Post on 07-Feb-2017
Stephen Lowe ASHRAE Hampton Roads Chapter Past President AECOM Design – Mechanical Engineering Discipline Manager, Virginia Beach Division
Professional Engineer Commonwealth of Virginia Professional Engineer – Commonwealth of Virginia, NCEES
BSME – University of Virginia, 2002 Sheet metal fabricator, equipment installer, refrigeration service technician during high school and collegeC l i E i f P Consulting Engineer from 2003 – Present
OverviewOverview Introduction Cooling Tower Basics
Principles of Operation Types of Cooling Towers Common Applications
Design Considerations Selection CriteriaSelection Criteria Open Piping and Pumping Systems Water Quality Tower Location Ancillary Systems Ancillary Systems
Construction Issues Constructability, Quality Control and Startup Rating and Certification
L L d Lessons Learned Conclusion
Resources Questions and Discussion
Cooling Tower Basics Principles of Operation
Cooling Via Evaporation – depends on Wet Bulb TemperatureTemperature
Design Dry Bulb Temperatures commonly exceed 95 deg F: poor delta T for cooling processesD i W b lb h l d F Design Wet bulb much less: ~80 deg F
Use a process that depends on WB to take advantage of greater delta Tg
Evaporative effect is a change of state and thus a large transfer of energy occurs relative to the required mass flowflow
Cl d Ci i Open Cooling Tower
Closed Circuit 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 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 jpresentation.
Basic HVAC Cooling To er ApplicationBasic HVAC Cooling Tower Application
Design Considerations Design Considerations
Selection Criteria Open Piping and Pumping Systems Water Quality Tower Location Tower Location Ancillary Systems
Design Considerations Selection Criteria
Process (Chiller Condenser) Temperature Requirements Hot Water Temperature (HWT): Temperature of water entering the p ( ) p g
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 ConditionsDesign Conditions
Design ConditionsDesign Conditions
YESNO YES
Design Conditions Use Evaporation Design WB for selection of evaporative equipmentU f % % % di i i h d i Use of 0.4%, 1%, or 2% condition is up to the designer based on specific application and design firm practice
A 2 degree F margin of safety is recommended to A 2 degree F margin of safety is recommended to account for recirculation (rule of thumb – further analysis is required for the specific application)y q p pp
Condenser water delta T depends on many factors such as the process that is cooled and optimization of h i the pumping system
Design Considerations Open Piping and Pumping Systems Cooling tower pumping application troubles are
ll l d f h f ll i igenerally related to one of the following issues: Incorrect Pump Head Estimation Pump Cavitation Pump Cavitation Air Introduction to Pump Suction Improper Bypass Configuration/ControlImproper Bypass Configuration/Control
Design Considerations Open Piping and Pumping Systems
Pump Head Estimationl l f h h Determine actual static lift height
Determine impact of siphon draw: either break the siphon or design to maintain it.
Determine friction head Pipe Equipment (condenser tower spray nozzle etc ) Equipment (condenser, tower spray nozzle, etc.) Components (valves, strainers, etc.)
Design Considerations Open Piping and Pumping Systems
Pump head estimation errors can cause:bl Unstable pump operation
Unstable condenser flow at the chiller Admission of air into the pump inletp p 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
d i ft f d ith i i t d ti i t th pressure and is often confused with air introduction into the pump suction.
Net Positive Suction Head (NPSH) is the measure of suction pressure 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 it ti ill If NPSHA < NPSHR, cavitationwill occur.
NPSHA Example Calculation
Design Considerations Open Piping and Pumping Systems
B & G Cooling Tower Pumping and Piping Manual has 4 l i th ti i irules concerning the pump suction piping:
1. Leave the suction Line Alone!2. Place the pump below tower pan water levelp p p3. Avoid “above the pump” air traps in the suction line4. Avoid fine mesh, high pressure drop strainers in the suction
lineline
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 arrangementyp g
Design Considerations Open Piping and Pumping Systems
Tower pan vortexing is caused by excessive velocity in th t it i i d b t ll d bthe tower exit piping and can be controlled by: Do not excessively oversize the pump Maintaining tower exit pipe size for a minimum of 10 pipe g p p p pdiameters
Increase pan depth Provide a vortex breaker in the tower pan 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 d th t fl d th t it i i d b id d depth to flood the tower exit piping and can be avoided by: Correctly sizing the makeup water liney g p
Evaporation Rate Blowdown Rate
Providing a check valve to keep the piping system flooded on Providing a check valve to keep the piping system flooded on pump shut down
Properly isolating tower cells in multiple cell installations
WRONGWRONGCondenser and a significant portion of the piping system will drain pack to pan on pump shutdown. Pan may flood and shutdown. Pan may flood and chiller could start with a dry condenser.
CORRECTCORRECTCondenser will remain flooded on pump shutdown. Minimal system volume will drain to pan on shutdown, therefore pan pshould not flood.
WRONGBypass connected to pump
i i i i suction piping can cause air introduction to pump inlet and unstable pump operation.
CORRECTB di h i t b i ith Bypass discharging to basin with balancing valve set equal to discharge nozzle pressure drop will result in constant pump head and flooded pump suction.p p
CORRECTSeparate operators allow valves to maintain
WRONGLinked valves will result in unstable flow.
a constant total flow as both valves modulate.
Bypass Configuration Example
Design Considerations Water Quality
Removal of Solids Chemical Treatment
Consult a chemical treatment professional for your specific applicationpp
Biological Contamination Minimize drift Site towers away from air intakes Avoid piping designs that result in stagnant water Chemically treaty
Design Considerations Water Quality
Removal of Solids What’s that floating in the basin????g Whatever it is, it has a good shot at ending up inside your chiller!!!!
Design Considerations Water Quality
Removal of Solidsh h l d b k 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
d land plume Need ample clearance to minimize recirculation and ensure proper performanceensure proper performance
Low frequency sound emitted from large fans travels great distancegreat distance
Drift can mark windows and paint jobs
Design Considerations Ancillary Systems
Freeze Protection Tower Sump Basin
Electric Steam Injection Recirculation
Piping Piping Water Treatment Systems
Chemical Systems Solids FiltrationSolids 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 itsure you get plenty of space for it
Consider laying out all of the piping based on a manufacturer’s particular towerU h f ’ h kli i Use the manufacturer’s startup checklist on your site visit
Read the installation and operation manual during p gdesign; 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 Protectionl d l d l l 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 pused instead.
Construction Issues Lessons Learned
Vertical Loop in Suction Pipingd d d h l 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 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 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 roomin 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 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 p g gunsightly 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 if ldmanifold
Construction Issues Lessons Learned
Air introduced to pump suction via tower discharge if ldmanifold
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 liline.
When adding or replacing towers connected through an equalization line, design the tower and framing to q , g gensure that the existing operating water level is matched with the new tower.If i i id ib i i l i f h If it is necessary to provide vibration isolation 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 F d lFundamentals
Baltimore Air Coil Product and Application HandbookE E i L M l 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?