Chiller Basics

What is a Chiller?

A chiller is a water-cooled air conditioning system that cools inside air, creating a more comfortable and

productive environment. Chillers are also used in the manufacturing environment to provide "process" cooling

to equipment in an effort to maximize productivity.

With large facilities, such as commercial buildings, hospitals, universities, government facilities and theme

parks, the cost of energy to generate cooling in excess of 50 tons is cost prohibitive with air-cooled units.

Water-cooled chillers produce higher tonnage at lower costs per ton, creating greater energy efficiency. A

typical home has 3-5 tons of cooling capacity.

How a Complete Chiller System Works

Chillers circulate chilled water to air-handlers in order to transfer heat from air to water. This water then returns

to the evaporator side of the chiller where the heat is passed from the water to a liquid refrigerant (freon). The

refrigerant leaves the evaporator as a cold vapor and enters the compressor where it is compressed into a hot

vapor. Upon leaving the compressor, the vapor enters the condenser side of the chiller where heat is

transferred from the refrigerant to the water side of the condenser where it is circulated to an open cooling

tower for the final removal of heat via evaporation in the cooling tower.

What is Chiller Efficiency?

Chiller efficiency is the amount of energy (electricity) it takes to produce a "ton" of cooling. It is expressed as

kw/ton. All chillers have a designed kw/ton efficiency that was established when the chiller was commissioned.

Plant design, water treatment, maintenance practices, chiller age, cooling tower design, cooling load and plant

operations dramatically effect chiller operating efficiency and operating costs.

Chiller Operation, Service and Maintenance A chiller "operator" is known by several titles, including Stationary

Engineer, HVAC Engineer and Service Technician. Operation and maintenance includes collecting and logging

data from various gauges, controls and meters located on or near the chiller. Service contractors, who

specialize in equipment repair, are contracted when major repairs or overhauls are required.

There are essentially three types of maintenance performed on chillers; water chemistry, mechanical

maintenance and operational procedures. Water chemistry is maintained to keep proper balance and minimize

the effects of scale, corrosion and micro-biological / debris fouling. Mechanical maintenance includes proper

lubrication, adequate liquid refrigerant, oil levels and pump curve tests. Operational procedures include eddy-

current tests, oil analysis, calibration of gauges and meters and other various tests.

Maximizing Chiller Efficiency

This article has been published in Maintenance Technology and Hotel Engineer Magazines.

Chillers are the single largest energy-using component in most facilities, and can typically consume over 50%

of the electrical usage. Chillers use approximately 20% of the total electrical power generated in North America

and the U.S. Department of Energy estimates that chillers expend up to 30% in additional energy through

inefficiency. With over 100,000 chillers in the United States alone, inefficiency costs industry billions of dollars

in energy annually.

Chillers running inefficiently also result in decreased equipment reliability, increased maintenance intervals and

shortened lifespan. The slightest decrease in chiller performance can have a major impact on efficiency. For

instance, every 1°F increase in condenser water temperature above full load design can decrease chiller

efficiency by 1% to 2%. A failing or neglected water treatment program can reduce efficiency 10% to 35% or

more in extreme cases.

What is Maximum Chiller Efficiency?

Contrary to popular belief, running the chiller at full load design and achieving the design kW/Ton does not

necessarily mean the chiller is running at maximum efficiency. This is due to the fact that most chillers rarely

operate at full load design (less than two percent of the time on average). Most chillers achieve the greatest

tonnage at the lowest kilowatt usage by operating at approximately 70-75% load along with the lowest Entering

Condenser Water Temperature (ECWT), based on design. Knowing a chiller's efficiency and the effects of load

and ECWT will help the facility determine the most efficient chiller configurations, saving the maximum on

energy costs.

Document Chiller Data

The first step in maximizing chiller efficiency is to establish a method for recording chiller operational data in a

daily log. It's common for facilities to maintain chiller logs, but unfortunately they rarely get reviewed, which is

critical. EffTrackTM can automatically collect and trend chiller operational data. EffTrack accurately measures

chiller performance at full and partial loads, calculates efficiency and diagnoses the causes of inefficiency.

Once the chiller status (baseline) has been determined, operational changes can be made to increase

efficiency and measure the results.

Ensure Accurate Data

Ensuring accurate data can be difficult. One of the most common assumptions made by a facility is that the

flow to the chiller is constant and always at design. Unfortunately, this may not be the case and there are

several reasons why. Chiller systems are dynamic, ever-changing models, which must adapt to the

environment around them. They expand and contract from the original design. They are subject to wear, tear

and age. The best advice is don't assume anything until proven by accurate, continuous verification.

The best way to provide precise data, obtain concrete results and minimize problems is to verify flow rates to

the chiller for tonnage measurements and other calculations to determine efficiency. Four methods for

determining flow are inline flow meter, external flow meter, delta pressure and delta temperature.

Flow meters can be a high quality turbine type, magmeter (inline) or ultrasonic (external), and gives the most

accurate Gallons Per Minute (GPM) flow readings. GPM can be determined by delta pressure using a gauge or

annubar. Delta temperature cannot actually measure the flow rate in GPM, but it can identify proper flow and

problems associated with flow. It can also be affected by other conditions not directly related to flow, such as a

scaled or fouled chiller barrel, non-condensable gasses and refrigerant level, making interpretation more

difficult. However, the use of delta temperature along with a flow meter or delta pressure gauge creates a

powerful diagnostic tool that can detect problems affecting efficiency in the chiller system.

Along with proper flow, check and calibrate temperature sensors/gauges, pressure sensors/gauges, electrical

meters, etc. periodically or when a problem is detected.

Increase the Chill Water Temperature and Lower the Entering Condenser Water Temperature For constant

speed chillers, every 1°F increase in chill water temperature can increase chiller energy efficiency 1 to 2%. For

variable speed chillers, every 1°F increase in chill water temperature can result in a 2 to 4% efficiency increase.

However, it may not be possible to increase the chill water temperature to save money due to design

constraints, occupant comfort levels or real-time energy pricing (sacrificing efficiency at one time to improve the

efficiency at another time).

Take advantage of wet bulb conditions in the cooling tower system to lower the chiller's entering condenser

water temperature. This can result in a 1 to 1.5% efficiency improvement for every 1°F below the chiller full load

design. It is important to note that part loads associated with chiller type (high or low pressure) and compressor

motor style (constant or variable speed) will affect the chiller's performance. Consult the chiller manufacturer to

establish appropriate guidelines for entering condenser water temperature.

Have an Aggressive Water Treatment Program

A good water treatment program is a necessity for efficiency. Maintaining the proper water treatment will

prevent costly problems. If a problem(s) already exists, take the necessary steps to correct it immediately. The

results can provide significant energy savings with greater chiller efficiency, maximized equipment life and

reduced overall maintenance costs. Remember, always wear appropriate Personal Protective Equipment

(PPE) when using chemicals or cleaning equipment.

Biocide and Scale/Corrosion Protection

A water treatment program provides a biocide program that minimizes microbiological growth along with

excellent scale/corrosion protection. Microbes, if not properly controlled, can cause numerous problems, such

as forming sticky slime deposits in the tube bundle of a chiller, possibly reducing heat transfer efficiency 15% or

more. The situation can be compounded by the formation of permanent scale or iron deposits on the sticky site.

If this occurs, an additional 10 to 20% loss in heat transfer efficiency may result. To fix the problem and restore

lost efficiency, an unscheduled shut down and physical cleaning of the chiller may be required. Furthermore, if

no action is taken to improve the water treatment, under deposit corrosion may occur throughout the condenser

system, which may create leaks in the transfer piping.

Cooling Tower Cleaning and Lay-Up

Chiller System Dead LegCooling tower system cleaning is essential for peak efficiency. A good time to consider

cleaning is fall and spring, just before and after winter lay-up. This usually means part or all of the condenser

system may lay-up dormant for several months. Dead legs (no circulation/stagnate water) in the condenser

system are potential areas for producing many types of microbes. One type of anaerobic bacteria of particular

importance is Sulfate Reducing Bacteria (SRB). SRB can cause significant localized pitting corrosion and

severe damage in a relativity short period of time. Treating these areas of a condenser system with biocides

and biodispersants prior to lay-up can help minimize microbial problems. Lay-up treatments also ensure an

easier start up in the spring, minimizing maintenance problems.

Chiller Tower System Basin WaterA lay-up treatment is designed to protect the equipment and piping by

reducing pipe chip scale (flash corrosion). This chip scale or flash corrosion can have a serious impact on start-

up, causing blockage of distribution holes on the tower hot deck, plugged strainers and in extreme cases,

blockage in the chiller. Any of these problems will reduce flow and heat transfer efficiency in the condenser

system.

When cleaning the tower basin, all debris should be removed (i.e. sand, silt, trash, and most importantly

biofilm). Biofilms are home to many living organisms. Some, of the more common organisms include

pseusdomonas slime, which can reduce heat transfer efficiency, and SRB. Tower cleaning should also include

inspection of the drift eliminators, fill and louvers to minimize airflow restriction across the cooling tower system.

Make sure the tower fans are working properly to produce the desired airflow for heat transfer removal. Chiller

Tower System CorrosionVisually inspect the wood and metal construction, looking for signs of deterioration.

Wood deterioration may be a sign of microbio problems (mold, yeast or fungi) or over feeding the oxidizing

biocide causing wood delignification/deterioration. Look for white rust on the metal construction caused by

either a tower that was never properly pretreated and passivated or a chemical program that may not fit the

water chemistry. A thorough spring-cleaning can assist in maintaining maximum efficiency throughout the

summer months.

Pretreatment

Pretreatment is recommended for a new system, (condenser, evaporator and tower system), or when there is a

new add-on to an existing system to ensure heat transfer efficiency and prolong equipment life. The purpose of

pretreatment is to remove oil and grease from new piping and chillers. If pretreatment is not performed, the oil

and grease may adhere to the heat exchanger, reducing heat transfer. Oil and grease can also provide food for

microbes to bloom, requiring additional costly biocide treatments. Pretreatment should passivate the new

metals and minimize white rust and flash corrosion.

Galvanic Corrosion

Galvanic corrosion is associated with dissimilar metal coupling and can exist in all areas of the HVAC system

(though it primarily occurs on the condenser side of a chiller), and if severe enough, can affect the life of the

chiller. Metal passivating chemicals commonly used in the evaporator minimize galvanic corrosion. Most

chillers have copper tubes with carbon steel tube sheets and end bells, in which a galvanic reaction can occur

between the copper and carbon steel. Installing sacrificial anodes and painting the inside of the chiller end bells

and tube sheet with an epoxy coating can also minimize this corrosion.

Preserve Design Flow Rates

Maintain condenser and evaporator design flow rates, checking them annually. A rule of thumb is to always

maintain flow greater than 90% of design because lower flow will reduce chiller efficiency. When the flow is

reduced or restricted, it can create undesirable laminar flow (less than 3 feet per second) through the chiller,

which can also cause a water treatment program to fail. Above design flow (greater than 12 feet per second)

through the chiller may cause vibration wear and erosion/corrosion of the tubes, reducing reliability and life.

Cracks and pitting holes can develop causing leaks in the tube bundle.

Curb Non-Condensable Gasses

Non-condensable gasses (air) are associated with low-pressure chillers with evaporators designed to use

refrigerants that operating in a vacuum. When a leak develops in the evaporator, air and moisture are pulled in,

which affects the compressor and reduces heat transfer efficiency. The compressor is working to move the

non-condensable but getting no refrigerant effect. In fact, non-condensables can blanket tubes in the

condenser lowering the overall efficiency up to 6 to 8% at 60% load and 8 to 14% at full load. To help minimize

the affect of non-condensables, purge units are required.

Maintain Refrigerant Levels

The ability of a chiller to efficiently remove heat directly correlates to the compressor's ability to move the

refrigerant per unit of time. It is important to maintain proper refrigerant levels because low levels cause the

compressor to work harder and less efficiently. Check for leaks regularly, especially when a chiller shows signs

of low refrigerant level. Trending refrigerant levels will help determine if the chiller has a leak(s), a bad purge

unit or refrigerant carryover (i.e., due to low ECWT).

Refrigerant Analysis

Regular refrigerant analysis is an important part of determining chiller inefficiencies. If oil content in the

refrigerant is above the chillers manufactures guidelines, it may be reducing heat transfer. Keeping good

maintenance records on oil usage in a chiller will help to avoid this condition.

Schedule Preventive Maintenance

Compressor oil analysis should be performed annually. Low-pressure chillers may require more frequent

analysis, based on purge run hours. This test should include a spectrometric chemical analysis containing

information on metals, moisture content, acids and other contaminants that can affect chiller performance.

Replace oil filters on an as-needed basis including high pressure drop or when the compressor oil is changed.

Consult your chiller manufacturer, lubricant supplier and/or oil analysis laboratory for oil and filter change

intervals.

Monitor Refrigerant Approach Temperature

One of the earliest signs of chiller inefficiency is an increase in Refrigerant Approach Temperature (RAT). The

RAT is determined by calculating the difference between the leaving fluid (water) and the saturated

temperature of the refrigerant being heated (evaporator) or cooled (condenser). Newer chillers or chillers that

have been retrofitted perform this function. Older chillers may require taking the suction pressure (evaporator)

and head pressure (condenser), then converting these pressures to temperature from a refrigerant

temperature/pressure table. Every chiller has a manufacturer design RAT. When it is exceeded, a problem with

heat exchange in the chiller exists. Problems associated with high RAT include low refrigerant level, non-

condensable gasses, low/high flow rates, part loads at low ECWTs and finally, a scaled or fouled chiller.

Critical Sensor Accuracy

How Sensor and Gauge Accuracy Impact Chiller Efficiency

Do You Really Know How Your Chillers Are Running?

Historically, plant engineers have kept chiller operating logs to measure chiller performance and determine

causes of problems. The data collected includes readings taken from the chiller during scheduled inspections

such as evaporator and condenser temperatures, pressures, flows, running load amps, volts, etc. This

schedule varies from every two hours to once a shift, depending on the type of operation and more importantly,

manpower constraints. In most facilities, logs were a vital tool in scheduling downtime, preventive maintenance

and inspections based on chiller run hours. Today, it is common for facilities to maintain logs, but they rarely

get reviewed until there is a problem, which is too late.

Garbage In

The old adage of "garbage in equals garbage out" holds especially true when determining chiller performance.

All temperature and pressure sensors/gauges lose their calibration and drift over time. The period of time and

amount of drift vary from one sensor to another. If your sensors haven't been calibrated in the last year and are

over three years old, the odds of them being inaccurate are almost 100%. Evaporator and condenser water

flows can fluctuate during seasonal changes, when circulating pumps begin to wear out or there are plant

changes from original design. Therefore, if flows aren't measured and adjusted as needed, they are probably

far from design - which is essential.

Add the potential for human error into the equation. When logging data, it can be very easy to flip flop readings,

use outlet voltage instead of inlet voltage, transpose values or use approximations when using mercury

thermometers or dial gauges that are not properly sized for the operating pressures. Digital gauges that

measure to the 1/10 are highly recommended for accuracy and the reduction of human error.

In most facilities today, manpower is a major problem. Cutting back on preventive maintenance and taking logs

has become a target for saving manpower resources. It's hard to argue with an operating engineer when they

say "I don't have time to take logs on a regular basis, much less evaluate the data." Compound this with

inaccurate data, which renders it almost worthless and makes any clear-cut analysis virtually impossible. One

can only hope that when problems occur they are not major, effecting operations and requiring expensive

repairs.

Determining Chiller Efficiency

A common method for determining chiller efficiency has been to calculate the actual kW/Ton, then determine

the difference between actual and design full-load kW/Ton. The problem with this method is it can only be

accurate if the chiller is operating at full load conditions when compared to design full-load. This occurs on

average less than 2% of the time. To calculate kW, take the square root of 3 (1.732), multiply by the actual

running load amps, multiply by the actual volts, divide by 1000, then multiply by the power factor (PF). To

calculate tonnage, take the evaporator delta temperature (Delta T), which is the difference between the

evaporator water temperature in and the evaporator water temperature out, multiply by the evaporator water

flow gallons per minute (GPM) and divide by 24 (Figure 1).

kW = [(1.732 x Amps x Volts) ÷ 1000] x PF

Tons = (Delta T x GPM) ÷ 24

Figure 1. Industry Standard kW/Ton Equation.

Efficiency Technologies, Inc. has taken these calculations to the next level and developed an Internet-based

chiller efficiency and trending tool called EffTrack™. EffTrack accurately measures chiller performance at all

part loads and any operating condition with a proprietary Calculated Part Load Value (CPLV). Comprehensive

reports include advanced diagnostics and fault detection that identify the cause of inefficiency (including bad

data) and provides detailed corrective action instructions.

Determining Accuracy

There are some obvious ways to tell if your sensors are out of calibration. If the Delta T's vary high or low from

design operating conditions, then it could be an indication that one or both of the temperature sensors are

inaccurate or water flow may be off-design. If the evaporator leaving refrigerant temperature is greater than the

evaporator chilled water out temperature, or the condenser leaving refrigerant temperature is less than the

condenser water out temperature then it indicates a problem in either the water temperature sensor or the

refrigerant pressure sensor. If leaving refrigerant temperatures are not recorded, then pressures can be

converted to temperature from a standard refrigerant/temperature chart. Without a tool like EffTrack, it can be

difficult to detect bad sensors and determine how far they are out of calibration.

Garbage Out

The impact of inaccurate data can have a dramatic effect on energy consumption. Every 1°F decrease in chill

water temperature caused by an inaccurate high reading creates a 2-4% increase in energy usage to maintain

that unnecessary low temperature. Not knowing the real temperatures can cost a fortune in wasted energy, not

to mention wear and tear on the chiller components by running out of intended parameters. The four main

contributors of bad data are temperature sensors, pressure sensors, water flow and human error.

Temperature

How do inaccurate temperature sensors/gauges affect efficiency calculations? Take an example of a chiller

with design specifications of: design tonnage = 600, full load amps = 500, volts = 460, PF = .9, design Delta T =

10°F, GPM = 1440 and design kW/Ton = .598. If this chiller's evaporator water in temperature sensor is reading

low 1°F and the evaporator water out temperature sensor is reading high 1°F, this gives a combined total of

2°F off or an 8°F Delta T at full load. When the kW/Ton is calculated, it equals .747. Divide .598 by .747 to get

80% efficiency. Just 2°F drift can make it appear that the chiller is 20% inefficient. This can alter scheduling of

maintenance, produce inaccurate cost analysis and skew the plant load profile by 20%, making decisions

concerning chiller sizing very difficult. At 80% efficiency, a 600 ton chiller running at 50% load, 24 hours / 365

days, at $0.06/kWh would indicate a ~$24,912 loss. This emphasizes why sensors and gauges must be

accurately calibrated to 1/10°F.

Temperature Calibration

Figure 2. Ice Bath 

Temperature Measurement.

Temperature gauge calibration is very easy. Make sure the gauges to be calibrated are clean and dirt free. If a

commercial temperature bath to perform a 3 or 5 point calibration is unavailable, freeze ~ 2 liters of deionized

(DI) water and crush or shave into as small of pieces as possible. Fill an insulated container, such as a Dewer

flask or thermos (that will maintain a temperature) with the crushed ice. Fill the container with chilled DI water,

and then drain the water out. Make sure the ice settles in the bottom of the container, minimizing air pockets in

the ice. Continue this process until the volume is filled with ice and minimal liquid/air space. Submerge the

entire length of the temperature probe into the ice bath until the readings stabilize (Figure 2). Adjust calibration

dial until it reads 32.0°F (Figure 3). If calibrating more gauges, use the same ice bath, which can last for 1-2

hours.

Chiller Digital Temperature Gauge Ice Bath Calibration Chiller Digital Temperature Gauge Ice Bath Calibration

Figure 2. Ice Bath Temperature Measurement. Figure 3. Calibrating the Temperature Sensor.

Temperature sensors for a chiller panel or building automation system (BAS) may not be able to be calibrated.

Typically, an offset can be entered into the BAS/chiller software to compensate for calibration. To do this, a

calibrated sensor must be used to get an accurate temperature reading and the difference between the

calibrated sensor and the BAS sensor entered as the offset. Entering an offset is not as accurate as using a

calibrated sensor and replacing the sensor with one that can be calibrated is recommended.

Pressure

Figure 3. Calibrating the 

Temperature Sensor.

When refrigerant pressure sensors/gauges calibration drifts, it affects the ability to diagnose problems.

Inaccurate pressures can give the indications of fouling or scaling, high or low refrigerant levels, refrigerant

stacking and non-condensable gasses. Misdiagnosis of these conditions can not only cost in wasted energy,

but also potentially damage the chiller.

Pressure Calibration

There are several ways to calibrate pressure sensors/gauges including hand pump calibrators, dead weight

testers (Figure 4), portable field calibrators and laboratory calibration services. These devices and services

range in price from hundreds to several thousand dollars. If a pressure calibration device is unavailable or cost

prohibitive, replacement of the sensor/gauge is recommended. A typical factory-calibrated gauge can cost

under $30. A high quality, coil-type gauge that will hold calibration for a long period of time will cost slightly

more.

Chiller Pressure Gauge Calibration with a Dead Weight Pressure Tester Figure 4. Dead Weight Tester for

Pressure Calibration.

Water Flow

Figure 4. Dead Weight Tester for Pressure Calibration.

One of the most common assumptions made by a facility is that water flow to the chiller is constant and always

at design. Unfortunately, this may not be the case because chillers are dynamic, ever-changing models, which

must adapt to the environment around them. They expand and contract from their original design and are

subject to wear, tear and age.

The impact of off-design flow can be illustrated by taking the same chiller design specifications as in the

temperature example with a design kW/Ton = .598, but with a pump that is oversized 20%, making the actual

GPM 1728, which would drop the Delta T to 8.33 at full load. Following the standard kW/Ton equation in Figure

1 (using design GPM of 1440 if flow is not actually measured), the calculated kW/Ton would be .717, giving an

apparent efficiency of 80% based on inaccurate data.

Measuring Water Flow

Four methods for determining flow are an inline flow meter, external flow meter, delta pressure (Delta P) and

Delta T. Flow meters can be a high quality turbine type, magmeter (inline) or ultrasonic (external), and gives the

most accurate GPM flow readings. GPM can be determined by Delta P using a manometer or annubar. Delta T

cannot actually measure GPM, but it can determine potentially high or low flows. If an accurate Delta T at full

load is greater than design, it can indicate low GPM. Conversely, if the Delta T at full load is less than design, it

can indicate high GPM. If inline or ultrasonic flow meters are not available, a handheld manometer can be

purchased for a few hundred dollars and are a very effective way to use Delta P to correct flow (see Sidebar).

Figure 6. Connect Pressure Lines 

and Open Valves.

Figure 5. Handheld Manometer 

with Couplings Connected.

An even more affordable, but less accurate way to measure Delta P is to make your own Delta P gauge (Figure

11). Take a pressure gauge, attach a "T" pipe, connect a ball valve to each end of the T, and then connect

couplings for the pressure lines to the ball valves. Connect the pressure lines to the device and the chiller's

pressure inlet and outlet. Open the inlet ball valve, take a pressure reading and then close the ball valve. Open

the outlet ball valve, take a reading and then close the valve. The difference between the inlet and outlet

pressures is the Delta P.

Using a Handheld Manometer to Measure Delta P

Figure 8. Pinching-Off Valve to 

Stabilize Readings.

Figure 7. Reading Manometer.

Connect the pressure hose couplings to the manometer (Figure 5). Connect the female coupling of the

pressure hose that is attached to the positive side of the manometer to the male coupling on the side of the

evaporator/condenser with the greatest pressure (the inlet). Connect the female coupling of the pressure hose

that is attached to the negative side of the manometer to the male coupling on the side of the

evaporator/condenser with the lowest pressure (the outlet). If the positive and negative are reversed, the

manometer will read in negative pressure. Turn pressure valves on for the inlet and outlet (Figure 6) and read

manometer Delta P (Figure 7). If the manometer reading fluctuates rapidly, reduce the pressure valve flow.

This will slow the fluctuation of readings (Figure 8). Make minor water flow valve adjustments on the

evaporator/condenser until the chiller's design Delta P is met (Figure 9).

IMPORTANT: The positive and negative (inlet and outlets) pressure connections on the chiller should be level,

or the same height in order to read the most accurate pressure. If a pressure reading is taken higher up on a

pipe than its counterpart, the reading may be inaccurate.

Self Defense

Figure 10. Makeshift Delta P Gauge.

Figure 9. Adjusting Water Flow Valves.

One technique that some service companies use to acquire business is to perform basic chiller efficiency

calculations. These calculations typically show problems and are followed by the promise of improvement or

correction if contracted. The ability to know your operations and accurately measure your performance is a

valuable asset. Use this asset to prevent being manipulated by the promise of unrealistic savings that may

have been produced by inaccurate data. Having calibrated temperatures, pressures and flows and knowing the

true performance of your chiller plant is an excellent way to keep everyone honest. or more in extreme cases.

Stacking and Carryover

The Impact of Refrigerant Stacking and Carryover in Water-Cooled Chillers

This article has been published in Energy & Power Management, RSES Journal and Industrial Equipment

News Magazines.

The Cooling Cycle

To understand refrigerant "stacking" and "carryover", you must first be familiar with the cooling cycle for water-

cooled chillers. To begin, chillers circulate "chilled water" in a closed loop system from the evaporator to air-

handlers where heat is transferred from the circulating air to the "chilled water" (Figure 1A). The warmed water

returns to the evaporator where heat is transferred from the water to a cold, low-pressure liquid refrigerant

(Figure 1B). The compressor creates a continuous low-pressure in the evaporator, making it possible for the

liquid refrigerant to boil into a low-pressure vapor (Figure 1C). This vapor absorbs the heat and transfers it out

of the evaporator into the compressor (Figure 1D). Once inside the compressor, the low-pressure vapor is

compressed into a hot, high-pressure vapor (Figure 1E). The high-pressure vapor discharges from the

compressor and enters the condenser where the heat is transferred to the colder condenser water circulating

from the cooling tower basin (Figure 1F). Removing the heat from the high-pressure vapor causes it to

condense into a warm, high-pressure liquid. Additional heat can be removed from the high-pressure liquid

refrigerant through a sub-cooler or storage vessel in the condenser prior to returning to the evaporator . . .

where the process begins again. The heat transfer is completed when the condenser water leaves the chiller

and circulates to the open cooling tower, where heat is removed by evaporation to the atmosphere, causing the

water temperature to drop prior to returning back to the condenser (Figure 1G).

What is Stacking and Carryover?

Figure 1. The Cooling Cycle

Stacking is the abnormal accumulation of refrigerant in the condenser, commonly caused by a decrease in the

difference in pressure or "lift" between the condenser and the evaporator. This reduced pressure drop prevents

the refrigerant's ability to physically flow back to the evaporator and maintain a normal refrigerant cycle.

Carryover is the presence of liquid refrigerant droplets in the cold, low-pressure vapor produced in the

evaporator. Generally, there are two types of carryover. The first type is by design and controlled with the intent

of enhancing the lubrication process in the compressor. The second type is not by design, and if excessive can

be detrimental to the performance and reliability of a chiller. When this occurs, liquid refrigerant droplets travel

from the evaporator to the compressor inlet (Figure 2). As these liquid droplets enter the compressor system

they vaporize on the metal internals, stripping away lubricant. Ultimately this oil stripped from the compressor

becomes entrained in the hot, high-pressure refrigerant vapor and enters the condenser. Once this happens, oil

from the sump is used to replace the stripped oil, causing the oil level to drop in the compressor oil reserve

(see Compressor Oil Levels Drop section). Entrained oil travels with the refrigerant as it condenses from the

hot, high-pressure vapor to the warm, high-pressure liquid in the condenser. The excessive oil in the warm

liquid refrigerant then returns to the evaporator where the oil separates from the refrigerant. Once separated,

the oil can be removed and return to the compressor oil reserve. Also, in severe cases, carryover can cause

excessive liquid refrigerant to accumulate (stack) in the condenser.

Although stacking and carryover occur under similar conditions in a chiller, they can develop together or

separately. Identifying and preventing these conditions are covered throughout this article.

What Makes a Chiller Susceptible?

Figure 2. Refrigerant Carryover

Stacking and carryover can be very common, affecting all types of water-cooled chillers. However, low-

pressure chillers, which operate the evaporator in a vacuum, are more susceptible. Compared to high-pressure

chillers, they do not have a large difference in refrigerant pressure between the condenser and evaporator.

Figure 3 illustrates the comparison of a low-pressure chiller using R-123 refrigerant and a high-pressure chiller

using R-134 refrigerant. At 65°F Entering Condenser Water Temperature (ECWT) and 40°F evaporator chill

water temperature, the pressure difference in the R-123 machine is 4.5 psi. The pressure difference in the R-

134 machine is 28.9 psi. It is this "lift" in both chillers that can be beneficial or detrimental if not properly

maintained. To be fair, the advantage of low-pressure chillers is the ability, in most cases, to achieve a lower

Full Load Design (FLD) kW/Ton. Depending on plant design and operations, both types have their advantages.

How Stacking and Carryover Occurs

Figure 3. Pressure/Temperature Chart Comparison

There are three conditions that cause stacking and carryover: mechanical, maintenance and the most common.

. . operational.

Mechanical Problems

There are three tower temperature control problems that can cause stacking and carryover in chillers. First is

the location of the tower temperature control. This temperature should mirror the ECWT. Second, historesis is

the delayed reaction time of the controller affecting the ability to maintain a tight temperature range. The third

and most common problem is tower operations maintaining temperatures too cold for the design of the chiller.

To remedy these problems, make sure the tower temperature is controlled in the basin, mirroring the ECWT.

Improve controls to keep a maximum 2°F high/low temperature range in the tower basin. Maintain the design

ECWT for the chiller. All will help contribute to a well-run, efficient plant.

A malfunctioning or un-calibrated refrigerant level control in the condenser can create stacking by not allowing

refrigerant to flow back to the evaporator. Refrigerant builds up in the condenser, while at the same time

lowering refrigerant levels in the evaporator due to an unbalanced refrigerant cycle. The chiller loses heat

transfer efficiency and may shut down on low evaporator refrigerant temperature.

Maintenance Problems

It can be difficult to add the proper amount of refrigerant to a chiller; therefore refrigerant overcharge may occur

when the chiller is serviced. This additional refrigerant is added to compensate for the loss during normal

operations or leaks (which have been repaired). As a result, stacking and carryover may occur. When this

happens, it is generally easy to identify. The obvious signs are higher than normal refrigerant pressures in both

the evaporator and condenser after the chiller was serviced. Extreme overcharge will cause the chiller to run

poorly or have difficulty staying online due to high condenser head pressure. A superheat test is an excellent

tool for verifying and ensuring proper refrigerant levels.

Operational Problems

The primary cause of stacking and carryover, as it relates to operations, is the operator running the ECWT too

low for the chiller load conditions. This can bring about stacking, or in extreme cases, carryover with stacking.

Both cause chiller inefficiency and damage over time.

ECWTs can have a good or bad impact on chillers. Lowering the ECWT can be a great energy management

practice if done within the chiller design parameters. For example, every 1°F drop in ECWT below FLD can

improve a chiller's efficiency up to 1.5% depending on the chiller design. Lowering the ECWT reduces the "lift",

making it possible for the compressor to use less energy to produce the desired capacity (tonnage). This is

extremely valuable, considering the cost of energy today and that chillers are typically the largest energy

consumer in most facilities. However, there is a fine line between good energy management and initiating

potential problems due to low ECWTs.

Chiller manufacturers today not only build chillers with more efficient FLD kW/Ton, but they can also design

chillers to maximize efficiency at part loads. Since most chillers run in part loads ~98% of the time, it is up to

the facility to decide which chiller best fits their needs and to master the operation of these chillers to achieve

the lowest kW/Ton under all conditions. They must also operate in a manner that protects this very expensive

equipment.

In terms of potential savings, a well-run chiller should achieve an overall operating kW/Ton below FLD kW/Ton

by 5% to 15% for constant speed drives, and by 20% to 30% for variable speed drives. How can plants achieve

this efficiency? Primarily, maintaining the design ECWT for the chiller load and using the operator's knowledge

of the chiller to maximize its performance. There is no substitute for well-trained operating engineers; however,

they need the required data and analysis to make informed decisions when operating a chiller to achieve its

best efficiency.

Identifying and Preventing Stacking and Carryover

Both stacking and carryover affect chiller performance and are not easily detected from a single sensor. In fact,

it requires multiple sensors, experienced operators and data analysis to positively identify. Therefore, it is

typically only diagnosed in severe cases, usually when the chiller shuts down or after an oil or refrigerant

analysis is performed. Furthermore, misunderstanding of these conditions may lead to false assumptions like

low oil and refrigerant levels, resulting in excess oil being added to the chiller or refrigerant overcharge.

Compressor Oil Levels Drop

When oil levels drop in the compressor sump due to low ECWT, it can prompt the operator to add additional oil.

Later, when the ECWT returns to normal, excess oil separates in the evaporator, where it is removed and

returns to fill the compressor oil sump with surplus oil. This surplus oil may need to be manually removed.

However, if the chiller continues to run with low ECWTs, low oil levels may persist and even more oil added.

This makes it possible for this excess oil to emulsify in the refrigerant and inhibit heat transfer on the evaporator

tubes.

Keep detailed records of oil additions between oil changes. When the additions are made, check for leaks. If no

leaks exist, examine operations. Look for foaming in the evaporator refrigerant sight gauge as a sign of high oil

levels in the refrigerant. Look for refrigerant boiling in the compressor oil sight gauge as a sign of stacking

and/or carryover.

Evaporator Liquid Refrigerant Levels Drop

This can happen very quickly or slowly depending on the disparity between the ECWT and chiller load. When

severe, the refrigerant is rapidly pulled out of the evaporator and stacked in the condenser. This may cause the

chiller to shut down on low refrigerant temperature. The refrigerant cycle is broken due to low lift. Increasing

ECWT restores the lift and the refrigerant will go back to a balanced cycle between the condenser and

evaporator.

Seasonal cold weather conditions can make tower water basin temperatures difficult to control and the potential

for stacking and carryover greater. During startup, lowering the demand limit on the chiller and/or reducing

condenser flow may be the easiest solution for providing the desired condenser water temperature.

Oil and Refrigerant Analysis

It is best practice to perform quarterly oil analysis and refrigerant analysis when problems are suspected.

Abnormally high metal content in the oil analysis can be an indicator of past refrigerant carryover and bearing

wear caused by increased friction from stripping the lubrication. The chiller representative and a physical

inspection of the compressor bearings can confirm this. There is an acceptable level of oil that can be in a

refrigerant analysis and not impede chiller heat transfer. When refrigerant carryover is chronic, the percentage

of oil in the refrigerant will increase, causing heat transfer problems.

Chiller Diagnostics Software

Investing in a chiller diagnostics program, such as EffTrack™, can make it easy to identify stacking and

carryover. EffTrack collects, stores, and analyzes chiller operating data to determine performance, diagnose

causes of inefficiency, and recommend corrective action. EffTrack notifies plant contacts if problems occur.

Plant operators and facility managers can review the hourly updated information by logging in to EffTrack from

any computer with Internet access. By following the EffTrack recommendations for improvement, plant

operators can eliminate chiller faults and significantly lower the chiller kW/Ton and plant kWh consumption.

These savings are identified and measured in the EffTrack eports. In addition to stacking and carryover,

EffTrack can identify high or low refrigerant levels, compressor problems, water flow rate problems including

plugged or restricted water flow, fouling and scaling, non-condensable gasses in low-pressure chillers, cycles

of concentration problems, and sensor calibration problems or bad data.

Get To Know Your Chillers

Every chiller has its own unique circumstances that create stacking and carryover. Ask your chiller

manufacturer, "what is the best operating conditions for ECWT and the matching part load values." Then

develop a program for monitoring efficiency and identifying chiller problems. Once achieved, the facility can

reap the rewards of peak efficiency and performance at the lowest possible expense.

Chiller Service Companies

Energy Management Consulting Companies

Mechanical / Industrial HVAC Contractors

Water Treatment Companies