 
|
 |
 |
Central
Refrigeration Guide
| Centralized refrigeration systems maintain
cold and freezing temperatures in food display cases and walk-in
storage coolers in supermarkets, convenience stores, florists,
liquor stores, delis, and restaurants. These systems account
for 40 to 50 percent of a supermarket's or convenience store's
total energy use and energy efficiency can generally be substantially
improved. For such businesses, where profit margins are very
low, reducing these costs can significantly improve profits.
To best serve these commercial customers, an understanding
of central commercial refrigeration systems is required. |
Description of
the Technology
 |
Figure 1 presents the
basic components of a commercial refrigeration system.
Heat is removed from the product display cases and
discharged to the outdoors. |
The basic steps of the vapor compression cycle and it's
primary components are:
- Evaporator: A heat exchanger inside the display
case in the sales area. Heat removed from the conditioned
space within the case causes the liquid refrigerant to
boil away at a very low temperature, producing a low-temperature,
low-pressure gas.
- Compressor: This unit pulls the refrigerant gas
from the evaporator through refrigerant piping and compresses,
or pumps it up, to a much higher pressure. Compression
also results in a much higher temperature for the gas.
Compressors are usually centrally located in a machine
room at the back of the store.
- Condenser: Typically located outside, this is
a heat exchanger which transfers the heat from the refrigerant
to the outdoors. High-pressure refrigerant gas from the
compressor is converted back into a liquid (i.e., condensed)
as it is cooled inside the condenser. All the refrigerant
vapor changes back into a liquid, but it is still at the
same pressure as when it entered the condenser. A high-pressure,
medium-temperature liquid refrigerant is now moving out
of the condenser.
- Receiver: A vessel which stores the liquid refrigerant
leaving the condenser until it is needed at the evaporators.
- Expansion Valve: This valve, also located in
the refrigerated display case, releases the high-pressure
liquid in a controlled fashion. When the pressure is removed
from the liquid, it rapidly be becomes a vapor that is
much colder than the liquid. Now a mixture of cold liquid
and vapor is coming out of the expansion device, getting
ready to enter the evaporator and start the cycle over.
- Refrigerants: Commercial refrigeration systems
have traditionally used R-12, R-22, and R-502 as refrigerants.
These are all chlorofluorocarbon (CFC) refrigerants and
as of January 1, 1997, R-12 and R-502 are no longer being
produced for U.S. consumption. The cost of existing quantities
has risen sharply and their future availability is uncertain.
Some stores are converting from R-502 to HFC-404A. New
stores or major remodels are using HFC-404A for the low-temperature
systems and HFC-134A for the medium-temperature systems.
Another solution is to stay with R-22 designs until firm
recommendations are available from chemical suppliers
and compressor manufacturers.
Back to Top |
Typical Applications
Commercial refrigeration systems maintain cold and freezing
temperatures for storing food and displaying it for self-service
sales. Food is stored in walk-in coolers typically located
in the back of the store. Display cases come in a variety
of configurations and maintain different temperatures depending
on what is being displayed. For convenience stores, and other
smaller commercial applications, walk-in coolers have glass
doors on one side to also serve as display cases. Table 1
shows the temperatures at which the cases are maintained for
various types of products. The evaporator temperatures will
run 10 to 15°F below the case temperature.
Typical Display Case Temperature
| Product |
Temperature |
| Ice Cream, Frozen Bakery |
-25°F to -10°F |
| Frozen Foods |
-15°F to 0°F |
| Meats, Seafood |
20°F to 30°F |
| Dairy, Produce, Beer/Juice |
25°F to 40°F |
| Walk-In Coolers, Prep Rooms |
35°F to 50°F |
| Produce, Flowers |
45°F to 60°F |
Back to Top |
Efficiency Opportunities
A variety of strategies can be used to reduce refrigeration
costs. These include:
- Night covers for display cases: The covers should have
small perforated holes to decrease moisture buildup.
- Strip curtains for walk-in boxes: Install on doors of
walk-in cases or doorways of refrigerated warehouses.
- Glass or acrylic doors: Install on existing upright
display cases.
- New refrigeration case with doors: Replace existing
older refrigeration case doors.
- Humidistat controls: Install a device that senses the
humidity in the air outside of the upright display case
and turns off the glass door and frame anti-sweat heaters
at low-humidity conditions. Dew-point or analog dew-point
controllers are recommended.
- Case lighting electronic ballasts: Install a fully electronic
ballast to control lights in a refrigerated display case.
- Insulate bare suction lines: These large diameter lines
that run from the evaporator to the compressor lose heat
to the environment.
- Cooler or freezer door gaskets: Replace worn gaskets.
- Auto-closers for coolers or freezers: Auto-closer must
be able to firmly close a door that is within one inch
of full closure.
- Heat recovery: Supermarkets can often take advantage
of the large amount of heat rejected through the refrigeration
system by using it for space heating or water heating.
This is referred to as heat recovery or heat reclaim.
Figure 2 shows a system with a heat recovery coil in the
air handler. During normal operation, discharge gas from
the compressors is piped directly to the condenser. When
space heat is needed, a three-way valve directs the refrigerant
gas to a coil in the air handler. Heat that would normally
be rejected in the condenser is now rejected to the store's
supply air. The refrigerant then passes to the condenser.
Back to Top |
Tips for Efficient
Operation and Maintenance
A regularly performed maintenance program will help ensure
that equipment not only serves a long and useful life but
also operates more efficiently. The following are a few tips:
Ongoing:
- Move delivered product quickly into the walk-in coolers
from the receiving dock. Allowing the product to warm
up to outdoor temperatures will increase the pull down
load on the refrigeration system.
Monthly:
- Clean the condenser coils to maintain optimum heat transfer
performance.
- Check to be sure that display case air curtains, grilles,
or registers are clean and free of obstructions to assure
proper air flow across the evaporator coils.
- Check door gaskets for leaks and tears to minimize infiltration
loads.
- Check the refrigerant charge; add more when needed.
- Check the defrost cycle time clocks to see that they
are set and operating properly. The number and length
of defrosts can be varied seasonally.
Yearly:
- Have a professional refrigeration contractor give the
system a thorough check-up, paying special attention to
the compressors and motors.
- Check and lubricate fans.
- Examine refrigerant lines for leaks.
Back to Top |
|
|
