Tải bản đầy đủ
Fig. 22 External Liquid-to-Suction Heat Exchanger(Walker 1992)

Fig. 22 External Liquid-to-Suction Heat Exchanger(Walker 1992)

Tải bản đầy đủ

This file is licensed to Abdual Hadi Nema (ahaddi58@yahoo.com). License Date: 6/1/2010

15.12
meat storage cooler, and the second compartment is refrigerated at
7 to 13°C and used as a cutting and packaging room. Best results are
attained when meat is cut and wrapped to minimize exposure to
temperatures above –2 to 0°C.

Licensed for single user. © 2010 ASHRAE, Inc.

Wrapped Meat Storage
At some point between the wrapping room and display refrigerator, refrigerated storage for the wrapped cuts of meat must be provided. Without this space, a balance cannot be maintained between
the cutting/packaging rate and the selling rate for each particular cut
of meat. Display refrigerators with refrigerated bottom storage compartments, equipped with racks for holding trays of meats, offer one
solution to this problem. However, the amount of stored meat is not
visible, and the inventory cannot be controlled at a glance.
A second option is a pass-through, reach-in cabinet. This cabinet
has both front and rear insulated glass doors and is located between
the wrapping room and the display refrigerators. After wrapping, the
meats are passed into the cabinet for temporary storage at –2 to 0°C
and then are withdrawn from the other side for restocking the display
refrigerator. Because these pass-through cabinets have glass doors,
the inventory of wrapped meats is visible and therefore controllable.
The third and most common option involves a section of the back
room walk-in meat storage cooler or a completely separate packaged meat storage cooler. The cooler is usually equipped with rolling racks holding slide-in trays of meat. This method also offers
visible inventory control and provides convenient access to both the
wrapping room and the display refrigerators.
The overriding philosophy in successful meat wrapping and merchandising can be summarized thus: keep it clean, keep it cold, and
keep it moving.

Walk-In Coolers and Freezers
Each category of displayed food product that requires refrigeration for preservation is usually backed up by storage in the back
room. This storage usually consists of refrigerated rooms with sectional walls and ceilings equipped with the necessary storage racks
for a particular food product. Walk-in coolers are required for storage of meat, some fresh produce, dairy products, frozen food, and
ice cream. Medium and large stores have separate produce and dairy
coolers, usually in the 2 to 4°C range. Meat coolers are used in all
food stores, with storage conditions between –2 and 0°C. Unwrapped meat, fish, and poultry should each be stored in separate
coolers to prevent odor transfer. Walk-in coolers, which serve the
dual purpose of storage and display, are equipped with either sliding
or hinged glass doors on the front. These door sections are often
prefabricated and set into an opening in the front of the cooler. In
computing refrigeration load, allow for the extra service load.
Moisture conditions must be confined to a relatively narrow
range because excessive humidity encourages bacteria and mold
growth, which leads to sliming. Too little moisture leads to excessive dehydration.
Air circulation must be maintained at all times to prevent stagnation, but it should not be so rapid as to cause drying of an unwrapped
product. Forced-air blasts must not be permitted to strike products;
therefore, low-velocity coils are recommended.
For optimum humidity control, unit coolers should be selected at
about a 6 K TD between entering air temperature and evaporator
temperature. Note that the published ratings of commercial unit
coolers do not reflect the effect of frost accumulation on the evaporator. The unit cooler manufacturer can determine the correct frost
derating factor for its published capacity ratings. From experience,
a minimum correction multiplier of 0.80 is typical.
A low-temperature storage capacity equivalent to the total volume of the low-temperature display equipment in the store is satisfactory. Storage capacity requirements can be reduced by frequent
deliveries.

2010 ASHRAE Handbook—Refrigeration (SI)
Generally, forced-air coils are selected for low-temperature coolers where humidity is not critical for packaged products. For lowtemperature coolers, gas or electric defrost is required. Off-cycle
defrosts are used in produce and dairy coolers. Straight time or timeinitiated, time- or temperature-terminated gas or electric defrosts
are generally used for meat coolers. For more details, see the section
on Walk-In Coolers/Freezers in Chapter 16.

REFRIGERATION SYSTEMS
Food stores sell all types of perishable foods and require a variety of refrigeration systems to best preserve and most effectively
display each product. Moreover, the refrigerating system must be
highly reliable because it must operate 24 h per day for 10 or more
years, to protect the large investment in highly perishable foods.
Temperature controls vary greatly, from a produce preparation
room (which may operate with a wet coil) requiring no defrost to
the ice cream refrigerator requiring induced heat to defrost the coil
periodically.

Design Considerations
When selecting refrigeration equipment to operate display refrigerators and storage rooms for food stores, consider (1) cost/space
limitations, (2) reliability, (3) maintainability and complexity, and
(4) operating efficiency. Solutions span from the very simple (one
compressor and associated controls on one refrigerator) to the complex (central refrigeration plant operating all refrigerators in a store).
Suction Groups. Various refrigerators have different evaporator
pressure/temperature requirements. Produce and meat wrapping
rooms, which have the highest requirements, may approach the suction pressures used in air-conditioning applications. Open ice cream
display refrigerators, which have the lowest, may have suction pressures corresponding to temperatures as low as –40°C. All other
refrigerators and coolers fall between these extremes.
Refrigeration Loads. Refrigerator requirements are often given
as refrigeration load per unit length, with a lower value sometimes
allowed for more complex parallel systems. The rationale for this
lower value is that peak loads are smaller with programmed defrost,
making refrigerator temperature recovery after defrost less of a
strain than on a single-compressor system.
Published refrigerator load requirements allow for extra capacity
for temperature pulldown after defrost, per ASHRAE Standard 72.
The industry considers a standard store ambient condition to be
24°C and 55% rh, which should be maintained with air conditioning. A portion of this air-conditioning load is carried by the open
refrigerators, and credit for heat removed by them should be considered in sizing the air-conditioning system.
Equipment Selection. The designer matches the load requirements of the refrigerator lineups to the capacity of the chosen refrigeration system. Manufacturers publish load ratings to help match
the proper refrigeration system with the fixture loads. For singlecompressor applications only, the ratings can be stated (for selection
convenience) as the capacity the condensing unit must deliver at an
arbitrary suction pressure (evaporator temperature). In general,
manufacturers of display refrigerators use ASHRAE Standard 72,
which specify standard methods of testing open and closed refrigerators for food stores. These standards establish refrigeration load
requirements at rated ambient conditions of 24°C and 55% rh in the
sales area with specific door-opening patterns. Display refrigerators
for similar applications are commercially available from many manufacturers. Manufacturers’ recommendations must be followed to
achieve proper results in both efficiency and product integrity.
Appropriate equipment selection depends on a number of factors.
Life-Cycle Cost. The total cost elements of the refrigeration system include not only the purchase price but also the operating cost
(energy), cost of installation and commissioning, cost of maintenance and service, and the environmental cost.

This file is licensed to Abdual Hadi Nema (ahaddi58@yahoo.com). License Date: 6/1/2010

Licensed for single user. © 2010 ASHRAE, Inc.

Retail Food Store Refrigeration and Equipment
Space Limitations. Store size, location, and price per square
metre play a role in determining the type and location of equipment.
Locations can include an equipment room at the back of the store,
on a mezzanine, in a machine house on the roof, or distributed
throughout or on top of the store.
Refrigerant Selection. Selection of a suitable refrigerant for
food stores has been affected by international concern about the
ozone-depleting effect of chlorine-containing refrigerants. International treaties no longer allow developed nations to manufacture
equipment that uses chlorofluorocarbon refrigerants.
Hydrochlorofluorocarbon refrigerants, such as R-22, are still
popular while their prices remain low and availability is assured for
a reasonable time, although their consumption and production are
scheduled to be phased out entirely by 2030. Current hydrofluorocarbon alternatives in the United States include R-404A, R-134a,
and R-507. Other refrigerants are listed in ASHRAE Standard 34.
Secondary loop systems are covered in the section on Low-Charge
Systems in this chapter.
Compressor performance and material compatibility are two
major concerns in selecting new refrigerants. Research has found
good equipment reliability. Retrofit recommendations have also
been developed by equipment and refrigerant manufacturers to
guide stores in converting from ozone-depleting substances to alternatives; close consultation with equipment manufacturers is necessary to stay current on this issue.
Concern about ozone depletion has led to U.S. Environmental
Protection Agency regulations to minimize refrigerant emissions.
Intentional venting of all refrigerants, including the substitutes, is
prohibited. Additional regulations apply to chlorine-containing
refrigerants such as R-22. If systems that contain more than 22.7 kg
of refrigerant leak at an annual rate exceeding 35%, equipment
repairs are required. Certain servicing and record-keeping practices
are also required (EPA 1990). Proposed regulations extend these
regulations to the hydrofluorocarbon substitutes and tighten the leak
repair requirements. These developments should be monitored.
Chapters 29 and 30 of the 2009 ASHRAE Handbook—Fundamentals have more information on refrigerants and their properties.
Refrigerant Lines. Sizing liquid and suction refrigerant lines is
critical in the average refrigeration installation, because of the typically long horizontal runs and frequent use of vertical risers. Correct liquid-line sizes are essential to ensure a full feed of liquid to the
expansion valve; oversizing must be avoided to prevent system
pumpdown or defrost cycles from operating improperly in singlecompressor systems.
Proper suction-line sizing is required to ensure adequate oil return
to the compressor without excessive pressure drop. Oil separates in
the evaporator and moves toward the compressor more slowly than
the refrigerant. Unless the suction line is properly installed, oil can
accumulate in low places, causing problems such as compressor
damage from liquid slugging or insufficient lubrication, excessive
pressure drop, and reduced system capacity. To prevent these problems, horizontal suction lines must pitch down as gas flows toward
the compressor, the bottoms of all suction risers must be trapped, and
refrigerant speed in suction risers must be maintained according to
piping practices described in Chapters 1 and 2. To overcome the
larger pressure drop necessary in suction risers, suction lines may be
oversized on long horizontal runs; however, they still must pitch
down toward the compressor for good oil return.
Manufacturers’ recommendations and appropriate line sizing
charts should be followed to avoid adding heat to either suction or
liquid lines. In large stores, both types of lines can be insulated profitably, particularly if subcooling is used.

Typical Systems
Refrigeration systems in use today can generally be categorized
into one of the following types: single (a single compressor connected to one or more evaporator loads), multiplex (or parallel

15.13
compressor) rack, loop, distributed, and secondary refrigerant.
Each type has distinct advantages and disadvantages, and may be
chosen based on the weight a designer assigns to the different components of equipment life-cycle cost.
The most common compressors used in a typical supermarket
refrigeration system include reciprocating, scroll, and screw compressors, which are discussed in Chapter 37 of the 2008 ASHRAE
Handbook—HVAC Systems and Equipment. Planning load management and sizing the compressors are very important to a successful refrigeration installation.
Single System. A single-compressor/single-evaporator system
is sometimes referred to as a conventional system. Each compressor
may be piped to an individual condenser, or several single compressors may be piped to a larger condenser with multiple circuits. Some
single-compressor systems are connected to two or more evaporator
systems, in which case each evaporator system uses its own liquid
and suction lines and is controlled independently.
A solid-state pressure control for single systems can help control
excess capacity when ambient temperature drops. The control
senses the pressure and adjusts the cutout point to eliminate shortcycling, which ruins many compressors in low-load conditions.
This control also saves energy by maintaining a higher suction pressure than would otherwise be possible and by reducing overall running time.
Multiplex System. Another common refrigeration technique
couples two or more compressors in parallel, piped together with
common suction and discharge lines. The compressors share a
common oil management system and usually operate connected to
one or more large condensers. The condensers are usually remotely
air- or evaporatively cooled, but they can also be built as part of the
compressor rack assembly. The multiplex rack system has several
evaporator systems, individually controlled and connected to the
compressor rack’s common suction line.
Multiple-evaporator systems are usually designed such that each
evaporator system operates at a different saturated suction pressure
(temperature). Because they are connected to one common suction
pressure, the compressors are forced to operate at the lowest evaporator pressure to achieve the coldest evaporator system temperature. The obvious result is a sacrifice in efficiency. Running all the
equipment at the low suction pressure required for ice cream (on
low-temperature systems) or for meat (on medium-temperature systems) causes all the compressors to operate at lower suction pressures than are necessary. To overcome this inefficiency, large parallel
systems frequently isolate ice cream and meat refrigeration. Satellite
compressors may be used for extreme loads. The satellite compressor has its own independent suction but shares the rack system’s
common discharge piping and oil management system. Split-suction
manifolds are often used for larger loads: different suction pressures
are obtained, but all compressors discharge into a common header
and share the oil management system.
Consult manufacturers to determine the appropriate suction pressure (temperature) at the fixture and the load that each system adds to
the total. The multiplex rack system must then be designed to deliver
the total of all the loads at a common suction pressure no higher than
the lowest system pressure requirement less the suction line pressure
drop. Systems designed to operate at suction pressures higher than
the common must use some means of suction line regulation to prevent higher-temperature evaporators from operating at temperatures
below what is necessary to maintain product temperatures.
Suction pressure can be regulated with either electronically [electric evaporator pressure regulating (EEPR)] or mechanically
actuated [evaporator pressure regulating (EPR)] valves. When
sized according to manufacturers’ recommendations, these valves
cause little or no pressure drop in the full-open position. When regulating, they create pressure drop to maintain the fixtures using them
at their design condition above the common rack suction pressure.
Larger pilot-operated EPR valves may use discharge pressure to

This file is licensed to Abdual Hadi Nema (ahaddi58@yahoo.com). License Date: 6/1/2010

Licensed for single user. © 2010 ASHRAE, Inc.

15.14
open and close the valves, or may be internally piloted, with
upstream pressure used to open and close. Although each type has
advantages and disadvantages, electric valves are being used more
frequently because of their ability to communicate with the rack’s
energy management system.
The suction gas temperature leaving display fixtures should be
superheated to ensure that only vapor enters the compressor suction
intake. Particularly on low-temperature fixtures, the suction line gas
temperature increase from heat gained from the store ambient can be
substantial and adversely affect both refrigeration system capacity
and compressor discharge gas temperature. This must be considered
for system design. One solution to reduce excessively high superheat
is to run the suction and liquid lines tightly together between the fixture and compressor system if the liquid is subcooled, with the pair
insulated together for a distance of 9 to 18 m from the fixture outlet.
This technique cannot be used with gas defrost or refrigerants requiring low suction superheat at the compressor suction (for example,
low-temperature single-stage R-22 systems). Suction-to-liquid line
heat exchangers can be installed in the display fixture. This technique allows the suction gas to pick up heat from the liquid instead of
the store ambient. Under all conditions, the suction line should be
insulated from the point where it leaves the display refrigerator to the
suction service valve on the compressor. The insulation and its installation must be vapor resistant.
To ensure proper thermostatic expansion valve operation, the
engineer should verify that liquid entering the fixture is subcooled.
Some refrigerator and/or system designs require liquid-line insulation, which is very important when ambient outdoor air or mechanical subcooling is used to improve system efficiency.
Parallel operation is also applied in two-stage or compound systems for low-temperature applications. Two-stage compression includes interstage gas cooling before the second stage of compression
to avoid excessive discharge temperatures. A multiplex rack system
with multiple compressors of equivalent capacity is called an even
parallel system; with compressors of different capacities, it is called
an uneven parallel system.
Parallel compressor systems must be designed to maintain proper
refrigerator temperatures under peak summer load. During the rest of
the year, store conditions can be easily maintained at a more ideal
condition, and refrigeration load will be lower. In the past, refrigeration systems were operated at 32°C condensing conditions or above
to maintain enough high-side pressure to feed the refrigerated display
fixture expansion valves properly. When outdoor ambient conditions
allow, current technology permits the condensing temperature to follow the ambient down to about 21°C or less. When proper liquid-line
piping practices and valve selection guidelines are observed, the
expansion valves will feed the evaporators properly under these low
condensing pressures (temperatures). Therefore, at partial load, the
system has excessive capacity to perform adequately.
Multiple compressors may be controlled or staged based on a
drop in system suction pressure. If the compressors are equal in size,
a mechanical device can turn off one compressor at a time until only
one is running. The suction pressure will be perhaps 35 kPa or more
below optimum. Microprocessors offer the option of remote control
and system operation for all types of compressors, managing compressor cycling and run time for each compressor, and ensuring the
common suction pressure is optimized. Satellite compressors can be
controlled accurately with one control that also monitors other components, such as oil pressure and alarm functions. To match changing
evaporator loads, rack capacity can be varied by cycling compressors, varying the speed of one or more compressors, and/or unloading compressor cylinders by closing valves or moving ports on screw
compressors.
Unequally sized compressors can be staged to obtain more steps
of capacity than the same number of equally sized compressors.
Figure 23 shows seven stages of capacity from a 5, 7, and 10 kW
compressor parallel arrangement.

2010 ASHRAE Handbook—Refrigeration (SI)
Fig. 23 Stages with Mixed Compressors

Fig. 23

Stages with Mixed Compressors

Loop Systems. A loop system is simply a variation of the multiplex rack system. Rather than piping the different evaporator systems (or circuits) back to the machine room, the loop system is
designed so that a single suction and liquid “loop” is piped out to the
store for each common suction pressure. The individual circuits are
then connected to the loop near the fixtures. If EPRs and solenoid
valves are used, they will typically be installed nearer the refrigerator lineups.
Factory-Assembled Equipment. Factory assembly of the necessary compressor systems with either a direct air-cooled condenser
or any style of remote condenser is common practice. Both single
and parallel systems can be housed, prepiped, and prewired at the
factory. The complete unit is then delivered to the job site for placement on the roof or beside the store.
Prefabricated Equipment Rooms. Many supermarket designers choose to have compressor equipment installed in factoryprefabricated housing, commonly called a mechanical center, to
reduce real estate costs for the building. The time requirements for
installation of piping and wiring may also be reduced with prefabrication. Most of the rooms are modular and prewired and include
some refrigeration piping. Their fabrication in a factory setting
should offer good quality control of the assembly. They are usually
put into operation quickly upon arrival at the site.
Energy Efficiency. A typical supermarket includes one or more
medium-temperature parallel compressor systems for meat, deli,
dairy, and produce refrigerators and medium-temperature walk-in
coolers. The system may have a satellite compressor for the meat or
deli refrigerators, or all units may have a single compressor. Energy
efficiency ratios (EERs) typically range from 2.3 to 2.6 W/W for the
main load. Low-temperature refrigerators and coolers are grouped on
one or more parallel systems, with ice cream refrigerators on a satellite or on a single compressor. EERs range from 1.2 to 1.5 W/W for
frozen-food units to as low as 1.0 to 1.2 W/W for ice cream units.
Cutting and preparation rooms are most economically placed on a
single unit because the refrigeration EER is nearly 2.9 W/W. Airconditioning compressors are also separate because their EERs can
range up to 3.2 W/W (Figure 24).

Low-Charge Systems
Over the last decade, different supermarket refrigeration system
configurations with lower refrigerant charges have been considered
in attempts to mitigate the environmental issues of ozone depletion

This file is licensed to Abdual Hadi Nema (ahaddi58@yahoo.com). License Date: 6/1/2010

Retail Food Store Refrigeration and Equipment

Licensed for single user. © 2010 ASHRAE, Inc.

Fig. 24

Typical Single-Stage Compressor Efficiency

Fig. 24

Typical Single-Stage Compressor Efficiency

and global warming. The Montreal Protocol established due dates to
phase out different refrigerants worldwide. The production and use
of hydrochlorofluorocarbons (HCFCs), such as R-22, in refrigeration systems will be totally phased out by the year 2030. Commercial refrigeration is one of the largest consumers of refrigerant
worldwide, and special attention has been devoted to minimizing
use of refrigerant in existing and new sites. This section discusses
the following three types of low-charge systems: secondary loop,
distributed, and liquid-cooled self-contained.
Secondary Loop. In secondary coolant systems, heat is removed
from refrigerated spaces and display cabinets by circulating a
chilled fluid in a secondary loop cooled by a primary refrigeration
system. Fluid circulation is typically provided by a centrifugal
pump(s) designed for the flow rate and pressure drop required by the
system load and piping arrangement.
Selection of secondary fluid is critical to system efficiency because viscosity and heat transfer properties directly affect system
performance. In most cases, the secondary fluid is in a single-phase
state, removing heat through a sensible temperature change. Inhibited propylene glycol solutions are most often used for mediumtemperature systems, typically at fluid temperatures not lower than
–9.4°C. Low-temperature fluids are commonly composed of solutions of various potassium-based organic salts and inhibitors, though
several alternatives are available and corrosion remains a concern.
Fluids involving a phase change, including carbon dioxide and
water-based ice slurries, are also possible. For an explanation of various options for secondary fluids, including safety considerations,
see Chapter 13.
Heat can be removed from the fluid using a chiller of any design,
but commonly a plate type is used for highest efficiency. Coils engineered to remove heat effectively from refrigerated spaces are generally designed differently from those used for volatile refrigerants.
Liquid should enter the bottom of the coil, leave at the top, and avoid
trapping air. Drain and vent valves must also be equipped to assist
air removal and service.
Typically, the entire refrigeration system for supermarkets is
divided into two temperature groups: one low-temperature (frozen
food, ice cream) and one medium-temperature (meat, dairy, produce, preparation rooms). To increase efficiency, the systems may be
further divided into additional temperature groups, although often at

15.15
a higher capital cost. Temperature is controlled by regulating flow
using a balance valve, or cycling flow around a set point using a solenoid valve. Piping may be in circuited or loop arrangement, or a
combination of the two. Circuited systems have the advantage of
containing most of the control valves in a central location, but at the
cost of a greater amount of installed piping.
Performance Characteristics. Secondary coolant systems have
several advantages. Because primary refrigeration piping is located
almost wholly within the machine room, the amount of piping and
refrigerant required can be reduced by as much as 80 to 90%. Because field piping of the primary system is typically limited to only
a few joints, the majority of the primary system piping joints are
factory-installed. Factory-installed joints are generally higherquality than field-installed joints, because they are formed in controlled conditions by skilled labor, using nitrogen and a variety of
pressure-testing and leak-identification methods. Higher-quality
joints combined with a lower refrigerant charge can significantly
lower refrigerant leakage rates, which reduce the environmental effects associated with the primary refrigerant. The compressors and
evaporator are close-coupled, so suction line pressure losses and
heat gains are minimized, enhancing system performance. Secondary coolant systems are inherently less complex than direct-expansion types, requiring fewer and less complicated valves and control
devices. Less expensive nonmetallic piping systems and components can also be used, because the system operating pressure is
low, typically less than 415 kPa (gage). Service of the refrigeration
system is basically limited to the machine room area, and maintenance costs can be reduced. Because a fluid loop is used, thermal
storage may be applied to reduce peak power demands and take advantage of lower off-peak utility rates. Ambient or free cooling
may be applied in areas with colder climates. Secondary systems
also can use primary refrigerants not typically suitable for direct
expansion systems, including ammonia and hydrocarbons.
Disadvantages of secondary systems include thermodynamic
loss inherent in the additional step of heat transfer in the chiller, as
well as the energy consumed by the fluid pump and the heat it transfers to the circulating fluid. Insulation must also be applied to both
coolant supply and return lines to minimize heat gain.
Distributed Systems. Distributed systems eliminate the long
lengths of piping needed to connect display fixtures with compressor racks in back-room parallel compressor systems. The compressors are located in cabinets, close-coupled to the display refrigerator
lineups, placed either at the end of the refrigerator lineup or, more
often, behind the refrigerators around the store’s perimeter.
Distributed systems are typically located in the store to provide
refrigeration to a particular food department, such as meat, dairy, or
frozen food. With this arrangement, the saturated suction temperature (SST) for each rack closely matches the evaporator temperature
of the display refrigerators and walk-in coolers. This is not always
the case for parallel-rack DX systems, because a single rack often
serves display refrigerators with three or four different evaporator
temperatures, and the parallel-rack DX system must operate at an
SST that will satisfy the requirements of the lowest-temperature
one connected. Better evaporator temperature matching with distributed systems can decrease the system’s overall energy consumption.
Distributed systems typically require a much lower refrigerant
charge than parallel-rack DX systems, because of the former’s
shorter suction and liquid lines to display refrigerators. Refrigerant
piping to remote condensers can be eliminated by using a closedloop water-cooled system.
Close-coupling display refrigerators to distributed systems has
other ramifications for energy consumption. Shorter suction lines
mean that pressure drop between evaporators and the compressor
suction manifold is less than with parallel-rack DX systems, so the
SST of distributed systems will be closer to the display refrigerator
evaporator temperature: about 0.6 to 1.1 K less than refrigerator