2010 ASHRAE Handbook - Refrigeration (I-P Edition)

698 Pages·2010·19.137 MB·English

2010 ASHRAE HANDBOOK
(cid:163)
REFRIGERATION
Inch-Pound Edition
American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
1791 Tullie Circle, N.E., Atlanta, GA 30329
[phone] http://www.ashrae.org
©2010 by the American Society of Heating, Refrigerating and Air-Conditioning Engineers,
Inc. All rights reserved.
DEDICATED TO THE ADVANCEMENT OF
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No part of this book may be reproduced without permission in writing from ASHRAE,
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with appropriate credit; nor may any part of this book be reproduced, stored in a retrieval
system, or transmitted in any form or by any means—electronic, photocopying, recording,
or other—without permission in writing from ASHRAE.
Volunteer members of ASHRAE Technical Committees and others compiled the infor-
mation in this handbook, and it is generally reviewed and updated every four years. Com-
ments, criticisms, and suggestions regarding the subject matter are invited. Any errors or
omissions in the data should be brought to the attention of the Editor. Additions and correc-
tions to Handbook volumes in print will be published in the Handbook published the year
following their verification and, as soon as verified, on the ASHRAE Internet Web site.
DISCLAIMER
ASHRAE has compiled this publication with care, but ASHRAE has not investigated,
and ASHRAE expressly disclaims any duty to investigate, any product, service, process,
procedure, design, or the like that may be described herein. The appearance of any technical
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ISBN 978-1-933742-81-6
ISSN 1930-7195
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using sustainable forestry practices.
ASHRAE Research: Improving the Quality of Life
The American Society of Heating, Refrigerating and Air- annually, enabling ASHRAE to report new data about material
Conditioning Engineers is the world’s foremost technical society in properties and building physics and to promote the application of
the fields of heating, ventilation, air conditioning, and refrigeration. innovative technologies.
Its members worldwide are individuals who share ideas, identify Chapters in the ASHRAE Handbook are updated through the
needs, support research, and write the industry’s standards for test- experience of members of ASHRAE Technical Committees and
ing and practice. The result is that engineers are better able to keep through results of ASHRAE Research reported at ASHRAE confer-
indoor environments safe and productive while protecting and pre- ences and published in ASHRAE special publications and in
serving the outdoors for generations to come. ASHRAE Transactions.
One of the ways that ASHRAE supports its members’ and indus- For information about ASHRAE Research or to become a mem-
try’s need for information is through ASHRAE Research. Thou- ber, contact ASHRAE, 1791 Tullie Circle, Atlanta, GA 30329; tele-
sands of individuals and companies support ASHRAE Research phone: [phone]; www.ashrae.org.
Preface
The 2010 ASHRAE Handbook—Refrigeration covers the refrig- plus revised figures on thermostatic expansion valves (TXVs) and
eration equipment and systems for applications other than human several revised examples.
comfort. This book includes information on cooling, freezing, and • Chapter 12, Lubricants in Refrigerant Systems, has new content
storing food; industrial applications of refrigeration; and low-tem- on pressure/viscosity coefficients, compressibility factors, and
perature refrigeration. Primarily a reference for the practicing engi- lubricants’ effects on system performance.
neer, this volume is also useful for anyone involved in cooling and • Chapter 17, Household Refrigerators and Freezers, has been reor-
storage of food products. ganized and updated for revised standards and new component
An accompanying CD-ROM contains all the volume’s chapters technologies, including variable-speed and linear compressors,
in both I-P and SI units. and has information on new configurations and functions, such as
This edition includes two new chapters: wine cooling units, rapid-chill/freeze/thaw, and odor elimination.
The section on performance evaluation has been revised and inte-
• Chapter 3, Carbon Dioxide Refrigeration Systems, describes the
grated with the section on standards.
history of this “natural refrigerant” and why it is the subject of
• Chapter 25, Cargo Containers, Rail Cars, Trailers, and Trucks, has
renewed interest today. The chapter contains discussion and dia-
been updated with information on multitemperature compart-
grams on CO refrigerant applications, system design, equip-
2 ments and air curtains.
ment, safety, lubricants, commissioning, operation, and
• Chapter 38, Fruit Juice Concentrates and Chilled Juice Products,
maintenance.
has added description of storage tank sterilization.
• Chapter 50, Terminology of Refrigeration, lists some of the com-
• Chapter 44, Ice Rinks, has extensive changes to the section on
mon terms used in industrial refrigeration systems, particularly
heat recovery and updated loads information based on ASHRAE
systems using ammonia as the refrigerant.
research project RP-1289.
Also new for this volume, chapter titles, order, and groupings
This volume is published, both as a bound print volume and in
have been revised for more logical flow and use. Some of the other
electronic format on a CD-ROM, in two editions: one using inch-
revisions and additions are as follows:
pound (I-P) units of measurement, the other using the International
• Chapter 2, Ammonia Refrigeration Systems, has added guidance System of Units (SI).
on avoiding hydraulic shock, on purging water and noncondens- Corrections to the 2007, 2008, and 2009 Handbook volumes can
ables, as well as on hot-gas defrost and defrost control. be found on the ASHRAE Web site at http://www.ashrae.org and in
• Chapter 6, Refrigerant System Chemistry, has added information the Additions and Corrections section of this volume. Corrections
on polyvinyl ether (PVE) lubricants and corrosion, plus updates for this volume will be listed in subsequent volumes and on the
for recent ASHRAE research on copper plating and material com- ASHRAE Web site.
patibility. Reader comments are enthusiastically invited. To suggest
• Chapter 8, Equipment and System Dehydrating, Charging, and improvements for a chapter, please comment using the form on
Testing, has new table data on dehydration and moisture-measuring the ASHRAE Web site or, using the cutout pages at the end of this
methods and a revised section on performance testing. volume’s index, write to Handbook Editor, ASHRAE, 1791 Tullie
• Chapter 9, Refrigerant Containment, Recovery, Recycling, and Circle, Atlanta, GA 30329, or fax [phone], or e-mail
Reclamation, has added a new table comparing sensitivities of [email protected].
various leak-detection methods and a procedure for receiver level
monitoring.
• Chapter 11, Refrigerant-Control Devices, has updated informa- Mark S. Owen
tion on electric expansion valves and discharge bypass valves, Editor
CONTENTS
Contributors vii
ASHRAE Technical Committees, Task Groups, and Technical Resource Groups ix
ASHRAE Research: Improving the Quality of Life x
Preface x
SYSTEMS AND PRACTICES
Chapter 1. Halocarbon Refrigeration Systems (TC 10.3, Refrigerant Piping) 1.1
2. Ammonia Refrigeration Systems (TC 10.3) 2.1
3. Carbon Dioxide Refrigeration Systems (TC 10.3) 3.1
4. Liquid Overfeed Systems (TC 10.1, Custom-Engineered Refrigeration Systems) 4.1
5. Component Balancing in Refrigeration Systems (TC 10.1) 5.1
6. Refrigerant System Chemistry (TC 3.2, Refrigerant System Chemistry) 6.1
7. Control of Moisture and Other Contaminants in Refrigerant Systems (TC 3.3, Refrigerant
Contaminant Control) 7.1
8. Equipment and System Dehydrating, Charging, and Testing (TC 8.1, Positive-Displacement
Compressors) 8.1
9. Refrigerant Containment, Recovery, Recycling, and Reclamation (TC 8.3, Refrigerant
Containment) 9.1
COMPONENTS AND EQUIPMENT
Chapter 10. Insulation Systems for Refrigerant Piping (TC 10.3) 10.1
11. Refrigerant-Control Devices (TC 8.8, Refrigerant System Controls and Accessories) 11.1
12. Lubricants in Refrigerant Systems (TC 3.4, Lubrication) 12.1
13. Secondary Coolants in Refrigeration Systems (TC 10.1) 13.1
14. Forced-Circulation Air Coolers (TC 8.4, Air-to-Refrigerant Heat Transfer Equipment) 14.1
15. Retail Food Store Refrigeration and Equipment (TC 10.7, Commercial Food and Beverage
Cooling, Display, and Storage) 15.1
16. Food Service and General Commercial Refrigeration Equipment (TC 10.7) 16.1
17. Household Refrigerators and Freezers (TC 8.9, Residential Refrigerators and Food Freezers) 17.1
18. Absorption Equipment (TC 8.3, Absorption and Heat-Operated Machines) 18.1
FOOD COOLING AND STORAGEs
Chapter 19. Thermal Properties of Foods (TC 10.9, Refrigeration Applications for Foods and Beverages) 19.1
20. Cooling and Freezing Times of Foods (TC 10.9) 20.1
21. Commodity Storage Requirements (TC 10.5, Refrigerated Distribution and Storage Facilities) 21.1
22. Food Microbiology and Refrigeration (TC 10.9) 22.1
23. Refrigerated-Facility Design (TC 10.5) 23.1
24. Refrigerated-Facility Loads (TC 10.8, Refrigeration Load Calculations) 24.1
REFRIGERATED TRANSPORT
Chapter 25. Cargo Containers, Rail Cars, Trailers, and Trucks (TC 10.6, Transport Refrigeration) 25.1
26. Marine Refrigeration (TC 10.6) 26.1
27. Air Transport (TC 10.6) 27.1
FOOD, BEVERAGE, AND FLORAL APPLICATIONS
Chapter 28. Methods of Precooling Fruits, Vegetables, and Cut Flowers (TC 10.9) 28.1
29. Industrial Food-Freezing Systems (TC 10.9) 29.1
30. Meat Products (TC 10.9) 30.1
31. Poultry Products (TC 10.9) 31.1
32. Fishery Products (TC 10.9) 32.1
33. Dairy Products (TC 10.9) 33.1
34. Eggs and Egg Products (TC 10.9) 34.1
35. Deciduous Tree and Vine Fruit (TC 10.9) 35.1
36. Citrus Fruit, Bananas, and Subtropical Fruit (TC 10.9) 36.1
37. Vegetables (TC 10.9) 37.1
38. Fruit Juice Concentrates and Chilled Juice Products (TC 10.9) 38.1
39. Beverages (TC 10.9) 39.1
40. Processed, Precooked, and Prepared Foods (TC 10.9) 40.1
41. Bakery Products (TC 10.9) 41.1
42. Chocolates, Candies, Nuts, Dried Fruits, and Dried Vegetables (TC 10.9) 42.1
INDUSTRIAL APPLICATIONS
Chapter 43. Ice Manufacture (TC 10.2, Automatic Icemaking Plants and Skating Rinks) 43.1
44. Ice Rinks (TC 10.2) 44.1
45. Concrete Dams and Subsurface Soils (TC 10.1) 45.1
46. Refrigeration in the Chemical Industry (TC 10.1) 46.1
LOW-TEMPERATURE APPLICATIONS
Chapter 47. Cryogenics (TC 10.4, Ultralow-Temperature Systems and Cryogenics) 47.1
48. Ultralow-Temperature Refrigeration (TC 10.4) 48.1
49. Biomedical Applications of Cryogenic Refrigeration (TC 10.4) 49.1
GENERAL
Chapter 50. Terminology of Refrigeration (TC 10.1) 50.1
51. Codes and Standards 51.1
Additions and Corrections
Index I.1
Composite index to the 2007 HVAC Applications, 2008 HVAC Systems and Equipment, 2009
Fundamentals, and 2010 Refrigeration volumes
Comment Pages
CHAPTER 2
AMMONIA REFRIGERATION SYSTEMS
System Selection......................................................................... 2.1 Screw Compressors.................................................................. 2.12
Equipment.................................................................................. 2.2 Condenser and Receiver Piping............................................... 2.14
Controls...................................................................................... 2.6 Evaporative Condensers.......................................................... 2.15
Piping......................................................................................... 2.7 Evaporator Piping.................................................................... 2.17
Reciprocating Compressors..................................................... 2.10 Multistage Systems................................................................... 2.20
Rotary Vane, Low-Stage Liquid Recirculation Systems................................................... 2.21
Compressors......................................................................... 2.12 Safety Considerations............................................................... 2.25
C
USTOM-ENGINEERED ammonia (R-717) refrigeration sys- reduces its enthalpy, resulting in a higher net refrigerating effect.
tems often have design conditions that span a wide range of Economizing is beneficial because the vapor generated during sub-
evaporating and condensing temperatures. Examples are (1) a food cooling is injected into the compressor partway through its com-
freezing plant operating from +50 to –50°F; (2) a candy storage pression cycle and must be compressed only from the economizer
requiring 60°F db with precise humidity control; (3) a beef chill port pressure (which is higher than suction pressure) to the dis-
room at 28 to 30°F with high humidity; (4) a distribution warehouse charge pressure. This produces additional refrigerating capacity
requiring multiple temperatures for storing ice cream, frozen food, with less increase in unit energy input. Economizing is most bene-
meat, and produce and for docks; and (5) a chemical process requir- ficial at high pressure ratios. Under most conditions, economizing
ing multiple temperatures ranging from +60 to –60°F. Ammonia is can provide operating efficiencies that approach that of two-stage
the refrigerant of choice for many industrial refrigeration systems. systems, but with much less complexity and simpler maintenance.
The figures in this chapter are for illustrative purposes only, and Economized systems for variable loads should be selected care-
may not show all the required elements (e.g., valves). For safety fully. At approximately 75% capacity, most screw compressors
and minimum design criteria for ammonia systems, refer to revert to single-stage performance as the slide valve moves such that
ASHRAE Standard 15, IIAR Bulletin 109, IIAR Standard 2, and the economizer port is open to the compressor suction area.
applicable state and local codes. A flash economizer, which is somewhat more efficient, may
See Chapter 24 for information on refrigeration load calculations. often be used instead of the shell-and-coil economizer (Figure 1).
However, ammonia liquid delivery pressure is reduced to econo-
Ammonia Refrigerant for HVAC Systems
mizer pressure. Additionally, the liquid is saturated at the lower
There is renewed interest in using ammonia for HVAC systems pressure and subject to flashing with any pressure drop unless
has received renewed interest, in part because of the scheduled phase- another means of subcooling is incorporated.
out and increasing costs of chlorofluorocarbon (CFC) and hydrochlo-
rofluorocarbon (HCFC) refrigerants. Ammonia secondary systems Multistage Systems
that circulate chilled water or another secondary refrigerant are a vi- Multistage systems compress gas from the evaporator to the
able alternative to halocarbon systems, although ammonia is inappro- condenser in several stages. They are used to produce temperatures
priate for direct refrigeration systems (ammonia in the air unit coils) of –15°F and below. This is not economical with single-stage com-
for HVAC applications. Ammonia packaged chilling units are avail- pression.
able for HVAC applications. As with the installation of any air-con- Single-stage reciprocating compression systems are generally
ditioning unit, all applicable codes, standards, and insurance limited to between 5 and 10 psig suction pressure. With lubricant-
requirements must be followed. injected economized rotary screw compressors, where the discharge
temperatures are lower because of the lubricant cooling, the low-
SYSTEM SELECTION suction temperature limit is about –40° F, but efficiency is very low.
Two-stage systems are used down to about –70 or –80°F evaporator
In selecting an engineered ammonia refrigeration system, sev-
temperatures. Below this temperature, three-stage systems should
eral design decisions must be considered, including whether to use
be considered.
(1) single-stage compression, (2) economized compression, (3)
multistage compression, (4) direct-expansion feed, (5) flooded
feed, (6) liquid recirculation feed, and (7) secondary coolants.
Single-Stage Systems Fig. 1 Shell-and-Coil Economizer Arrangement
The basic single-stage system consists of evaporator(s), a com-
pressor, a condenser, a refrigerant receiver (if used), and a refriger-
ant control device (expansion valve, float, etc.). Chapter 2 of the
2009ASHRAE Handbook—Fundamentals discusses the compres-
sion refrigeration cycle.
Economized Systems
Economized systems are frequently used with rotary screw com-
pressors. Figure 1 shows an arrangement of the basic components.
Subcooling the liquid refrigerant before it reaches the evaporator
The preparation of this chapter is assigned to TC 10.3, Refrigerant Piping. Fig. 1 Shell-and-Coil Economizer Arrangement
2.1
2.2 2010 ASHRAE Handbook—Refrigeration
power requirements for 100% load. The compressor’s unloading
Fig. 2 Two-Stage System with High- and
mechanism can be used to prevent motor overload. Electric motors
Low-Temperature Loads
should not be overloaded, even when a service factor is indicated.
For screw compressor applications, motors should be sized by add-
ing 10% to the operating power. Screw compressors have built-in
unloading mechanisms to prevent motor overload. The motor
should not be oversized, because an oversized motor has a lower
power factor and lower efficiency at design and reduced loads.
Steam turbines or gasoline, natural gas, propane, or diesel inter-
nal combustion engines are used when electricity is unavailable, or
if the selected energy source is cheaper. Sometimes they are used in
combination with electricity to reduce peak demands. The power
output of a given engine size can vary as much as 15% depending on
Fig. 2 Two-Stage System with High- and the fuel selected.
Low-Temperature Loads Steam turbine drives for refrigerant compressors are usually lim-
ited to very large installations where steam is already available at
moderate to high pressure. In all cases, torsional analysis is required
Two-stage systems consist of one or more compressors that oper-
to determine what coupling must be used to dampen out any pulsa-
ate at low suction pressure and discharge at intermediate pressure
tions transmitted from the compressor. For optimum efficiency, a
and have one or more compressors that operate at intermediate pres-
turbine should operate at a high speed that must be geared down for
sure and discharge to the condenser (Figure 2).
reciprocating and possibly screw compressors. Neither the gear
Where either single- or two-stage compression systems can be
reducer nor the turbine can tolerate a pulsating backlash from the
used, two-stage systems require less power and have lower operat-
driven end, so torsional analysis and special couplings are essential.
ing costs, but they can have a higher initial equipment cost.
Advantages of turbines include variable speed for capacity con-
trol and low operating and maintenance costs. Disadvantages
EQUIPMENT
include higher initial costs and possible high noise levels. The tur-
bine must be started manually to bring the turbine housing up to
Compressors
temperature slowly and to prevent excess condensate from entering
Compressors available for single- and multistage applications in- the turbine.
clude the following:
The standard power rating of an engine is the absolute maximum,
• Reciprocating not the recommended power available for continuous use. Also,
Single-stage (low-stage or high-stage) torque characteristics of internal combustion engines and electric
Internally compounded motors differ greatly. The proper engine selection is at 75% of its
maximum power rating. For longer life, the full-load speed should
• Rotary vane
be at least 10% below maximum engine speed.
• Rotary screw (low-stage or high-stage, with or without
Internal combustion engines, in some cases, can reduce operating
economizing)
cost below that for electric motors. Disadvantages include (1) higher
The reciprocating compressor is the most common compressor initial cost of the engine, (2) additional safety and starting controls,
used in small, 100 hp or less, single-stage or multistage systems. The (3) higher noise levels, (4) larger space requirements, (5) air pollu-
screw compressor is the predominant compressor above 100 hp, in tion, (6) requirement for heat dissipation, (7) higher maintenance
both single- and multistage systems. Various combinations of com- costs, and (8) higher levels of vibration than with electric motors. A
pressors may be used in multistage systems. Rotary vane and screw torsional analysis must be made to determine the proper coupling if
compressors are frequently used for the low-pressure stage, where engine drives are chosen.
large volumes of gas must be moved. The high-pressure stage may be
a reciprocating or screw compressor. Condensers
When selecting a compressor, consider the following: Condensers should be selected on the basis of total heat rejection
at maximum load. Often, the heat rejected at the start of pulldown is
• System size and capacity requirements.
several times the amount rejected at normal, low-temperature oper-
• Location, such as indoor or outdoor installation at ground level or
ating conditions. Some means, such as compressor unloading, can
on the roof.
be used to limit the maximum amount of heat rejected during pull-
• Equipment noise.
down. If the condenser is not sized for pulldown conditions, and
• Part- or full-load operation.
compressor capacity cannot be limited during this period, condens-
• Winter and summer operation.
ing pressure might increase enough to shut down the system.
• Pulldown time required to reduce the temperature to desired con-
ditions for either initial or normal operation. The temperature Evaporators
must be pulled down frequently for some applications for a pro-
Several types of evaporators are used in ammonia refrigeration
cess load, whereas a large cold-storage warehouse may require
systems. Fan-coil, direct-expansion evaporators can be used, but they
pulldown only once in its lifetime.
are not generally recommended unless the suction temperature is
Lubricant Cooling. When a reciprocating compressor requires 0°F or higher. This is due in part to the relative inefficiency of the
lubricant cooling, an external heat exchanger using a refrigerant or direct-expansion coil, but more importantly, the low mass flow rate
secondary cooling is usually added. Screw compressor lubricant of ammonia is difficult to feed uniformly as a liquid to the coil.
cooling is covered in detail in the section on Screw Compressors. Instead, ammonia fan-coil units designed for recirculation (overfeed)
Compressor Drives. The correct electric motor size(s) for a systems are preferred. Typically, in this type of system, high-pressure
multistage system is determined by pulldown load. When the final ammonia from the system high stage flashes into a large vessel at the
low-stage operating level is –100°F, the pulldown load can be three evaporator pressure, from which it is pumped to the evaporators at an
times the operating load. Positive-displacement reciprocating com- overfeed rate of 2.5 to 1 to 4 to 1. This type of system is standard and
pressor motors are usually selected for about 150% of operating very efficient. See Chapter 4 for more details.
Ammonia Refrigeration Systems 2.3
Flooded shell-and-tube evaporators are often used in ammonia discharge gas as it enters above the liquid level. Heat removed from
systems in which indirect or secondary cooling fluids such as water, the discharge gas is absorbed by evaporating part of the liquid and
brine, or glycol must be cooled. eventually passes through the high-stage compressor to the con-
Some problems that can become more acute at low temperatures denser. Disbursing the discharge gas below a level of liquid refrig-
include changes in lubricant transport properties, loss of capacity erant separates out any lubricant carryover from the low-stage
caused by static head from the depth of the pool of liquid refrigerant compressor. If liquid in the intercooler is to be used for other pur-
in the evaporator, deterioration of refrigerant boiling heat transfer poses, such as liquid makeup or feed to the low stage, periodic lubri-
coefficients caused by lubricant logging, and higher specific vol- cant removal is imperative.
umes for the vapor. Another purpose of the intercooler is to lower the liquid temper-
The effect of pressure losses in the evaporator and suction piping ature used in the low stage of a two-stage system. Lowering refrig-
is more acute in low-temperature systems because of the large erant temperature in the intercooler with high-stage compressors
change in saturation temperatures and specific volume in relation to increases the refrigeration effect and reduces the low-stage compres-
pressure changes at these conditions. Systems that operate near or sor’s required displacement, thus reducing its operating cost.
below zero gage pressure are particularly affected by pressure loss. Intercoolers for two-stage compression systems can be shell-
The depth of the pool of boiling refrigerant in a flooded evapo- and-coil or flash. Figure 3 depicts a shell-and-coil intercooler incor-
rator exerts a liquid pressure on the lower part of the heat transfer porating an internal pipe coil for subcooling high-pressure liquid
surface. Therefore, the saturation temperature at this surface is before it is fed to the low stage of the system. Typically, the coil sub-
higher than that in the suction line, which is not affected by the liq- cools liquid to within 10°F of the intermediate temperature.
uid pressure. This temperature gradient must be considered when Vertical shell-and-coil intercoolers perform well in many appli-
designing the evaporator. cations using ammonia refrigerant systems. Horizontal designs are
Spray shell-and-tube evaporators, though not commonly used, possible but usually not practical. The vessel must be sized properly
offer certain advantages. In this design, the evaporator’s liquid depth to separate liquid from vapor that is returning to the high-stage com-
penalty can be eliminated because the pool of liquid is below the pressor. The superheated gas inlet pipe should extend below the liq-
heat transfer surface. A refrigerant pump sprays liquid over the sur- uid level and have perforations or slots to distribute the gas evenly
face. Pump energy is an additional heat load to the system, and more in small bubbles. Adding a perforated baffle across the area of the
refrigerant must be used to provide the net positive suction head vessel slightly below the liquid level protects against violent surg-
(NPSH) required by the pump. The pump is also an additional item ing. A float switch that shuts down the high-stage compressor when
that must be maintained. This evaporator design also reduces the the liquid level gets too high should always be used. A means of
refrigerant charge requirement compared to a flooded design (see maintaining a liquid level for the subcooling coil and low-stage
Chapter 4). compressor desuperheating is necessary if no high-stage evaporator
overfeed liquid is present. Electronic level controls (see Figure 10)
Vessels can simplify the use of multiple float switches and float valves to
High-Pressure Receivers. Industrial systems generally incorpo- maintain the various levels required.
rate a central high-pressure refrigerant receiver, which serves as the The flash intercooler is similar in design to the shell-and-coil
primary refrigerant storage location in the system. It handles refrig- intercooler, except for the coil. The high-pressure liquid is flash-
erant volume variations between the condenser and the system’s low cooled to the intermediate temperature. Use caution in selecting a
side during operation and pumpdowns for repairs or defrost. Ideally, flash intercooler because all the high-pressure liquid is flashed to
the receiver should be large enough to hold the entire system charge, intermediate pressure. Though colder than that of the shell-and-coil
but this is not generally economical. The system should be analyzed intercooler, liquid in the flash intercooler is not subcooled and is
to determine the optimum receiver size. Receivers are commonly susceptible to flashing from system pressure drop. Two-phase liquid
equalized to the condenser inlet and operate at the same pressure as feed to control valves may cause premature failure because of the
the condenser. In some systems, the receiver is operated at a pres- wire-drawing effect of the liquid/vapor mixture.
sure between the condensing pressure and the highest suction pres-
sure to allow for variations in condensing pressure without affecting Fig. 3 Intercooler
the system’s feed pressure. This type is commonly referred to as a
controlled-pressure receiver (CPR). Liquid from the condenser is
metered through a high-side control as it is condensed. CPR pres-
sure is maintained with a back-pressure regulator vented to an inter-
mediate pressure point. Winter or low-load operating conditions
may require a downstream pressure regulator to maintain a mini-
mum pressure.
If additional receiver capacity is needed for normal operation,
use extreme caution in the design. Designers usually remove the in-
adequate receiver and replace it with a larger one rather than install
an additional receiver in parallel. This procedure is best because
even slight differences in piping pressure or temperature can cause
the refrigerant to migrate to one receiver and not to the other.
Smaller auxiliary receivers can be incorporated to serve as
sources of high-pressure liquid for compressor injection or thermosi-
phon, lubricant cooling, high-temperature evaporators, and so forth.
Intercoolers (Gas and Liquid). An intercooler (subcooler/
desuperheater) is the intermediate vessel between the high and low
stages in a multistage system. One purpose is to cool discharge gas
of the low-stage compressor to prevent overheating the high-stage
compressor. This can be done by bubbling discharge gas from the
low-stage compressor through a bath of liquid refrigerant or by
mixing liquid normally entering the intermediate vessel with the Fig. 3 Intercooler
2.4 2010 ASHRAE Handbook—Refrigeration
Fig. 4 Arrangement for Compound System with Vertical Intercooler and Suction Trap
Fig. 4 Arrangement for Compound System with Vertical Intercooler and Suction Trap
Figure 4 shows a vertical shell-and-coil intercooler as piped into
Fig. 5 Suction Accumulator with Warm Liquid Coil
the system. The liquid level is maintained in the intercooler by a
float that controls the solenoid valve feeding liquid into the shell
side of the intercooler. Gas from the first-stage compressor enters
the lower section of the intercooler, is distributed by a perforated
plate, and is then cooled to the saturation temperature correspond-
ing to intermediate pressure.
When sizing any intercooler, the designer must consider (1) low-
stage compressor capacity; (2) vapor desuperheating, liquid make-
up requirements for the subcooling coil load, or vapor cooling load
associated with the flash intercooler; and (3) any high-stage side
loading. The volume required for normal liquid levels, liquid surg-
ing from high-stage evaporators, feed valve malfunctions, and liq-
uid/vapor must also be analyzed.
Necessary accessories are the liquid level control device and
high-level float switch. Though not absolutely necessary, an auxil-
iary oil pot should also be considered.
Suction Accumulator. A suction accumulator (also known as a
knockout drum, suction trap, pump receiver, recirculator, etc.) pre-
vents liquid from entering the suction of the compressor, whether on
the high or low stage of the system. Both vertical and horizontal ves-
sels can be incorporated. Baffling and mist eliminator pads can
enhance liquid separation.
Suction accumulators, especially those not intentionally main-
taining a level of liquid, should have a way to remove any build-up of
ammonia liquid. Gas boil-out coils or electric heating elements are Fig. 5 Suction Accumulator with Warm Liquid Coil
costly and inefficient.
Although it is one of the more common and simplest means of trap can start and stop the liquid ammonia pump, sound an alarm in
liquid removal, a liquid boil-out coil (Figure 5) has some draw- case of excess liquid, and sometimes stop the compressors.
backs. Generally, warm liquid flowing through the coil is the source When the liquid level in the suction trap reaches the setting of
of liquid being boiled off. Liquid transfer pumps, gas-powered the middle float switch, the liquid ammonia pump starts and re-
transfer systems, or basic pressure differentials are a more positive duces the liquid level to the setting of the lower float switch, which
means of removing the liquid (Figures 6 and 7). stops the liquid ammonia pump. A check valve in the discharge line
Accessories should include a high-level float switch for com- of the ammonia pump prevents gas and liquid from flowing back-
pressor protection along with additional pump or transfer system ward through the pump when it is not in operation. Depending on
controls. the type of check valve used, some installations have two valves in
Vertical Suction Trap and Pump. Figure 8 shows the piping of a series as an extra precaution against pump backspin.
a vertical suction trap that uses a high-head ammonia pump to trans- Compressor controls adequately designed for starting, stopping,
fer liquid from the system’s low-pressure side to the high-pressure and capacity reduction result in minimal agitation, which helps sep-
receiver. Float switches piped on a float column on the side of the arate vapor and liquid in the suction trap. Increasing compressor
Ammonia Refrigeration Systems 2.5
Fig. 6 Equalized Pressure Pump Transfer System
Fig. 8 Piping for Vertical Suction Trap and High-Head Pump
Fig. 6 Equalized Pressure Pump Transfer System
Fig. 7 Gravity Transfer System
Fig. 8 Piping for Vertical Suction Trap and High-
Head Pump
Fig. 9 Gage Glass Assembly for Ammonia
Fig. 7 Gravity Transfer System
capacity slowly and in small increments reduces liquid boiling in the
trap, which is caused by the refrigeration load of cooling the refrig-
erant and metal mass of the trap. If another compressor is started
when plant suction pressure increases, it should be brought on line
slowly to prevent a sudden pressure change in the suction trap.
A high level of liquid in a suction trap should activate an alarm or
stop the compressors. Although eliminating the cause is the most
effective way to reduce a high level of excess surging liquid, a more
immediate solution is to stop part of the compression system and Fig. 9 Gage Glass Assembly for Ammonia
raise plant suction pressure slightly. Continuing high levels indicate
insufficient pump capacity or suction trap volume.
Liquid Level Indicators. Liquid level can be indicated by visual Fig. 10 Electronic Liquid Level Control
indicators, electronic sensors, or a combination of the two. Visual indi-
cators include individual circular reflex level indicators (bull’s-eyes)
mounted on a pipe column or stand-alone linear reflex glass assemblies
(Figure 9). For operation at temperatures below the frost point, transpar-
ent plastic frost shields covering the reflex surfaces are necessary. Also,
the pipe column must be insulated, especially when control devices are
attached to prevent false level readings caused by heat influx.
Electronic level sensors can continuously monitor liquid level.
Digital or graphic displays of liquid level can be locally or remotely
monitored (Figure 10).
Level indicators should have adequate isolation valves. High-
temperature glass tube indicators should incorporate stop check or
Fig. 10 Electronic Liquid Level Control
excess-flow valves for isolation and safety.