Transcript Slide 1
Two Primary Types Types Of Refrigeration Systems Cryogenic System Mechanical System An open system that uses the refrigerant once and then vents it into the atmosphere. A sealed system that uses the same refrigerant over and over. Examples: A freezing system using liquid nitrogen (LN2) or carbon dioxide (CO2). Examples: A system that uses Ammonia (NH3) or Freon as the refrigerant. We’ll focus on mechanical systems today. How It Works How Does Mechanical Refrigeration Work? 1. To cool the air or liquid, it moves through a heat exchanger (evaporator), transferring its heat into the refrigerant. 2. The refrigerant inside the evaporator must be maintained at a lower temperature than the air or liquid. Heat (high energy) moves to cold (low energy). 3. Temperature of refrigerant is controlled by the pressure inside the evaporator. (Pressure-Temperature relationship) 4. The liquid refrigerant inside the evaporator absorbs the heat, changing state to vapor. (boiling) How It Works How Does Mechanical Refrigeration Work? Examples: At 33.5 PSIG, Ammonia boils at +20°F. At 3.6 PSIG, it boils at -20°F. All fluids have a “Pressure – Temperature” Relationship. How It Works That means… By maintaining the desired pressure inside the evaporator we get the desired temperature. The temperature at that boiling point determines how much heat is absorbed into the refrigerant, changing the liquid refrigerant to a vapor. Primary Components Four Primary Components Of A Mechanical System Evaporator: This is where the heat from the product or space is transferred into the refrigerant. It’s main function is to remove heat from air or liquid. There are several types of evaporators used in food processing industry. Primary Components Types of Evaporators Air Units: a common type of evaporator found in cold storages, pre-coolers, and mechanical freezers. They are a combination of fans and a coil. Air is blown over the tubes which contains refrigerant. Other names: evaporators, coils Primary Components Types of Evaporators Shell And Tube Heat Exchangers: primarily used to cool fluids. Fluid being cooled is pumped through the tubes while the liquid refrigerant is in the shell. Applications: chill glycol, wine, water Primary Components Types of Evaporators Plate and Frame Heat Exchangers: plates stacked together containing refrigerant or product in alternating plates. These have extremely good heat transfer characteristics. Applications: chill glycol, wine, water Primary Components Four Primary Components Of A Mechanical System Compressor: the largest electrical user in a food storage facility is the compressor. It maintains the evaporator refrigerant pressure required to give desired evaporator temperature and to compress the refrigerant gas from the evaporator to a higher pressure. There are primarily three types of compressors… Primary Components Types of Compressors Rotary Screw Compressor: The most common type used. These units are typical 100 HP to 1,000 HP. Wide range of applications and can be used in a single stage system or as the high stage or booster in a twostage system. Primary Components Types of Compressors Reciprocating Compressor: these units have pistons going up and down compressing gas, similar to a car engine. Typically 50 HP to 200 HP and can be used in a single stage system or as the high stage or booster in a twostage system. Limited compression ratio. Primary Components Types of Compressors Rotary Booster Compressor: These are no longer manufactured but there are still many in operation. They are used only as boosters in two stage systems. Primary Components Four Primary Components Of Any Refrigeration System Condenser: high pressure gas is cooled and condenses back to liquid. This is where all of the heat from the cold storage and from the food product is transferred out of the refrigeration system. Fans blow air through coils as water is pumped over them. The water evaporates, absorbing the heat in the refrigerant gas, and the energy is rejected into the atmosphere. Primary Components Four Primary Components Of Any Refrigeration System Expansion Device: function is to control the flow of refrigerant from high (condensing pressure) to low (evaporating pressure) and controls the flow of liquid refrigerant into the evaporator. Several Types: hand expansion valve, motorized modulating valve, thermal expansion valve, etc. Hand Expansion Valve Motorized Modulating Valve Thermal Expansion Valve Refrigeration System Refrigeration System Refrigeration System Commercial System - Freon Only Ex: supermarkets, convenience stores, and small cold storages Condensing Unit: The outdoor condensing unit contains both the compressor and condenser. Air Unit: Standard air unit with thermal expansion valve and electric defrost. Applications Cold Storage Cold Storage: Large rooms/buildings, typically 0°F to -20°F, for storing frozen foods. Found in large processing plants and as food distribution centers. Refrigeration systems can be shut off, sometimes for hours, without temperature gain that damages frozen food. Applications Cooler Cooler: Typically +32°F to +40°F for storing perishable, refrigerated food products. Because product temperature has a very tight tolerance, length of time the refrigeration can be off is very limited. Applications Types Of Mechanical Freezers IQF Belt Freezer: A long tunnel type freezer with one or two belts. To freeze mostly small, IQF products such as peas, cut corn, French fries, onions, pizza topping, shredded cheese, etc. Usual retention time is under 20 minutes. Applications Types Of Mechanical Freezers: Spiral Freezer: spiral wound belt with many tiers to freeze items requiring a longer retention time such as chicken parts, fish fillets, hamburger patties, potato co-products, entrees, pizzas, etc. Usual retention time is 30-60 minutes. Applications Types Of Mechanical Freezers: Variable Retention Freezer: also called a carton freezer, is a large unit with moving conveyors or trays. For long retention time products like pies, tubs of ice cream, or turkeys and cartooned (boxed) products like chicken parts, turkey parts, meat. Retention time 2-12 hours or possibly longer. Applications Types Of Mechanical Freezers: Blast Freezer: small room with large coil inside. This is a batch operation with racks, loaded with product, manually put into and removed from the freezer. Retention time more than an hour, sometimes up to 20 hours. Applications Types Of Mechanical Freezers: Plate Freezer: vertical or horizontal plates. Also called contact freezers because the product is in direct contact with the heat exchange plate surface. Product is pumped or placed between the movable plates and refrigerant is pumped through the plates. Vertical plate freezers are used to freeze meat, offal, and fruit products into 50-60 lb blocks. Usual retention time is 2-4 hours. Horizontal Vertical Applications Types Of Mechanical Freezers: Flat Product Freezer: specific type freezer for quickly freezing thin product like hamburgers and fish fillets. Designed specifically to replace LN2 freezers. Applications Food Freezing Load Calculation: Product: Green Peas Quantity: 10,000 lbs/hr Temperature: Entering +70F Exiting: 0F (equilibrated) Applications Food Freezing Load Calculation: 1) Product Load Specific Heat above freezing is 0.79 btu/lb F 39F (+70F - +31F) x 0.79 Latent Heat (from chart) = 30.8 btu/lb = 106.0 btu/lb Specific Heat below freezing is 0.42 btu/lb F +31F (+31F - 0F) x .42 TOTAL: = 13.0 btu/lb = 139.8 btu/lb 10,000 lb/hr x 139.8 BTU/lb = 116.5 TR 12,000 BTU/TR Applications Food Freezing Load Calculation: 2)Base Load Motors (116 HP x 3,000 btu/h/HP) = 348,000 btu/h Enclosure = 60,000 btu/h Other (lights, etc.) = 84,000 btu/h TOTAL: 492,000 btu/h 12,000 btuh/TR = 492,000 btu/h = 41.0 TR 3) Safety Factor 5% (water, uneven product feed): = 6.0 TR TOTAL LOAD = 163.5 TR