping and Piping Manual has 4 rules concerning the pump suction piping: i 1. Leave the suction Line Alone! 2. Place the pump pbelow tower pan water level 3. Avoid above the pump air traps in the suction line 4. Avoid fine mesh, high pressure drop strainers in the suction line
Design Considerations Open Piping and Pumping Systems Air admittance to the pump inlet is typically caused by: Tower pan vortexing Tower pan drain down Incorrect bypass arrangement
Design Considerations Open Piping and Pumping Systems Tower pan vortexing is caused by excessive velocity in the tower exit piping i and can be controlled by: Do not excessively oversize the pump Maintaining tower exit pipe pp size for a minimum of 10 pipe pp diameters Increase pan depth Provide a vortex breaker in the tower pan Install tower on the equator
Design Considerations Open Piping and Pumping Systems Tower pan drain down is caused by insufficient water depth to flood the tower exit piping i and can be avoided d by: Correctly sizing the makeup water line Evaporation Rate Blowdown Rate Providing a check valve to keep the piping system flooded on pump shut down Properly isolating tower cells in multiple cell installations
WRONG Condenser and a significant portion of the piping system will drain pack to pan on pump shutdown. Pan may flood and chiller could start with a dry condenser. CORRECT Condenser will remain flooded on pump shutdown. Minimal system volume will drain to pan on shutdown, therefore pan should not flood.
WRONG Bypass connected to pump suction piping i can cause air introduction to pump inlet and unstable pump operation. CORRECT Bypass discharging i to basin with balancing valve set equal to discharge nozzle pressure drop will result in constant pump head and flooded pump psuction.
CORRECT Separate operators allow valves to maintain a constant total flow as both valves modulate. WRONG Linked valves will result in unstable flow.
Bypass Configuration Example
Design Considerations Water Quality Removal of Solids Chemical Treatment Consult a chemical treatment professional for your specific application Biological Contamination Minimize drift Site towers away from air intakes Avoid piping designs that result in stagnant water Chemically treat
Design Considerations Water Quality Removal of Solids What s that floating in the basin???? Whatever it is, it has a good shot at ending up inside your chiller!!!!
Design Considerations Water Quality Removal of Solids Guy with shovel and bucket In line strainers Side stream filters Basin sweeping system
Design Considerations Tower Location Visibility Sound Drift
Getting the proper Real Estate Location, location, location Cooling Towers are highly visible due to physical size and plume Need ample clearance to minimize recirculation and ensure proper performance Low frequency sound emitted from large fans travels great distance Drift can mark windows and paint jobs
Design Considerations Ancillary Systems Freeze Protection Tower Sump Basin Electric Steam Injection Recirculation Piping Water Treatment Systems Chemical Systems Solids Filtration Control Systems Pneumatic operators will require freeze protection in some climates Large valves require lots of torque electric operators may need to be industrial grade
Construction Issues Constructability, Startup, Quality Control There is lots of large piping underneath the tower, make sure you get plenty of space for it Consider laying out all of the piping based on a manufacturer s particular tower Use the manufacturer s startup checklist on your site visit Read the installation and operation manual during design; there are lots of options that affect the cost of the tower. You need to specify them if your application needs them.
Construction Issues Ratings and Certification Manufacturer s Rated Performance CTI Certification Performance Based Commissioning
Construction Issues Lessons Learned Tower Basin Freeze Protection Direct steam injection is popular in industrial and large scale HVAC applications. Chemical used for boiler feedwater may react with condenser water chemical treatment resulting in the release of noxious gases. A hot water circulator pump coupled to a heat source can be p p p used instead.
Construction Issues Lessons Learned Vertical Loop in Suction Piping Tower mounted on grade adjacent to mechanical room Cold water pipe exits bottom of basin and passes through mechanical room wall Pipe is the run up the wall to the roof structure to get the pipe across the room to the pump Upon shutdown, the system equalizes and the piping at the roof level becomes air locked The pump will require manual priming in order to start
Construction Issues Lessons Learned Flooding Tower on Pump Shutdown Tower is mounted on grade adjacent to mechanical room. There is a significant volume of water located above the tower basin due to a long horizontal run of hot condenser water pipe in the mechanical room. The diverting valve is installed in the horizontal with the bypass connection facing down. Whenever the pump shuts down, all of the water in the horizontal pipe runs through the diverting valve and back to the basin. The basin overflows in the parking lot resulting in an unsightly puddle and wasted chemical treatment
Construction Issues Lessons Learned Air introduced to pump suction via bypass
Construction Issues Lessons Learned Air introduced to pump suction via bypass AIR CAN BE DRAWN INTO PUMP SUCTION
Construction Issues Lessons Learned Air introduced to pump suction via tower discharge manifold
Construction Issues Lessons Learned Air introduced to pump suction via tower discharge manifold MUST ISOLATE TOWER DISCHARGE
Construction Issues Lessons Learned Flooding hot water pans on manifolded towers
Construction Issues Lessons Learned Flooding hot water pans on manifolded towers AUTOFLOW VALVE
Construction Issues Lessons Learned Multiple connected towers should have an equalization line. When adding or replacing towers connected through an equalization line, design the tower and framing to ensure that the existing operating water level is matched with the new tower. If it is necessary to provide vibration i isolation i for the tower, the piping system must be isolated from the tower.
Resources B & G Cooling Tower Pumping and Piping Manual Marley Cooling Tower Company Cooling Tower Fundamentals Baltimore Air Coil Product and Application Handbook Evapco Equipment Layout Manual www.cti.org ASHRAE H db k 8 HVAC S t d ASHRAE Handbook 2008 HVAC Systems and Equipment, Chapter 39
Questions?