Cold storage or refrigerated storage is a family of equipments provided with mechanical refrigeration cycle (see Basics section for more information about refrigeration cycles). Cold storage means healthy food where it prevents food damaging by bacteria, either for raw or cooked foods. All refrigeration equipments have the same basic cycle components (compressor, condenser, evaporator and expansion valve, see Basics) regardless the capacity of equipment; the differences will be in control system and accessories.
General selection factors for refrigeration equipments:
- Storage capacity; expressed in dims L x W x H or in cubic meters for cold rooms, in litres or GN containers quantity for reach-in refrigerators).
- Equipment type; Reach-in refrigerator, reach-in freezer, cold room (chiller type), cold room (freezer type), blast chiller or blast freezer (see explanations below).
- Cooling capacity in KW (should be selected according to actual temperature surrounding condenser if it is air cooled, higher condenser surrounding temperature than tabulated in manufacturer catalogue means less cooling capacity than tabulated. Manufacturer catalogue gives de-rating table for refrigeration equipment to get actual cooling capacity at different surrounding temperatures).
- Internal cooling temperature in °C.
- Should have automatic defrost; automatic defrost means periodic defrosting of ice created on evaporator coil to improve heat transfer between refrigerated products and evaporator coil. This can be achieved either by using of an electric heater or hot gas (refrigerant) cycle, controlled by a timer. Small units can be provided with re-evaporation system for condensate water after defrosting, while large capacity units requires drain pipe to nearest drain point.
- Should have Chlorofluorocarbon (CFC) free refrigerant; previously the common refrigerants for refrigeration equipments are R-12 & R-502, both are causing penetration of Ozone layer, now all new refrigerants are CFC free such as R-134a (for refrigerators) or R-404a (for freezers).
- Insulation type, density & thickness; Injected polyurethane (PU) insulation with density of around (40 kg/m3) is commonly used for most of refrigeration equipments, where it is CFC free, injection gives homogeneous distribution for insulation with no gaps. Thickness of insulation affects K-factor (heat transfer coefficient, measured in (W/m2°K), table shown in Fig.5.2.1 shows different PU insulation thicknesses with related approximate value for K-factors
|PU insulation thickness (mm)||K-factor (W/m2 °C)|
Fig.5.2.1 PU insulation thickness & K-factors
- Air or water cooled condenser; selecting of condenser type depends on several factors such as refrigeration capacity, capital cost, energy cost, maintenance cost, water availability, space availability, etc. Simply; air cooled condenser reduces equipment capital cost, more suitable for small capacities, lower maintenance cost and higher running energy costs. Water cooled condenser is either cooled by water from cheap source like a river or by using of a cooling tower (common selection) which cools water and return it by a pump to cool condenser again, this allow reusing of water with small make up to balance for evaporated water, cooling tower has better performance in dry climates. Another type of water cooling is evaporative condenser in which water temperature is decreased in a similar way to cooling tower, but with condenser coil is exist inside the tower (see Basics). Water cooled condenser as explained has higher capital cost, more suitable for larger capacities, requires additional space for cooling tower, has little higher maintenance cost and has noticeable lower running energy cost which is the main advantage. Water cooled condenser is recommended for large capacity cold stores, large super markets refrigeration system (central system for refrigerated displays).
- If air cooled condenser is provided, it should be suitable for actual air temperature surrounding condenser, higher condenser temperature than tabulated in manufacturer catalogue will reduce cooling capacity tend to compressor trip after certain limit stated in manufacturer catalogue.
- Built in or remote air cooled condenser; remote air cooled condenser is usually located outside the building to decrease noise level inside the space, and to avoid space heating by condenser coil.
Important note: Air conditioning system designer should consider location of condensing unit, if condenser is built-in, designer should add condenser heat to space loads, also in case of any modification during execution, the designer attention should be invited to modify his system accordingly.
- Coefficient of performance COP; indicates equipment efficiency, higher COP is an advantage; it means less electric energy consumption for same cooling capacity.
- Capacity control; for large units, can be achieved by employing more than one compressor, providing semi-hermetic compressor with capacity un-loaders (semi-hermetic compressor means compressor and motor are combined together in one accessible casing), etc.
- Condenser coil pipes preferred to be of copper.
- Internal materials should be food grade; aluminium, plastic coated galvanized steel, stainless steel, plastic, etc.
- Should have internal lighting.
- Electric control system; depends on equipment capacity, in general control system should include thermostat control, thermal overload relay for all motors (compressor, condenser fan and evaporator fan), automatic defrost with timer, time delay relay for large unit’s compressors, automatic pump down with low pressure cut-out – LPCO – and solenoid valve (for large capacity units), high pressure cut-out (HPCO, for large capacity units), high temperature alarm (for large units), temperature/time memory (optional for large units), phase failure/sequence relay (for large capacities 3 phase units, especially when phase loss or inversion is expected), under voltage protection relay (for large capacities, especially in areas with un-stable power supply), signal lamps, run hour meter for compressor, etc.
Reach-in refrigerators & freezers
Moisture maintenance systems