Cooling Tower Applications

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.)


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!


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


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


Air admittance to the pump inlet is typically caused by: Tower pan vortexing Tower pan drain down Incorrect bypass arrangementyp g


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


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


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!!!!


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.


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


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


Air introduced to pump suction via bypass


Air introduced to pump suction via bypass

AIR CAN BE DRAWN INTO PUMP SUCTION


Air introduced to pump suction via tower discharge if ldmanifold


Air introduced to pump suction via tower discharge if ldmanifold

MUST ISOLATE TOWER DISCHARGE


Flooding hot water pans on manifolded towers


Flooding hot water pans on manifolded towers

AUTOFLOW VALVE


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?