Trane trg-trc005-en User Manual

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Air Conditioning Clinic

Refrigeration System Components

One of the Fundamental Series

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Refrigeration System Components

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The Trane Company • Worldwide Applied Systems Group 3600 Pammel Creek Road • La Crosse, WI 54601-7599 www.trane.com

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Refrigeration System Components

One of the Fundamental Series

A publication of The Trane Company— Worldwide Applied Systems Group

Preface

Refrigeration System Components

A Trane Air Conditioning Clinic

Figure 1

The Trane Company believes that it is 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 is intended to acquaint a nontechnical audience with various fundamental aspects of heating, ventilating and air conditioning. We have 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 vapor-compression refrigeration system components.

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

period one Refrigeration Cycle .............................................. 2

period two Condensers ............................................................ 5

Air-Cooled Condensers .............................................. 5

Evaporative Condensers ............................................ 7

Water-Cooled Condensers ......................................... 8

Condenser Control ................................................... 10

period three Evaporators .......................................................... 16

Finned-Tube Evaporators ......................................... 16

Shell-and-Tube Evaporators ......................................19

Evaporator Control ................................................... 20

period four Expansion Devices ............................................. 26

period five Accessories .......................................................... 34

Solenoid Valve .......................................................... 34

Liquid-Line Filter Drier .............................................. 36

Moisture-Indicating Sight Glass ............................... 38

Suction Line Filter .................................................... 39

Hot Gas Muffler ....................................................... 40

Shutoff Valve ............................................................ 41

Access Port .............................................................. 42

period six Review ................................................................... 43

Quiz ......................................................................... 48

Answers ................................................................ 50

Glossary ................................................................ 51

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notes

Introduction

Vapor-Compression Refrigeration

condenser

expansion

device

evaporator

The major components of a vapor-compression refrigeration system include the compressor, condenser, expansion device, and evaporator. The latter three will be discussed in this clinic— the compressor is discussed in a separate clinic.

This clinic will also discuss many of the common accessories used in a comfortcooling refrigeration system.

compressor

Figure 2

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notes

period one

Refrigeration Cycle

Refrigeration System Components

period one

Refrigeration Cycle

Figure 3

First, a brief review of the vapor-compression refrigeration cycle will help to relate these components.

Refrigeration Cycle

pressure

evaporator

enthalpy

A diagram of a typical vapor-compression refrigeration cycle can be superimposed on a pressure-enthalpy (P-h) chart to demonstrate the function of each component in the system. The pressure-enthalpy chart plots the properties of a refrigerant— refrigerant pressure (vertical axis) versus enthalpy (horizontal axis). Enthalpy is a measure of the heat content, both sensible and latent, per pound [kg] of refrigerant.

The cycle starts with a cool, low-pressure mixture of liquid and vapor refrigerant entering the evaporator (A) where it absorbs heat from the relatively warm air, water, or other fluid that is being cooled. This transfer of heat boils the liquid refrigerant in the evaporator, and this superheated refrigerant vapor is drawn to the compressor (B).

Figure 4

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notes

period one

Refrigeration Cycle

pressure

compressor

evaporator

enthalpy

The compressor draws in the superheated refrigerant vapor (B) and compresses it to a pressure and temperature (C) high enough that it can reject heat to another fluid. This hot, high-pressure refrigerant vapor then travels to the condenser.

Figure 5

Refrigeration Cycle

condenser

pressure

condenser

evaporator

enthalpy

compressor

Figure 6

Within the condenser, heat is transferred from the hot refrigerant vapor to relatively cool ambient air or cooling water. This reduction in the heat content of the refrigerant vapor causes it to desuperheat, condense into liquid, and further subcool before leaving the condenser (D) for the expansion device.

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notes

period one

Refrigeration Cycle

condenser

pressure

expansion

device

compressor

evaporator

enthalpy

Finally, the high-pressure liquid refrigerant (D) flows through the expansion device, causing a large pressure drop that reduces the pressure of the refrigerant 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.

The cooled mixture of liquid and vapor refrigerant then enters the evaporator (A) to repeat the cycle.

Figure 7

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notes

period two

Condensers

Refrigeration System Components

period two

Condensers

Figure 8

The first major component to be discussed is the condenser. The condenser is a heat exchanger that rejects heat from the refrigerant to air, water, or some other fluid.

The three common types of condensers are air-cooled, water-cooled, and evaporative.

Air-Cooled Condenser

propeller

fan

outdoor

air

condenser

coil

Air-Cooled Condensers

A typical air-cooled condenser uses propeller-type fans to draw outdoor air over a finned-tube heat transfer surface. The temperature difference between the hot refrigerant vapor that is flowing through the tubes and the cooler outdoor air induces heat transfer. 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 subcooler), the liquid refrigerant is further cooled below the temperature at which it was condensed.

subcooler

Figure 9

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period two

Condensers

notes

The air-cooled condenser is very popular in both residential and commercial applications because of its convenience. It requires very little maintenance and does not require the freeze protection and water treatment that is necessary with a water-cooled condenser. Additionally, it is favored in areas that have an inadequate or costly water supply, or where the use of water for air conditioning is restricted.

Effect of Subcooling

subcooling

{

pressure

The benefit of subcooling on system performance can be demonstrated by comparing the performance of a system with and without subcooling.

refrigeration

effect

enthalpy

Figure 10

The change in enthalpy (the line from A to B) that occurs in the evaporator is called the refrigeration effect. This is the amount of heat that each pound [kg] of liquid refrigerant will absorb when it evaporates.

In comparison, the same system without subcooling produces less refrigeration effect (the line from A’ to B). The system without subcooling must evaporate substantially more refrigerant within a larger coil to produce the same capacity as the system with subcooling.

Instead of subcooling in the condenser, some packaged refrigeration equipment, such as water chillers, may use an economizer or liquid/vapor separator to increase this refrigeration effect.

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notes

period two

Condensers

Centrifugal Fan Air-Cooled Condenser

condenser

coil

centrifugal

fan

Figure 11

An alternative air-cooled condenser uses a centrifugal fan to draw or blow air over the condensing coil. The principal advantage of this design is that the centrifugal fan is capable of overcoming the higher static-pressure losses associated with ductwork. Therefore, if the condenser is to be located indoors and uses a duct system to deliver air to and from the condenser coil, the centrifugal fan air-cooled condenser is probably best suited for this application.

Evaporative Condenser

fan

refrigerant

vapor

condenser

coil

pump

sump

subcooler

liquid refrigerant

subcooler

Evaporative Condensers

A modification of the air-cooled condenser is the evaporative condenser. Within this device, the refrigerant flows through tubes and air is drawn or blown over the tubes by a fan. The difference is that water is sprayed on the tube surfaces. As the air passes over the coil, it causes a small portion of the water to evaporate. This evaporation process absorbs heat from the coil,

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pump

Figure 12

period two

Condensers

notes

causing the refrigerant vapor within the tubes to condense. The remaining water then falls to the sump to be recirculated and used again.

Subcooling of the refrigerant can be accomplished by piping the condensed liquid back through another few rows of coil tubing, located either in the condenser airstream or in the water sump, where additional heat transfer reduces the temperature of the liquid refrigerant.

Water-Cooled Condenser

hot, refrigerant vapor

95ºF

[35ºC]

cooling water

85ºF

[29ºC]

Water-Cooled Condensers

The shell-and-tube is the most common type of water-cooled condenser. With this design, water is pumped through the tubes while the refrigerant vapor fills the shell space surrounding the tubes. As heat is transferred from the refrigerant to the water, the refrigerant vapor condenses on the tube surfaces. The condensed liquid refrigerant then falls to the bottom of the shell, where it flows through an enclosure that contains additional tubes (the subcooler). More heat is transferred from the liquid refrigerant to the water inside these tubes, subcooling the refrigerant.

After the warm water leaves the condenser, it must either be disposed of (as in the case of using water from a well) or it must be cooled before it can be reused by the condenser. In this example, the condenser brings in 85°F [29°C] water and warms it up to 95°F [35°C]. Before this water can be used again, it must be cooled back down to 85°F [29°C].

hot, refrigerant vapor

subcooled

refrigerant

, liquid

subcooler

Figure 13

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notes

period two

Condensers

Cooling Tower

propeller

fan

outdoor

air

sump

85ºF

[29ºC]

condenser

A cooling tower is a device commonly used to cool condensing water. In this design, warm water is sprayed over the fill inside the cooling tower while a propeller fan draws outdoor air upward through the fill. The movement of air through the spray causes some of the water to evaporate, a process that cools the remaining water. This cooled water then falls to the tower sump to be returned to the condenser.

from

condenser

sprays

95ºF

[35ºC]

fill

Figure 14

The final temperature of the water leaving the tower is determined, in part, by the humidity of the outdoor air. If the outdoor air is dry, the final water temperature can be considerably lower than the ambient dry-bulb temperature. If the outdoor air is humid, however, the final temperature will be near the ambient dry-bulb temperature.

While a cooling tower can reclaim much of the condensing water, it cannot reclaim it all. The evaporation process uses up water to dissipate heat contributed by the cooling load plus the heat of compression. In addition, as the water evaporates, the dissolved minerals and water treatment chemicals become concentrated in the sump. To prevent this solution from becoming concentrated and possibly corrosive, water is periodically bled from the sump and an equal amount of fresh water is added.

In the past, some water-cooled condensers used water from either a municipal or a natural water supply as the condensing water. After rejecting the condenser heat to this water, it was dumped into the sewer or back into the body of water. Environmental and economic restrictions have made this method uncommon.

Finally, a geothermal well system can be used to reject the heat from the condenser by circulating the condensing water through a series of underground pipes . This method takes advantage of the naturally-cool ground temperatures.

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notes

period two

Condensers

Condenser Control

I Condenser capacity is influenced by:

K Temperature difference between refrigerant and

cooling media

K Flow rate of cooling media through condenser

K Flow rate of refrigerant through condenser

Figure 15

Condenser Control

The heat rejection capacity of a condenser is influenced by (1) the temperature difference between the refrigerant and the cooling media (air, water, or other fluid), (2) the flow rate of the cooling media through the condenser, and (3) the flow rate of the refrigerant through the condenser.

To balance the rate of heat rejection (in the condenser) with the changing system load, at least one of these variables may be controlled.

Condenser Capacity

full load

condenser heat

rejection capacity

full load

part load

pressure

enthalpy

As the system load decreases, the heat rejection capacity of the condenser is greater than the load. Because of this excess capacity, the condenser matches the decreasing load by operating at progressively lower pressures. Additionally, a reduction in outdoor air temperature allows the temperature of the air or water flowing through the condenser to drop. This also has the effect of lowering the condensing pressure.

part load

condenser heat

rejection capacity

Figure 16

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period two

Condensers

notes

A reduction in condensing pressure lessens the power required to compress the refrigerant. Unfortunately, if the condensing pressure falls too low, the expansion valve may not be able to produce the flow of liquid refrigerant needed to satisfy the demand at the evaporator. In some systems, as the condensing pressure drops, the compressor suction pressure also drops, resulting in evaporator frosting and possible compressor shutdown due to a low-pressure safety device.

While it is not essential to control condensing pressure to a constant value, provisions should be made to control it within acceptable limits.

Condensing Temperature Control

flow-regulating

valve

condenser

water pump

Figure 17

One common method of controlling the capacity of a water-cooled condenser is to vary the rate at which water flows through the condenser.

For example, assume a water-cooled condenser is piped to a municipal water system. To control the capacity of the condenser, a flow-regulating valve is installed on the leaving-water side of the condenser. As the load on the system decreases, the regulating valve senses the lowering condensing pressure. The valve reduces the flow rate of the water until the heat-rejection rate of the condenser balances the system load at an acceptable pressure and temperature.

TRG-TRC005-EN 11

notes

period two

Condensers

Vary Condenser Water Flow Rate

cooling tower

condenser

diverting

variable-speed

drive

diverting

valve

It is more common, however, for a water-cooled condenser to be connected to a cooling tower.

cooling tower

bypass pipe

condenser

Figure 18

In this case, typical methods for modulating the water flow through the condenser include either using a variable-speed drive on the condenser water pump or using a diverting valve and pipe to bypass the condenser. The variable-speed drive on the pump modulates the amount of water pumped through the condenser. The diverting valve modulates the water flow through the condenser bundle by diverting some of the cooling water around the condenser through the bypass pipe, directly back to the cooling tower.

Each of these options has the effect of varying the flow rate of water through the condenser, ensuring an acceptable condensing pressure and temperature.

Vary Entering Water Temperature

variable-

speed

drive

cooling tower

condenser

cooling tower

bypass pipe

condenser

diverting

valve

Figure 19

Another method of controlling the capacity of a water-cooled condenser is to vary the temperature of the water entering the condenser.

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Trane trg-trc005-en User Manual

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Contents

Introduction

Refrigeration Cycle

Condensers

Evaporative Condensers

Water-Cooled Condensers

Condenser Control