Trane TRG-TRC012-EN User Manual

Air Conditioning Clinic

Helical-Rotary Water Chillers

One of the Equipment Series

TRG-TRC012-EN

Helical-Rotary Water Chillers

One of the Equipment Series

A publication of

The Trane Company— Worldwide Applied Systems Group

Preface

Helical-Rotary Water Chillers

A Trane Air Conditioning Clinic

Figure 1

The Trane Company believes that it’s incumbent on manufacturers to serve the industry by regularly disseminating information gathered through laboratory research, testing programs and field experience.

The Trane Air Conditioning Clinic series is one means of knowledge sharing. It’s intended to acquaint a nontechnical audience with various fundamental aspects of heating, ventilating, and air conditioning.

We’ve taken special care to make the clinic as uncommercial and straightforward as possible. Illustrations of Trane products only appear in cases where they help convey the message contained in the accompanying text.

This particular clinic introduces the reader to the concept of helical-rotary water chillers.

TRG-TRC012-EN

Contents

Introduction ............................................................1

period one Components ...........................................................3

Compressor .............................................................4

Oil Separator ............................................................8

Condenser ...............................................................9

Expansion Device ...................................................11

Liquid/Vapor Separator ...........................................12

Evaporator .............................................................13

Controls and Starter ...............................................15

period two Refrigeration Cycle .............................................16

period three Compressor Capacity Control .........................21

period four Maintenance Considerations ...........................25

period five Application Considerations ..............................31

Air-Cooled or Water-Cooled Condensing .................31

Condensing Temperature Control ...........................33

Constant or Variable Evaporator Water Flow ...........35

Short Evaporator Water Loops ................................36

Equipment Certification Standards ..........................38

period six Review ....................................................................40

Quiz ..........................................................................44

Answers .................................................................46

Glossary .................................................................47

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notes

Introduction

Chilled Water System

Figure 2

Water chillers are used in a variety of air conditioning and process cooling applications. They are used to make cold water that can be transported throughout a facility using pumps and pipes. This cold water can be passed through the tubes of coils in order to cool the air in an air conditioning application or it can provide cooling for a manufacturing or industrial process.

Systems that employ water chillers are commonly called chilled water systems.

absorption

helical-rotary

There are several types of water chillers that are differentiated by the refrigeration cycle they use or the type of compressor.

Absorption water chillers make use of the absorption refrigeration cycle and do not have a mechanical compressor involved in the refrigeration cycle. Absorption water chillers are the subject of a separate clinic.

centrifugal

Figure 3

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Introduction

notes

Water chillers using the vapor-compression refrigeration cycle vary by the type of compressor used. Reciprocating and scroll compressors are typically used in smaller chillers. Helical-rotary (or screw) compressors are typically used in medium-sized chillers. Centrifugal compressors are typically used in larger chillers.

As mentioned earlier, this particular clinic discusses helical-rotary water chillers.

Helical-Rotary Water Chillers

water-cooled

air-cooled

Figure 4

Helical-rotary water chillers can be either air-cooled or water-cooled, referring to the method of rejecting heat to the atmosphere. Both air-cooled and watercooled helical-rotary chillers are generally available from 70 to 450 tons [200 to 1500 kW].

The primary focus in Period 1 is on the water-cooled chiller, although it includes some discussion of air-cooled chiller components. A comparison of air-cooled versus water-cooled chiller applications is included in Period 5.

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period one

Components

Helical-Rotary Water Chillers

period one

Figure 5

Many of the components of the helical-rotary water chiller are similar to those of other chiller types.

components of a

Helical-Rotary Water Chiller

motor

oil supply

system

oil separator

condenser

This particular helical-rotary water chiller makes use of a shell-and-tube evaporator where refrigerant evaporates inside the shell and water flows inside tubes. The compressor is a twin-rotor, helical-rotary compressor. It uses a suction-gas-cooled motor to operate the compressor. Another shell-and-tube heat exchanger is used for the condenser, where refrigerant is condensed inside the shell and water flows inside tubes. Refrigerant is metered through the system using an electronic expansion valve. A liquid/vapor separator can be used to enhance the effectiveness of the refrigeration cycle. An oil supply system provides near oil-free refrigerant to the shells to maximize heat transfer performance while providing lubrication and rotor sealing to the helical-rotary compressor. A control panel is also provided on the chiller, and a starter connects the chiller motor to the electrical distribution system.

compressor

motor

evaporator

liquid/vapor

separator

control

panel

starter

Figure 6

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notes

period one

Components

compressor...

Helical Rotors

Figure 7

Compressor

The helical-rotary chiller uses 2 screw-like rotors to perform the compression process.

compressor...

Helical Rotors

female rotor

male rotor

housing

slide valve

The rotors are meshed and fit, with very close tolerances, within a housing.

Only the male rotor is driven by the compressor motor. The lobes of the male rotor engage and drive the female rotor, causing the 2 parts to counter-rotate.

Figure 8

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notes

period one

Components

compressor...

Helical Rotors

intake

port

Figure 9

In the operation of the helical-rotary compressor, refrigerant vapor enters the compressor housing through the intake port. The intake port in this example is at the top of the compressor housing.

compressor...

Helical Rotors

intake

port

pocket of refrigerant vapor

The entering refrigerant vapor is at a low, suction pressure and fills the grooves or pockets formed by the lobes of the rotors. As the rotors turn, they push the pockets of refrigerant toward the discharge end of the compressor.

Figure 10

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notes

period one

Components

compressor...

Helical Rotors

intake port

discharge port

Figure 11

Viewing the compressor from the side shows that after the pockets of refrigerant travel to the right past the intake port area, the vapor, still at suction pressure, is confined within the pockets by the compressor housing.

compressor...

Helical Rotors

discharge port

meshing point

Viewing the compressor from the top shows that rotation of the meshed rotor lobes drives the trapped refrigerant vapor (to the right) ahead of the meshing point.

Figure 12

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notes

period one

Components

compressor...

Helical Rotors

discharge port

meshing point

Continued rotation of the rotors causes the meshing point to travel toward the discharge end of the compressor, driving the trapped refrigerant vapor ahead of it. This action progressively reduces the volume of the pockets, compressing the refrigerant.

Figure 13

compressor...

Helical Rotors

discharge port

meshing point

Finally, when the pockets of refrigerant reach the discharge port the compressed vapor is released. As the rotors continue to rotate, the volume of the pockets is further reduced, squeezing the remaining refrigerant from the cavities.

Notice that the refrigerant vapor enters and exits the compressor through

ports—no valves are used. Compressors of this design are called ported compressors.

Figure 14

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notes

period one

Components

Oil Separator

refrigerant vapor

to condenser

refrigerant vapor

and oil mixture

oil return to sump

Oil Separator

The oil leaves the compressor entrained within the discharged refrigerant vapor.

Figure 15

The oil is recovered from the discharged refrigerant by an oil separator, which can have an efficiency of greater than 99%. The separator consists of a vertical cylinder surrounding an exit passageway. As the refrigerant-and-oil mixture is discharged into this passageway the oil is forced outward by centrifugal force, collects on the walls of the cylinder, and drains to the bottom. This accumulated oil drains out of the cylinder and collects in the oil sump located near the bottom of the chiller.

The oil sump is heated to ensure proper lubrication and minimize refrigerant condensation in the sump.

Oil Supply System

oil

refrigerant

vapor to

condenser

oil tank

sump

oil filter

oil

separator

master

solenoid

valve

rotor bearings

rotor

compressor

Figure 16

Oil that collects in the oil sump is at condensing pressure during compressor

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period one

Components

notes

operation and is, therefore, constantly moving to lower pressure areas of the chiller. In this system, oil flows in 2 distinct paths, each performing a separate function: 1) bearing lubrication and cooling and 2) rotor oil injection.

Oil leaves the sump and passes through an oil filter and master solenoid valve. The master solenoid valve is used to isolate the sump from the low-pressure side of the system when the compressor is shut down, preventing oil migration.

The first path is for lubricating and cooling the compressor bearings ➀. Oil is

routed to the bearings located in the rotor and bearing housing. Each housing is vented to the suction side of the compressor so that oil leaving the bearings is routed through the rotors, to the oil separator, and then back to the oil sump.

The second path is for lubricating and sealing the compressor rotors ➁. Oil is

injected along the bottom or top of the compressor rotors inside the housing. Its main purpose is to seal the rotor-to-rotor and rotor-to-housing clearances. This seal provides a barrier between the high- and low-pressure cavities of the compressor. Additionally, oil lubricates the male-to-female rotor drive arrangement.

water-cooled

Condenser

refrigerant vapor

baffle

cooling

tower

water

tube bundle

subcooler

liquid

refrigerant

Figure 17

Condenser

The high-pressure refrigerant vapor, now stripped of oil droplets, leaves the oil separator and continues on to the condenser.

In a water-cooled condenser, water is pumped through the tubes of this shell-and-tube heat exchanger while refrigerant vapor fills the shell space surrounding the tubes. The condenser has a baffle plate that helps distribute the refrigerant evenly within the shell. As heat is transferred from hot, highpressure refrigerant vapor to the water, refrigerant condenses on the tube surfaces.

The condensed liquid refrigerant then collects in the bottom of the shell where the lower tubes are now submerged, resulting in further cooling, or subcooling, of the refrigerant. This arrangement is called an integral subcooler.

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period one

Components

notes

Cooling water flows first through the lower tubes of the condenser and then through the upper tubes. This produces a nearly constant temperature difference between the downward moving refrigerant and the tube surfaces, resulting in a uniform heat transfer rate within the tube bundle.

Subcooled liquid refrigerant leaves the condenser (subcooler) and flows through the liquid line to the expansion device.

air-cooled

Condenser

propeller fan

subcooler

In a typical air-cooled condenser, propeller-type fans are used to draw outdoor air over a fin-tube heat transfer surface. The hot, high-pressure refrigerant vapor flows through the tubes as heat is transferred to the cooler outdoor air. The resulting reduction in the heat content of the refrigerant vapor causes it to condense into liquid. Within the final few lengths of condenser tubing the condensed liquid refrigerant is subcooled.

condenser

coil

outdoor air

Figure 18

Again, subcooled liquid refrigerant leaves the condenser (subcooler) and flows through the liquid line to the expansion device.

The differences between water-cooled and air-cooled chiller applications will be discussed further in Period 5.

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notes

period one

Components

expansion device…

Electronic Expansion Valve

Figure 19

Expansion Device

An expansion device is used to maintain the pressure difference between the high-pressure (condenser) and low-pressure (evaporator) sides of the system, as established by the compressor. This pressure difference allows the evaporator temperature to be low enough to absorb heat from the water being cooled, while also allowing the refrigerant to be at a high enough temperature in the condenser to reject heat to air or water at normally available temperatures. High-pressure liquid refrigerant flows through the expansion device, causing a large pressure drop that reduces the refrigerant pressure to that of the evaporator. This pressure reduction causes a small portion of the liquid to boil off, or flash, cooling the remaining refrigerant to the desired evaporator temperature.

In this chiller, the expansion device used is an electronic expansion valve. In addition to maintaining the high- and low-side pressure difference, the electronic expansion valve controls the quantity of liquid refrigerant entering the evaporator to ensure that it will be completely vaporized within the evaporator.

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notes

period one

Components

expansion device…

Orifice Plates

orifice plates

The orifice plate is another type of expansion device found in helical-rotary chillers. The column of liquid refrigerant creates a head pressure at its base, allowing it to pass through the orifices and undergo a pressure drop equal to the head (H) before it flashes. As the load decreases, less refrigerant is moved throughout the chiller, causing the level of the liquid column to drop. This causes additional flashing at the orifice plate which, in turn, results in feeding less liquid to the evaporator.

evaporator

Figure 20

Liquid/Vapor Separator

refrigerant vapor

to compressor

liquid refrigerant

from

expansion

device

Liquid/Vapor Separator

The mixture of liquid and vapor refrigerant that leaves the expansion device enters a liquid/vapor separator. Here the liquid refrigerant settles to the bottom of the chamber and the vapor is drawn off the top and routed directly to the suction side of the compressor. The remaining liquid refrigerant is then routed to the evaporator.

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liquid refrigerant

to evaporator

Figure 21

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