آرشیو خرداد ماه 1394

Chiller

Device that heat from the liquid (usually water) or absorbed by the vapor compression refrigeration cycle Myzdayd. The air or liquid cooling can be Dstgahhaastfadh usually cycles and flows through a heat exchanger. As an important byproduct, the temperature of the liquid has been absorbed or excreted to the outside or to a higher functionality is used for heating purposes. There are concerns about the design and selection of chillers. These concerns include effectiveness, efficiency, maintenance, environmental vulnerabilities

A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required. As a necessary byproduct, refrigeration creates waste heat that must be exhausted to ambient or, for greater efficiency, recovered for heating purposes. Concerns in design and selection of chillers include performance, efficiency, maintenance, and product life cycle environmental impact

In air conditioning systems, chilled water is typically distributed to heat exchangers, or coils, in air handling units or other types of terminal devices which cool the air in their respective space(s), and then the water is re-circulated back to the chiller to be cooled again. These cooling coils transfer sensible heat and latent heat from the air to the chilled water, thus cooling and usually dehumidifying the air stream. A typical chiller for air conditioning applications is rated between 15 and 150 short tons (13 and 134 long tons; 14 and 136 t) (180,000 to 18,000,000 BTU/h or 53 to 5,275 kW), and at least one manufacturer can produce chillers capable of up to 8,500 tons of cooling.^[1] Chilled water temperatures can range from 35 to 45 °F (2 to 7 °C), depending upon application requirements.^[2]

Use in industry

In industrial application, chilled water or other liquid from the chiller is pumped through process or laboratory equipment. Industrial chillers are used for controlled cooling of products, mechanisms and factory machinery in a wide range of industries. They are often used in the plastic industries , injection and blow molding, metal working cutting oils, welding equipment, die-casting and machine tooling, chemical processing, pharmaceutical formulation, food and beverage processing, paper and cement processing, vacuum systems, X-ray diffraction, power supplies and power generation stations, analytical equipment, semiconductors, compressed air and gas cooling. They are also used to cool high-heat specialized items such as MRI machines and lasers, and in hospitals, hotels and campuses.

Chillers for industrial applications can be centralized, where a single chiller serves multiple cooling needs, or decentralized where each application or machine has its own chiller. Each approach has its advantages. It is also possible to have a combination of both centralized and decentralized chillers, especially if the cooling requirements are the same for some applications or points of use, but not all.

Decentralized chillers are usually small in size and cooling capacity, usually from 0.2 to 10 short tons (0.179 to 8.929 long tons; 0.181 to 9.072 t). Centralized chillers generally have capacities ranging from ten tons to hundreds or thousands of tons.

Chilled water is used to cool and dehumidify air in mid- to large-size commercial, industrial, and institutional (CII) facilities. Water chillers can be water-cooled, air-cooled, or evaporatively cooled. Water-cooled chillers incorporate the use of cooling towers which improve the chillers' thermodynamic effectiveness as compared to air-cooled chillers. This is due to heat rejection at or near the air's wet-bulb temperature rather than the higher, sometimes much higher, dry-bulb temperature. Evaporatively cooled chillers offer higher efficiencies than air-cooled chillers but lower than water-cooled chillers.

Water-cooled chillers are typically intended for indoor installation and operation, and are cooled by a separate condenser water loop and connected to outdoor cooling towers to expel heat to the atmosphere.

Air-cooled and evaporatively cooled chillers are intended for outdoor installation and operation. Air-cooled machines are directly cooled by ambient air being mechanically circulated directly through the machine's condenser coil to expel heat to the atmosphere. Evaporatively cooled machines are similar, except they implement a mist of water over the condenser coil to aid in condenser cooling, making the machine more efficient than a traditional air-cooled machine. No remote cooling tower is typically required with either of these types of packaged air-cooled or evaporatively cooled chillers.

Where available, cold water readily available in nearby water bodies might be used directly for cooling, place or supplement cooling towers. The Deep Lake Water Cooling System in Toronto, Canada, is an example. It uses cold lake water to cool the chillers, which in turn are used to cool city buildings via a district cooling system. The return water is used to warm the city's drinking water supply, which is desirable in this cold climate. Whenever a chiller's heat rejection can be used for a productive purpose, in addition to the cooling function, very high thermal effectiveness is possible

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VALVES AND
REFRIGERANT
CONTROLS

11.1 TYPES OF VALVES
All devices considered in this chapter, even those called controls, are valves in
that they are placed in refrigerant lines and can restrict or even completley
block the flow of refrigerant. Two-position shutoff valyes are expected to operate
either in a completely open or a completely closed position. Other valves
modulate the flow rate of refrigerant in response to some variables, such as
temperature, pressure, or liquid level.
The specific types of valves explored in this chapter are:

manual shutoff valves

manual expansion valves

check valves

solenoid valves

level controls

pressure-regulating valves

superheat controlling expansion valves
A discussion of safety relief valves appears in Chapter 13, Safety

11.2 MANUAL SHUTOFF VALVES
A basic valve type distributed liberally throughout an industrial refrigeration
system is a manual shutoff valve. In the completely open position, this valve
should allow a free flow of refrigerant and when closed completely block the
flow. The usual function of the shutoff valve is to isolate a component or a
section of the system. Some major categories of manual shutoff valves are globe,
angle, inline, and ball valves, as shown schematically in Fig. 11.1. Three
desirable characteristics of manual shutoff valves are:

that they permit no pasغير مجاز مي باشدe of refrigerant when closed

that they cause only a low-pressure drop of refrigerant flowing through
them when they are open

that they do not leak to atmosphere
Several other types of valves are gate and butterfly valves which meet the lowpressure-
drop requirement, but in general do not seal as well as other valves
when closed. Consequently they are not widely used in industrial refrigeration
service. All valves shown in Fig. 11.1 have accessible handles, but in recent
years a strong preference has developed for capped valves when the valve does
not need to be opened and closed often.
Shutoff valves are oriented so that they close against the flow which usually
means that they close against the high pressure. With this orientation the
upstream pressure assists in the opening of the valve. If the valve must open
against high pressure, cases have been reported where the pressure holds the
disc with such force that the stem may pull away from the disc in an attempted
opening. When the valve closes against the flow the highest pressure is kept off
the stem and bonnet when the valve is shut off. Globe valves should be mounted
with the stem horizontal so that any vapor in the line cannot form a pocket at the
valve inlet which would periodically release, causing noise and unsteady flow.
Ball valves have become very popular in the past few years, primarily because
of the low pressure drop that they cause in their completely open position. A
further advantage of ball valves in certain situations is that they are quarterturn
valves so that a quarter turn of the handle permits quick opening or closing
of the valve. An undesirable characteristic of the basic ball valve is that of
trapping liquid within the ball when the valve is shut off. A ball valve in a cold
liquid line traps cold liquid inside the ball when the valve is closed, and this
liquid is likely to warm up when the flow is interrupted. The trapped liquid
expands which could blow out the valve seat or even rupture the valve body.
Two methods1 used most commonly to relieve pressure of trapped liquid in the
ball and prevent damage are upstream-venting and self-relieving seats. In upstream
venting a small hole is drilled through one side of the ball, connecting the upstream
line with the cavity when the valve is in its closed position. This configuration
bypasses the upstream seat, and provides a continuous vent path for cavity pressure.
In the self-relieving seat design, the seats act as internal relief valves to open a

vent path from the valve body cavity to the line. Self-relieving valve seats serve
as normal valve seats unless the pressure within the ball rises to an extreme
level, in which case they permit leakage of a few drops of liquid.
Judgment should be used in whether and what type of valves should be
incorporated in the lines. Even valves that are rarely shut off may be invaluable in
isolating a certain component or even another valve on rare occasions. On the
other hand, extra valves, particularly those placed in vapor lines, may represent a
persistent demand for extra compressor power when ever the system operates.
One estimate2 calculated that a fully open valve in the liquid/vapor line between
the evaporator and the low-pressure receiver causing 7.5-kPa (1.1-psi) pressure

drop could add ,400 to the annual operating غير مجاز مي باشدt of a 2100 kW (600 ton) system.
A fully open ball valve would add only to the annual operating غير مجاز مي باشدt.
Some pressure drop is expected when refrigerant flows through open valves,
and this pressure drop adds to the pressure drop occurring in straight sections of
pipe. Methods for calculating the pressure drop in straight pipes were presented
in Chapter 9, and Table 11.1 provides data for computing the pressure drop in
fittings and valves. Table 11.1 gives the values of the c-terms in the equation
An important observation that can be made from Table 11.1 is the relative
pressure drops of globe and angle valves. An angle valve causes anywhere from
1/2 to 1/8 the pressure drop of a globe valve (depending upon the valve and pipe
size) and should be considered if the physical arrangement permits. The
pressure-drop coefficients for ball valves are likely to be approximately the
same as gate valves.
11.3 MANUAL EXPANSION OR
BALANCING VALVES
Manual regulating valves are designed to adjust the flow rate through their entire
stem travel. Shutoff valves, on the other hand, are not intended for use as regulating
valves since they provide most of their regulation in the first turn of the valve from
its closed position. Two frequent applications of manual expansion or balancing
valves are at the evaporator coils of liquid-recirculation systems and in conjunction
with on-off liquid level control valves, as illustrated schematically in Fig. 11.2. In
the liquid recirculation system of Fig. 11.2a the function of the valves is to throttle
the flow rate to coils whose unthrottled coil-and-piping circuit has a lower pressure
drop than others. The liquid supply pressure ahead of the valves is increased,

EVAPORATORS
AIR COILS
AND LIQUID
CHILLERS

6.1 WHERE THE REFRIGERATION
LOAD ENTERS THE SYSTEM
The evaporator is the component of the refrigeration system where a fluid stream
or a product is cooled. As Fig. 6.1 shows, the evaporator is the interface between
the process and the refrigeration system. With the exception of direct cooling a
product, such as in a plate freezer, most evaporators chill air or such liquids as
water, brine, or antifreeze.
6.2 TYPES OF EVAPORATORS
Two major categories of evaporators used in industrial refrigeration practice
are air coils and liquid chillers. A typical air coil is shown in Fig. 6.2 and several
types of liquid chillers are shown in Fig. 6.3. In the air coil, refrigerant flows
through the tubes and air passes over the outside of the tubes. For effective
heat transfer, fins are fastened to the outside of the tubes and the air flows
between the fins.
The liquid-chilling evaporators in Figs. 6.3a and 6.3b are of the shell-andtube
design, while Figs. 6.3c is a plate-type chiller. In Fig. 6.3a the refrigerant
boils in the shell while the liquid flows through the tubes. In Fig. 6.3b the roles

of the tubes and shell are reversed. The plate-type evaporator in Fig. 6.3c is
growing in market share and is an adaptation of the plate-type heat exchanger
used for many years in the food industry. Some of its popularity is attributable
to its compactness and also that the refrigerant charge is less than in a shelland-
tube evaporator. The reduced charge characteristic is attractive both when
ammonia and the new chlorine-free refrigerants are used. A low-charge ammonia
system is desirable for safety reasons, and a low-charge chlorine-free system
minimizes the غير مجاز مي باشدt of these expensive refrigerants.
This chapter is divided into three parts: Part I, Evaporator Performance,
addresses the principles applicable to all types of evaporators; Part II, Air Coils

PART I. EVAPORATOR PERFORMANCE
6.3 EVAPORATOR HEAT TRANSFER
This chapter is not intended for the engineers of evaporator manufacturers
who must decide on the refrigerant circuiting, tube arrangement, and other
important details. These designers consider the performance, غير مجاز مي باشدt of materials,
and ease of manufacturing in deciding the configuration of the evaporator.
Instead the audience of this chapter is intended to be the user of evaporators
who should understand the basic performance of the evaporator as a heat
exchanger, how to properly select the evaporator from manufacturer’s catalogs,
and how to install, operate, and maintain the evaporator properly.
As a heat exchanger, the evaporator follows the rules of heat transfer. In the
evaporator of Fig. 6.4 heat flows in series through three resistances—the fluid
side, the metal of the tube, and the refrigerant.
Many engineers visualize a heat-flow process as an analogy to the electric
flow process with the correspondence of terms shown in Table 6.1.
Ohm’s law for electricity states that

Application of the electrical analogy provides a simple means of deriving an
expression for the overall U-value, where U is a term which when multiplied by
the overall temperature difference, tf - tr, and the area yields the rate of heat
transfer q, W (Btu/hr

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ELECTRICAL
CONTROL AND
INSTRUMENTATION

14.1 INSTRUMENTATION AND
CONTROL
This chapter concentrates on the electrical equipment associated with the
refrigeration system. It focuses on controlling the electrical supply to the system
and on electrical and/or electronic sensors and actuators. Because the full range
of electrical topics is so vast, a selection has been made, as illustrated in Fig.
14.1, such that the subjects treated in this chapter are those most intimately
connected with the refrigeration system. Those topics not covered or only
mentioned in passing include, for example, electrical circuits (single- and threephase,
wye and delta), conductor sizing, circuit protection, power factors and
their corrections, and transformers. Another important body of knowledge that
is not covered is that of motors—the types of industrial motors and their
characteristics.
The electrical principles that will be explained are those related to controlling
the power to electric motors and other equipment and the electrical types of
instrumentation and actuators. The first major topic presented will be ladder
diagrams, which are the widely accepted means of showing control logic and
are the plans followed by electricians. Means of executing ladder diagrams
include electromechanical relays, programmable controllers, and computer
controllers. Instruments providing visual indications (pressure gauges,
thermometers, etc.) are standard and should continue to be installed. The shift

in recent years to electronic and/or computer control, however, stimulates a
greater need for electric and electronic sensors, transducers, and actuators.
These devices will be explained in the latter sections of this chapter.
14.2 LADDER DIAGRAM SYMBOLS
Ladder diagrams serve two purposes—they represent the plan for hardwiring
a panel of electromechanical devices and they also represent the logic of the
control plan. Logic means the conditions that must be met before a certain
action is taken. The ultimate action of most ladder diagrams is to provide electric
power or to interrupt power to motors and other electrical devices. The symbols1
that are probably the most used in ladder diagrams are shown in Fig. 14.2 for
manual switches, Fig. 14.3 for switches controlled by physical variables and
other conditions, Fig. 14.4 for timing switches, Fig. 14.5 for symbols referring
to the controls for the power portion of the electric system, and Fig. 14.6 for
miscellaneous symbols.
A toggle switch, as shown in Fig. 14.2, retains its position (open or closed)
until manually changed. Push buttons are designed for momentary contact or
interruption. A dashed line indicates a mechanical linkage between two push
buttons, which in Fig. 14.2 shows one push button opening and the other closing
a contact when the button is pressed.
In Fig. 14.3, the switch changes from its normal position when the sensed
variable increases above its setting. For example, the normally closed (NC)
temperature switch set for 40°C (104°F) is closed when the sensed temperature

is below 40°C (104°F) and open when the temperature is above the setting. If a
low-temperature cutout is to open a switch when the temperature drops below
0°C (32°F), for example, a normally open (NO) switch would be chosen and set
for the temperature. During satisfactory operation above 0°C (32°F), the switch
is in its non-normal state (closed).
The symbols for another class of components used in ladder diagrams are
shown in Fig. 14.4 and apply to timing switches. Most timing switches are
single-throw, but double-throw switches are also available, as shown in Figs.
14.4e and 14.4f. One class of timing switches is indicated by the upward-pointing
arrow ↑ and another by a downward-pointing arrow ↓, representing a delay on
and a delay off, respectively. The energizing of the coil initiates the time delay
of a delay-on switch, while the denergizing of the coil initiates the time delay of
a delay-off timing switch.
Figures 14.4a and 14.4c show NC switches, and Figs. 14.4b and 14.4d show
NO switches. The NC, timed-open, delay-on switch of Fig. 14.4a begins the
timing upon energizing of the coil and opens the contacts following the specified
delay. When the coil is deenergized, the contacts immediately return to their
NC status. Should the coil be deenergized during the delay period, the switch
remains closed and the timer is reset to zero. The NO, timed-closed, delay-on
timing switch of Fig. 14.4b begins the timing operation upon energizing of the
coil and closes the switch following the delay. When the coil is deenergized, the
contacts immediately return to their NO status. Should the coil be deenergized
during the delay period, the contacts remain open and the timer is reset to zero.
The down arrow ↓ timer switches in Fig. 14.4c and 14.4d are in the status
shown (normally closed or normally open, respectively) when the coil has been
deenergized for some time. When the coil is energized, the switch changes
instantly to its nonnormal status, at which condition it remains so long as the
coil is energized. When the coil is deenergized, timing begins, and following the
specified delay, the contacts revert to their normal position. If the coil should

be energized during the delay period, the contacts return to their nonnormal
status and the timer resets to zero.
The double-throw timing switch of Fig. 14.4e is a combination of the switches
in Figs. 14.4a and 14.4b. The status of the blade shown occurs when the coil has
been deenergized for a period of time. When the coil is energized, the timer
begins, and following the specified delay, the blade changes from the NC contact
to the NO contact, where it remains so long as the coil is energized.
Deenergization of the coil returns the blade to the NC position instantly. Some
commercial timing switches of this type are supplied with power continuously,
and what is referred to as energizing and deenergizing of the coil is achieved by
closing and opening, respectively, external contacts.
The double-throw, delay-off timing switch of Fig. 14.4f combines the functions
of the switches in Figs. 14.4c and 14.4d in the following manner. The position of
the switch shown is what occurs when the coil has been deenergized for a period

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CHAPTER

CONDENSERS

7.1 TYPES OF CONDENSERS USED
IN INDUSTRIAL REFRIGERATION
The three main types of condensers used in general refrigeration systems are:

air-cooled

water-cooled

evaporative
All of these serve the industrial refrigeration field as well. In comparison to
the air-conditioning industry, however, a lower percentage of air-cooled
condensers and a higher percentage of evaporative condensers are operating in
industrial refrigeration plants. In industrial refrigeration practice, it is common
to connect the evaporative condensers in parallel—a concept not normally used
in air conditioning.
The three types of condensers are shown schematically in Fig. 7.1a, 7.1b,
and 7.1c. The air-cooled condenser in Fig. 7.1a condenses refrigerant vapor by
rejecting heat to ambient air blown over the finned condenser coil with the aid
of a fan, usually a propeller type.
Most all water-cooled condensers (Fig. 7.1b) condense refrigerant in the shell and
on the outside of tubes through which water passes. The condenser cooling water
picks up heat in passing through the condenser and this warm water is cooled by
circulating through a cooling tower (Section 7.6). While the shell-and-tube

construction predominates for water-cooled condensers, plate-type condensers,
sister of the plate-type evaporator explained in Sec. 6.31, are now appearing.
The evaporative condenser of Fig. 7.1c might be considered a cooling tower,
with the condenser tubes washed by the water spray. Ultimately, the heat
rejected from the refrigeration plant is discharged to ambient air, except where
the condenser is cooled by water from a well, lake, or stream.
This chapter first explores the condensing process outside and inside tubes.
Next, the overall performance of water-cooled condensers and the translation
of performance to noncatalog ratings is examined. An explanation of the
performance of cooling towers, the constant companions of water-cooled
condensers, is given. Because of their prevalence in industrial refrigeration
plants, the emphasis of this chapter is on the performance, selection, application,
and operation of evaporative condensers.
7.2 THE CONDENSING PROCESS
Nearly a century ago, heat-transfer pioneer, Willhelm Nusselt, proposed a model
to predict the magnitude of a condensing coefficient for a special geometric
situation1. Nusselt envisioned the condensation of vapor on a cold vertical plate,
Fig. 7.2, as a process where vapor condenses on the plate and the condensate
drains downward, with the condensate film becoming progressively thicker as
it descends. The local condensing coefficient is taken to be the conductance
through the condensate film—the conductivity of the liquid divided by the film
thickness at that point. Nusselt developed the expression for the mean
condensing coefficient as

The immediate question is where, if at all, does condensation occur on a
vertical plate in industrial practice? Actually, a very old condenser design
oriented the tubes vertically and water flowed by gravity down the inside of the
tubes to ease their cleaning. The refrigerant in the shell condensed on the outside
of the vertical tubes.
A slight modification of Eq. 7.1 applies to the widely used horizontal shelland-
tube condenser, Fig. 7.1b. The product of the number of tubes in a vertical

FIGURE 7.2
Condensation of a vapor on a cold vertical surface

row multiplied by the diameter of the tubes replaces the vertical length of the
plane L. White2 found by experimental tests that the coefficient is 0.63 and Goto3
measured 0.65, so the equation for N tubes of diameter D in a vertical row is

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