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Air Cooled Water Chillers – How They Work

مجموعة تكنولاب البهاء جروب

عميد دكتور

بهاء بدر الدين محمود

استشارى معالجة المياه

01229834104

Air cooled water chillers are vapour compression refrigeration systems.

The main components of a vapour compression refrigeration system are the compressor, condenser, expansion valve & evaporator.

Vapour compression refrigeration systems have a refrigeration cycle.

The cycle starts with a cool low pressure mixture of liquid & vapour refrigerant entering the chiller evaporator.

Once inside the chiller evaporator it absorbs the heat from the relatively warm water or fluid that the fluid chiller is cooling.

This transfer of heat boils the liquid refrigerant in the chillers evaporator and the super-heated vapour is pulled into the chillers compressor.

The chillers compressor compresses the refrigerant to a high temperature & pressure, high enough to allow the chillers condenser to give up its heat to the cooler ambient air.

Within the chillers condenser, heat is transferred from the hot refrigerant to the relatively cool ambient air this reduction in the chillers refrigerant causes it to de-superheat and condense into a liquid, then further sub-cool before leaving the chiller condenser.

The high pressure liquid refrigerant then enters the chiller expansion valve causing a large pressure drop across the chillers refrigerant circuit.

The pressure reduction causes a small portion of the refrigerant to boil off, or flash, this would be seen in the chillers site glass.

The site glass indicates if the chiller is short of gas, if the chiller is short of refrigerant gas the flashing inside the chillers site glass will increase.

The boiled off refrigerant helps cool the remaining refrigerant to the desired temperature before the mixture enters the chiller evaporator to start the cycle again.

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Air Cooled Chiller Condensers How They Work

The condenser is a major component of a water chiller.

The condenser is a heat exchanger that rejects heat from the chillers refrigerant to air.

Some chillers are water cooled.

A Water cooled chiller gives up its heat into relatively cool water from a cooling tower, air-blast cooler or adiabatic cooler.

A typical air cooled condenser fitted to an air cooled water chiller will use axial fans (propeller type) to draw outdoor air over a finned tubed heat transfer surface (heat exchanger).

The temperature difference or "delta T" between the hot refrigerant gas that is flowing through the condenser and the cooler outdoor air induces heat transfer.

The heat reduction of the chillers refrigerant vapour causes it to condense into liquid.

The last part of the chiller condenser is called a sub-cooler.

The chiller sub-cooler reduces the liquid gas temperature still further, under its condensing temperature.

The air cooled chiller is best suited to chiller hire applications.

Chiller hire projects require systems to be delivered and installed quickly with the minimum of fuss.

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Chiller Evaporators – How They Work

The chillers evaporator is a heat exchanger that transfers heat from a process or air conditioning water circuit to the chillers cooler liquid refrigerant.

Most air cooled chillers in the UK chiller hire market will be fitted with a shell and tube evaporator, a plate heat exchanger evaporator or a coil in tank evaporator.

A shell and tube evaporator is used primarily on chilled water applications.

When a chiller is fitted with this type of evaporator the chillers cool liquid refrigerant flows through tubes encased in a shell.

The process or air-conditioning circuit water fills the shell flowing around the tubes.

As heat is transferred from the water to the chillers refrigerant the gas boils inside the tubes and the resulting vapour is drawn into the chillers compressor.

Hot water will enter the shell at one end, chilled water leaving at the opposite end.

A Plate heat exchanger evaporator can be used for chilled water or fluid cooling applications.

Stainless steel evaporators are especially suited in food and beverage applications such as batch cooling or potable water systems.

A coil in tank evaporator comprises a coil block, usually copper, with aluminium fins to provide an efficient heat transfer surface.

The coil block is then submerged in a chilled water tank which acts as a thermal buffer for the refrigeration system.

This type of coil is suited to hire chillers used on process cooling applications, typically reactor cooling systems in chemical cooling, petrochemical cooling & pharmaceutical cooling systems.

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Summary Of Components Found On A Water Chiller

Air cooled condenser –

a type of condenser where refrigerant flows through the tubes and rejects heat into a flow of ambient air, most chiller hire units are fitted with air cooled condensers

Capillary Tube –

A type of expansion devise typically fitted on small capacity hire chillers, it comprises a long tube which reduces the pressure of the refrigerant.

Centrifugal Fan –

A type of fan fitted to an air cooled chiller allowing the fitting of ductwork onto the hot side of an air cooled condenser.

The fan is designed to work against a static pressure.

Compressor –

The main component in a chiller system, the compressor is used to increase the pressure & temperature of the refrigerant vapour.

Condenser –

The part of a chiller system where the refrigerant vapour is converted to liquid as it rejects heat.

Cooling Tower –

Used for process cooling applications & water cooled chiller condenser cooling

Distributor –

A devise used to supply uniform gas supply through a submerged coil in tank chiller evaporator

Enthalpy –

The property of a refrigerant indicating its heat content per Kg of refrigerant

Evaporator –

The part of the chiller system where cool liquid refrigerant absorbs heat from the chilled water circuit

Expansion valve –

A devise used to maintain the pressure difference between the high pressure & low pressure sides of the chiller system

Fill –

The heat transfer surface inside a cooling tower

Flash –

The process of liquid refrigerant being vaporised by a sudden reduction is pressure when entering the low pressure side of the chiller system

Hot Gas Muffler –

A device installed at the discharge side of the chiller compressor to reduce noise and vibration in reciprocating compressors

Liquid Line Filter Drier –

A devise installed in the liquid line to remove moisture and foreign matter, designed to protect the chillers compressor.

Pump Down Cycle –

A control sequence used in a chiller system to pump the refrigerant from the low pressure side of the system to the high pressure side.

Shell & Tube Evaporator –

A type of evaporator where refrigerant flows through the tubes & chilled water fills the surrounding shell

Shut Off Valve –

Used to isolate on part of the chiller system from the rest

Sub-Cooler –

The lower portion of the chillers condenser that further cools the saturated liquid refrigerant

Suction Header –

A section of pipe within the chiller system used to collect the refrigerant vapour when it leaves the tubes of a submerged coil evaporator

Suction Line Filter –

A devise installed into the chillers suction line to remove foreign matter from the refrigeration system

Superheat –

The amount of heat added to the chillers refrigerant vapour after it has vaporised within the evaporator.

Water Cooled Condenser –

A type of chiller condenser coil which water is used to remove heat from the refrigerant, this is normally a shell & tube type design.

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.

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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.

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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

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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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