Most grocery and convenience store owners have realized the tremendous profit potential derived from the use of impulse sales merchandisers placed at or near the checkout counters. Our industry has met the challenge by offering cabinets ranging as small as one glass door units only 24 inches wide, to seven foot long four sided, three tier island cases with LED lighting under the canopy. In between are the popular Grab ‘n Go, which is 44 inches high, available in lengths of 39, 51, or 75 inches, which can be placed back to back; and the Happy Pro, a 46 inch long wall case which is 84 inches high, with six lighted display shelves. The Happy Pro can also be placed at aisle ends. All feature self-contained refrigeration systems ready to be plugged in to a dedicated receptacle, and require no drain.
When you go into the average super market to buy your groceries, you may pick from various display cases maintaining temperatures of from minus 10 degrees to plus 40 degrees Fahrenheit, because of the product they are preserving. In today’s markets, there may be as many as thirty to forty cabinets, all being cooled by as few as six refrigeration systems on the roof. These are single stage refrigerant cycles, as explained in a previous article posted earlier. With a single stage system, minus 40 degrees is about as low as you can achieve.
Cryogenics is the scientific study of what may happen to certain substances well below those limits. Examples are the preservation of fertilized embryos, or even full human remains. This study developed the need for refrigeration capacities achieving these ultra-low temperatures, and led to the development of two stage or tandem refrigeration systems, often called Cascade Systems. Now engineers are working to bring this to the commercial refrigeration field.
In such systems, the condensing unit (heat disbursing side) becomes the evaporator unit (heat accumulating side) of the second unit. You may see the principle illustrated below. This probably represents the systems which will be available commercially in the near future.
The compilation of heat loads in regard to designing a walk-in refrigerator can be complex. For the design of a cooler which only maintains the temperature of a pre-cooled product, you need only figure the heat gain through every square foot of exposed panel based on the temperature differential between the outside (Ambient) air and what you want to maintain within the structure. However, many people want to reduce the temperature of a warm product (such as a side of beef) several degrees or even to a state of being frozen.
This brings us to the study of specific heat and latent heat. Every substance has a specific heat factor above freezing, another below freezing, and a latent (hidden) factor in between to change into a frozen state. This latent factor which changes the state of a product from fresh to frozen is considerably greater than the factor required to lower the same product just one degree either above or below freezing. These heat loads all must be combined by the engineer to determine the power necessary to accomplish the desired result in the hours of time desired for accomplishment.
The engineer knows that one British Thermal Unit (BTU) is the energy required to raise or lower the temperature of one pound of pure liquid water one degree Fahrenheit. If you are familiar with the terms used in relation to window air conditioners (half-ton, three quarter ton, one ton), the terms developed about the time cooling went from ice melting to mechanical refrigeration; it means the cooling capacity which is derived by melting one ton of ice within 24 hours, or 12,000 BTU’s/hour.
Humans realized the value of trying to insulate the walls of their residences as early as Roman times. Generally, it consisted of the use of flax or reeds. When the natural ice industry started building ice storage vaults, they used primarily straw and saw dust. When small ice boxes were built, often they used paper or cellulose. Eventually they used mineral wool made from metal slag, which included sulfur; followed by rock wool, which is limestone heated to 3500 degrees, and made into fiber.
By the early 20th century, cork became the insulation of preference, because of its properties and its “R” value (degree with which its slows heat transfer). It is made from the bark of the cork tree, and while you think of the light fishing bob, large sheets of it are relatively heavy. By the time of the Second World War, it was replaced by spun glass fiber, or fiberglass, pioneered by Owens Corning. In the latter 20th century, fiberglass was replaced by either closed cell polystyrene or foamed polyurethane. The latter has higher “R” value upon manufacture, but tends to deteriorate quicker then closed cell polystyrene because it is more susceptible to moisture. Moisture is the deadly enemy of insulation because water is a good transferor of heat.
When industrial ice making became popular in the late 19th century, ice houses were huge buildings with massive refrigeration equipment. The refrigerant gas used was Ammonia. You know how it smells, so you can imagine how dangerous an unexpected release of the refrigerant would be. Still, it powers industrial ice making plants in many parts of the world.
In the late 1800s, when smaller commercial refrigeration units became available, the first popular refrigerant was sulfur dioxide. You know how rotten eggs smell, so you know sulfur dioxide. Methyl chloride was then used, it is colorless and smells slightly sweet, but it is highly inflammable and toxic in large quantities, so it’s use ended about the time of World War II.
In the 1920s, after tragic accidents with refrigerants; General Motors, who owned Frigidaire, and DuPont Chemical set about looking for a better refrigerant. They developed chlorofluorocarbons with several derivatives, which were branded as “Freon” This became the refrigerant of choice, replacing methyl chloride; and other manufacturers produced similar products with all becoming known as Freon in the trade.
Regretfully, decades later it was discovered that chlorofluorocarbons were extremely dangerous to the earth’s ozone layer, which protects us from too much sun radiation. Freon manufacture is now illegal in the US, but licensed technicians are allowed to recapture it from a system, where it is then recycled and used again. The cost of the recycled gas is many times the cost of the original and highly taxed, so as present systems wear out, it will eventually cease to exist.
R-134a and R-404a are now popular as refrigerants because they have zero ozone depletion potential as well as a low direct global warming potential. There are other refrigerant derivatives being developed as we progress.
For centuries, man used natural frozen water ice to preserve perishable products. By the 19th century the eastern US and the country of Norway were both harvesting natural ice for storage, distribution and sales, and it became known as the ice trade. As early as 1750, inventors began to conceive that it would be possible to produce ice artificially by means of mechanical devices which used vapor and compression.
By the 1850s, men in Ohio and remote Australia had set up industrial ice making machines. The effort in Ohio failed because of competition with natural ice; but in warm Australia, 115 days by ship from frigid New England, the operation was successful. By the years following the Civil War in the US, the huge urban populations of the Northeast were consuming approximately 1400 pounds of natural ice per household per year.
During the latter years of the 1800s, the natural ice trade was supplanted by commercially produced plant ice, which required the use of massive sized refrigeration machines. This led to the development of smaller insulated boxes which could be built and installed practically anywhere, and the US icebox industry was begun.
Site powered by Weebly. Managed by Domain.com