Refrigeration and Cold Storage Notes

Refrigeration and Cold Storage

Application of Refrigeration and Cold Storage of Crops

• Preservation and storage of farm produce such as eggs, milk, meat, vegetables, fruits, or flowers.
• Lengthen the storage period of grain seeds.
• Retains the freshness of newly caught fish and preserve it prior to processing.
• Provide proper ventilation for various farm structures such as poultry, dairy, other crop conditioning enterprises.
• Provide a better environment for farm houses and offices.

Principles of Refrigeration

• Refrigeration involves removing heat to maintain a substance or space at a temperature lower than its normal temperature.
• Refrigeration systems remove heat from an enclosed space and release it outside.

Refrigerant

• The agent which conducts heat from the refrigerated coils and releases it to the outside air.
• Sources of Refrigerants:
  • Ammonia – oldest and widely used for large installation.
  • Methyl chloride.
  • Sulfur dioxide.
  • Freons – commonly used for refrigeration system nowadays.
• Properties of Some Refrigerants
  • Ammonia: Refrigerating Effect = $475.5$ BTU/lb. Remarks: Highly toxic, flammable.
  • Carbon dioxide: Refrigerating Effect = $55.5$ BTU/lb. Remarks: Nontoxic, nonflammable, high operating pressure.
  • Methyl chloride: Refrigerating Effect = $150.3$ BTU/lb. Remarks: Somewhat toxic, flammable.
  • Sulfur dioxide: Refrigerating Effect = $141.4$ BTU/lb. Remarks: Very toxic, nonflammable.
  • Freon 12: Refrigerating Effect = $51.1$ BTU/lb. Remarks: Nontoxic, nonflammable.
  • Freon 21: Refrigerating Effect = $89.4$ BTU/lb. Remarks: Nontoxic, nonflammable.
  • Freon 22: Refrigerating Effect = $69.5$ BTU/lb. Remarks: Nontoxic, nonflammable.

Evaporation

• Evaporation makes liquid change to gas.
• To cause evaporation, heat must be added to the refrigerant.
• Evaporation of refrigerant cools the surrounding area.
• The evaporator is located inside the refrigerated space (to pick up the heat).

Condensation

• It makes the gaseous refrigerant into liquid.
• Heat must be absorbed from the refrigerant in order to cause condensation.
• The cooler space subtracted heat from the refrigerant thereby causing condensation.
• Condenser are located outside the refrigerated space (to release heat).

Refrigeration Cycle

• It is the complete series of events occurring in order as the refrigerant travels the route.
• Expansion – high pressure liquid escapes through the valve into low pressure pipe and refrigerant expands.
• Evaporation – liquid refrigerant begins to evaporate and secures necessary heat from the air inside the refrigerated room.
• Compression – allows the refrigerant to give up the heat picked up from the refrigerated room by drawing it with the compressor.
• Condensation – high-pressure high temperature gaseous refrigerant is forced out by the compressor to the condenser.

Quantities in Refrigeration

• Heat to Removed for Cooling the Product
  • It is the heat required to remove when lowering the temperature of the product.
• Heat to Remove for Cooling the Space
  • Heat transferring from the outside to the inside of wall floors, ceiling, and loose fitting joints.
  • Heat lost when door is opened.
  • Heat lost due to electric light or appliances.
  • Heat lost due to person’s working inside the cooler.

Heat Removed from the Product

• $Q_{\text{product}} = \text{specific heat} \times \text{mass of product} \times \text{temperature difference}$
• Example: Determine the number of BTU that must be removed in order to lower the temperature of 200 lb of fresh beef from 90 to 40 F. Assume specific heat for beef above freezing point equal to 0.75 BTU/lb-F.
  • $BTU = 0.75 \frac{BTU}{lb-F} \times 200 lb \times (90F – 40F) = 7,500 BTU$

Specific Heat of Farm Products

Heat to Remove for Cooling the Space

• For chill room (35-40F) having a volume of 300 to 400 ft3 and 4 in. cork insulation on all surfaces use $6.5 \frac{BTU}{ft^2-hr}$
• For zero-degree rooms having a volume of 300 to 400 ft3 and 5 in. of cork insulation on all surfaces, use $8.5 \frac{BTU}{ft^2-hr}$
• Select the refrigeration that will remove the required number of BTU/hr by operating only 2/3 of the time.

Rating of Refrigerating System

• The total amount of heat that must be removed can be obtained by adding the product load and the space load express in BTU/hr.
• Tons of refrigeration can be obtained by dividing the total amount of heat required with 12,000 BTU/hr.
• Rule of thumb giving the relationship between the horsepower and the ton of refrigeration is
  • 1 hp = 1 ton refrigeration if dealing with chill rooms
  • 2 hp = 1 ton of refrigeration if dealing with a zero-degree room

Sample Calculations

Determine the size of refrigeration unit for a proposed walk-in cooler having a floor space of 6 x 8 ft and a height of 7 ft. It is to have 4 in. of cork insulation on all surfaces and an inside temperature of 35 F. The product to be cooled are: cabbage, 100 lb/day, and fresh beef, 200 lbs/day. Assume the products are to be cooled from 85 to 35 F in 24 hours.

• Product load:
  • $0.9 \times 100 \times \frac{(85-35)}{24} = 187 \frac{BTU}{hr}$
  • $0.75 \times 200 \times \frac{(85-35)}{24} = 312 \frac{BTU}{hr}$
• Space Load: $6 \times 8 + 6 \times 7 \times 2 + 8 \times 7 \times 2 + 6 \times 8 = 292 ft^2$
  • Using $6.5 \frac{BTU}{ft^2-hr}$
  • $292ft^2 \times 6.5 \frac{BTU}{ft^2-hr} = 1,898 \frac{BTU}{hr}$
• Total Load: $187 + 312 + 1,898 = 2,397 \frac{BTU}{hr}$

Room Air Conditioning

• Types are (1) console, (b) built-in unit, and (c) window model.
• General recommendations for selecting the proper size is given in table below.
• Electrical provision of the installation should include an individual-equipment branch circuit.
• Individual circuit makes the installation safe, assure proper unit operation, and minimizes the chances of light flicker and circuit overload.
• Safe operation requires that the air conditioner should be provided with ground

Selection Data for Air Conditioner

Rule of Thumb for Air Conditioning Design

• For every floor area of 20 m2 with a height of 8 ft and room temperature of 27 C, one hp air conditioning system is required.