Industrial Food Freezing Systems: - 18 - 35°C. Chapter 10 Has Frozen Storage Temperatures For Spe
Industrial Food Freezing Systems: - 18 - 35°C. Chapter 10 Has Frozen Storage Temperatures For Spe
Industrial Food Freezing Systems: - 18 - 35°C. Chapter 10 Has Frozen Storage Temperatures For Spe
BLAST FREEZERS
Blast freezers use air as the heat transfer medium and depend on
contact between the product and the air. Sophistication in airflow
control and conveying techniques varies from crude blast freezing
chambers to carefully controlled impingement-style freezers.
The earliest blast freezers consisted of cold storage rooms with
extra fans and a surplus of refrigeration. Improvements in airflow
control and mechanization of conveying techniques have made heat
Fig. 1 Typical Freezing Curve
transfer more efficient and product flow less labor-intensive.
While batch freezing is still widely used and appropriate for cer-
The preparation of this chapter is assigned to TC 10.9, Refrigeration Appli- tain production lines, more sophisticated freezers allow integrate
cation for Foods and Beverages. the freezing process into the production line. This integration,
15.2 1998 ASHRAE Refrigeration Handbook (SI)
Thus, packages are even and square within close tolerances. Auto-
CRYOMECHANICAL FREEZERS
matic plate freezers accommodate up to 200 packages per minute, Although the technique is not new, cryomechanical freezing
with freezing times from 10 to 150 min. When greater capacities are (combination of cryogenic and blast freezing) applications are
required, the freezers are placed in series with associated conveyor increasing. High-value, sticky products, such as IQF shrimp, and
systems to handle the loading and unloading of packages. wet, delicate products, such as IQF strawberries and IQF cane
berries, are common applications for these systems.
Specialized Contact Freezers A typical cryomechanical freezer has an initial immersion step in
A combination of air and contact freezing is used for wet fish which the product flows through a bath of liquid nitrogen to set the
fillets and similar soft, wet products with relatively large, flat sur- product surface. This rapid surface freezing reduces dehydration
faces. The continuous, solid stainless steel belt is typically 1.2 to 2 and improves the handling characteristics of the product, thus min-
m wide and may be 30 m long. Product is loaded onto the belt at imizing sticking and clumping. The cryogenically crust-frozen
one end of the freezer and then travels in a fixed position through product is then transferred directly into a mechanical freezer, where
the freezing zone to the discharge end. Freezing is usually accom- the remainder of the heat is removed and the product temperature is
plished both by conduction through the belt to a cooling medium reduced to -18°C or lower. The cryogenic step is sometimes retro-
below the belt and by convection through controlled airflow above fitted to existing mechanical freezers to increase their capacity. The
the belt. This freezer design produces attractive product, but a use of the mechanical freezing step makes operating costs lower
drawback is the physical size of the freezer. Capacities for typical than for cryogenic-only freezing.
products are generally limited to 0.9 to 2.3 kg/h, with a freezing
time of less than 30 min. OTHER FREEZER SELECTION CRITERIA
Another specialized contact freezer conveys food products on a Reliability
continuous plastic film over a low-temperature (-40°C) refriger-
ated plate. Upon contacting the film, the bottom surface of the prod- Due to the harsh operating conditions, the freezing system is
ucts is frozen approximately 1 mm deep in about 1 min. This probably the most vulnerable equipment in a process line. A process
equipment is used to eliminate product deformation or wire mesh line usually incorporates only one freezer, which makes reliability a
belt markings on products that are flat, wet and sticky, soft, or in major concern.
need of hand shaping before freezing. Another benefit of contact In order to achieve normal equipment life expectancy, freezing
prefreezing is that it reduces dehydration losses in the subsequent systems must be designed and constructed with adequate safety fac-
freezing step. Examples of products suitable for contact prefreezing tors for electrical/mechanical components and with materials that
are marinated, boneless chicken breasts and thin fish fillets. can withstand harsh environments and rugged usage.
Hygiene
CRYOGENIC FREEZERS
Cleanability and sanitary design are as important as reliability.
Cryogenic (or gas) freezing is often an alternative for (1) small- Freezing systems should (1) have a minimum number of locations
scale production, (2) new products, (3) overload situations, or where the product can hang up, (2) be constructed of noncorrosive,
(4) seasonal products. Cryogenic freezers use liquid nitrogen or liq- safe materials, and (3) be equipped with manual and/or automatic
uid carbon dioxide (CO2) as the refrigeration medium, and the sanitation systems for wash-down and cleanup. If the equipment is
freezers may be batch cabinets, straight belt freezers, spiral convey- not or cannot be properly cleaned and sanitized, product contamina-
ors, or liquid immersion freezers. tion can result. These features are particularly important for chilled,
partially cooked, and fully cooked products that may not be fully
Liquid Nitrogen Freezers reheated or properly prepared prior to consumption.
The most common type of liquid nitrogen freezer is a straight-
through, single-belt, process-line tunnel. Liquid nitrogen at - Quality
195°C is introduced at the outfeed end of the freezer directly onto The quality of processed food products is affected by physical
the product; as the liquid nitrogen vaporizes, those cold vapors are changes and by rates of microbiological activity and chemical reac-
circulated toward the infeed end, where they are used for precooling tions, each of which is influenced by the rate of temperature change.
and initial freezing of the product. The “warmed” vapors (typically The freezing process physically changes the food product; the rate
15.6 1998 ASHRAE Refrigeration Handbook (SI)
of physical change or the freezing time determines the size of ice REFRIGERATION SYSTEMS
crystals produced. See Chapter 11 for more information on food
microbiology. Most mechanical food freezers use ammonia as the refrigerant
and are equipped with either liquid overfeed or gravity flooded
At a slow freezing rate, initially formed ice crystals can grow to
evaporators. The choice of evaporator system depends on freezer
a relatively large size; fast freezing forces more crystals to be
size and configuration, space limitations, freezer location, existing
seeded with a smaller average size. However, different sized ice
plant systems (where applicable), relative cost, and end user prefer-
crystals are formed because the product’s surface freezes faster
ence. For systems with three or more evaporators, a liquid overfeed
than its inner parts. Large ice crystals may puncture the cell walls of
system is usually less costly to install and operate.
the product, usually increasing loss of juices during thawing. For
some products, this can greatly affect the texture and flavor of the The evaporators may be defrosted with water, hot gas, or a com-
remaining product tissue. bination of both. The defrost systems can be manual, manual
start/automatic run, or fully automatic. Coil defrost can take place at
The influence of freezing time is more apparent in some products
a shift change or be sequential, so that the freezer remains in con-
than in others. For strawberries, a shorter freezing time can signifi-
tinuous operation for long periods. Selection of a defrost system
cantly reduce drip loss. Drip loss is 20% for strawberries frozen in
depends on plant and product requirements, water supply and dis-
12 h but only 8% for strawberries frozen in 15 min. Cryogenic sys-
posal situation, sanitation regulations, and end user preference.
tems perform the same freezing function in 8 min or less, reducing
drip loss to less than 5%. With a liquid overfeed system, close consideration should be
given to refrigeration line sizing and potential static pressure penal-
A well-applied mechanical freezer or a cryogenic freezer can
ties if the liquid overfeed recirculator is remote from the freezer. See
crust-freeze products rapidly, minimizing loss of natural juices, aro-
Chapter 1 for design considerations.
matics, and flavor essences and maintaining higher, more market-
able product quality. Also, lower storage temperature and fewer, A design evaporator temperature for the freezer should be
less severe temperature fluctuations tend to help preserve quality. selected to achieve the lowest overall capital and operating cost pos-
sible for the freezer and the other high- and low-side refrigeration
Economics components while still remaining consistent with product require-
ments and other plant operating conditions.
Ironically, freezing equipment is considered both the most
expensive and the least expensive link in the modern processing Operation
chain. Although the freezer frequently represents the single largest
investment in a line, its operating costs are usually only 3 to 5% of Modern conveyor freezers are equipped with programmable
the total. Packaging costs may vary widely but generally are several logic controls (PLCs) and/or computer control systems that can
times greater than total freezing cost. monitor and control key elements of freezer operation to maximize
One essential factor to consider when choosing freezing equip- productivity, product quality, and safety. Items to be monitored and
ment is the loss in product mass that occurs during freezing. The controlled include belt speeds, air and refrigerant temperatures, air
cost of this loss may be about the same as the operating cost of the and refrigerant pressures, evaporator defrost cycles, belt washers
freezer for inexpensive products such as peas; the loss is even more and dryers, amperage for electric motors, safety and alarm func-
significant for expensive products such as seafood. tions, and other variables specific to the products being frozen.
Loss of product mass during freezing may be caused by mechan- The presence of electronic controls alone does not guarantee
ical losses, downgrading, and dehydration. Mechanical losses freezer performance; human operators are still needed. The number
include juice dripping or products dropping to the floor or sticking and specialty of operators required depends on the size of the plant
to conveyor belts, all of which are specific to each processing plant. and the quantity of freezers. A small plant may have a combination
A modern freezer should produce minimal losses in this category. operator covering belt production and refrigeration. In larger plants,
Downgrading losses refer to product damage, breakage, and other a freezer operator may oversee production while a specialist attends
occurrences that render the product unsalable at the top-quality to the refrigeration cycle. Long-term success requires well-trained,
price. For most products, a modern freezing system should incur knowledgeable operators who make the proper adjustments as
minimal losses from damage and breakage. changes occur in the process.
Dehydration losses occur in any freezing system. The evapora-
Maintenance
tion of water vapor from unpackaged products during freezing
becomes evident as frost builds up on evaporator surfaces. Frost is Freezing systems operate in a harsh environment in which some
also caused by excessive infiltration of warm, moist air into the of the components are hidden from view by the enclosure and
freezer. Still air inside a diffusiontight carton often creates larger product. Many freezers operate 5000 to 7000 h per year. The best
dehydration losses than individual quick freezing of unpackaged freezers are ruggedly constructed and well designed for ease of
products. Heat transfer is poor because no circulation of air occurs maintenance. Nevertheless, a well-run maintenance program is
within the package. The resulting evaporation of moisture can be essential to productivity and safety.
significant; however, the frost stays inside the carton. Freezer manufacturers supply operation and maintenance man-
A poorly designed freezing system operates with dehydration uals with key instructions, information on components, parts lists,
losses of easily 3 to 4%, while well-designed mechanical or cryo- and suggestions regarding maintenance and safety inspections and
genic freezing systems can be built to operate with losses near 0.5%. tasks on a planned-frequency basis. Manufacturers also provide
Liquid nitrogen tunnels normally operate with a dehydration loss of training programs for maintenance technicians.
about 0.4 to 1.25%, which occurs when the nitrogen gas is circu- It is important for plants to have a sufficient number of properly
lated over the product at the infeed end of the freezer. Infeed circu- trained maintenance technicians to maintain all systems. Duties
lation is sometimes needed to temper the product and to use the heat include prescribed inspections, routine maintenance tasks, trouble-
capacity in the nitrogen most efficiently. Nitrogen immersion freez- shooting, and required maintenance during nonproduction periods.
ers have lower dehydration losses but use more liquid nitrogen. A Depending on plant size, the technicians may be individual mechan-
CO2 freezer using jet impingement operates with a dehydration loss ics, electricians, and refrigeration specialists or combinations of the
of about 0.5 to 1.25%. three.