7.6 Food Processing & Cold Storage Refrigeration: Product Load, Pull-Down & Storage Design

7.6 Food Processing & Cold Storage Refrigeration: Product Load, Pull-Down & Storage Design

NCEES sub-topic 2E names "food processing and storage" explicitly among the refrigeration systems you must be able to work. This is the part of refrigeration practice that comfort-cooling engineers most often skip, and it is also the part where the arithmetic is most different: a cold room load is dominated by the product, not by the envelope, and the design question is usually "how fast" rather than "how cold."

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1. The Six Components of a Cold Storage Load

Two of these are unique to refrigeration and neither appears in comfort load calculations. Respiration heat applies to fruits and vegetables that are still alive: apples at 32 F respire at roughly 600 to 900 Btu per ton of product per 24 hours, and the same apples at 60 F respire at four to six times that rate. Evaporator fan motor heat is not a rounding error in a low-temperature room, because the fans run continuously and every watt they draw becomes a refrigeration load.

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2. Product Load: The Three-Step Calculation

Above the freezing point, the product behaves like any other mass:

$Q_1 = m \, c_{p,above} \, (T_1 - T_f)$

At the freezing point, the water in the product changes phase at constant temperature:

$Q_2 = m \, h_{if}$

Below the freezing point, the specific heat drops by roughly half because ice has about half the specific heat of liquid water:

$Q_3 = m \, c_{p,below} \, (T_f - T_2)$

The freezing point is not 32 F. Dissolved sugars, salts, and acids depress it. Lean beef freezes near 28 F, apples near 29 F, and ice cream mix near 27 F. Using 32 F shifts energy between the sensible and latent terms and is a favorite distractor.

Representative Property Values

A useful sanity relationship: cp_above is approximately 0.008 times the percent water content plus 0.20, and the latent heat of fusion is approximately 144 Btu/lb times the water fraction.

Worked Example A - Cooling Produce Without Freezing

40,000 lb of apples enter a cooler at 80 F and must be pulled down to 32 F within 24 hours. Respiration at storage temperature is 750 Btu per ton per 24 hours.

• Sensible product load: 40,000 x 0.87 x (80 - 32) = 1,670,400 Btu per 24 hours
• Respiration: (40,000 / 2,000) x 750 = 20 x 750 = 15,000 Btu per 24 hours
• Product subtotal: 1,685,400 Btu per 24 hours, or 70,225 Btu/hr, which is 5.85 tons of refrigeration

Note that no latent term appears. Apples must not freeze - the design intent is to stop exactly at 32 F, above the 29 F freezing point.

Worked Example B - Freezing Beef

20,000 lb of lean beef enters at 40 F and must reach 0 F.

• Above freezing: 20,000 x 0.77 x (40 - 28) = 184,800 Btu
• Latent: 20,000 x 100 = 2,000,000 Btu
• Below freezing: 20,000 x 0.40 x (28 - 0) = 224,000 Btu
• Total product load: 2,408,800 Btu

The latent term is 83% of the total. That single fact explains why blast freezers are enormous relative to coolers of the same volume, and why any error in the assumed freezing point barely matters compared with an error in the latent heat.

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3. From Load to Equipment Capacity

Refrigeration equipment is not sized on the 24-hour average. Compressors need off-time for defrost, for pull-down recovery after door openings, and for control stability, so cold storage design uses an assumed run time of 16 to 18 hours per day:

$\text{Required capacity} = \frac{\text{Total 24-hour load (Btu)}}{\text{design run hours}}$

Continuing Example B, if the freezing must be accomplished in 24 hours and the design run time is 18 hours per day, the product portion alone requires 2,408,800 / 18 = 133,800 Btu/hr, which is 11.2 tons - versus 8.4 tons if you had naively divided by 24. Add transmission, infiltration, internal gains, and defrost, and the connected capacity typically lands 40% to 70% above the raw product number.

Pull-down time is the design specification. A blast freezer that must take beef from 40 F to 0 F in 8 hours rather than 24 needs three times the capacity for the same tonnage of product. When an exam question gives you a pull-down period, that period - not 24 hours - is the denominator.

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4. Evaporator TD, Humidity, and Frost

In a refrigerated room the evaporator does double duty: it holds temperature and it sets humidity. The controlling parameter is the evaporator temperature difference (TD), the difference between room air temperature and refrigerant saturation temperature.

A small TD means a large, expensive coil operating at a warm suction temperature - which also means better compressor efficiency. Chasing a small TD is therefore not only a product-quality decision but an energy decision, and it is one of the few places in HVAC where the humidity target and the efficiency target point the same direction.

Below 32 F the coil accumulates frost rather than draining condensate. Frost insulates the surface and blocks airflow, so capacity decays between defrosts. The three common defrost methods trade first cost against parasitic load:

• Air (off-cycle) defrost works only in rooms held above about 35 F, and adds no heat beyond the room air.
• Electric defrost is simple and reliable, but every watt is delivered directly into the refrigerated space and must be pumped back out.
• Hot gas defrost diverts discharge gas to the evaporator, is the most efficient at scale, and is standard in industrial ammonia plants.

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5. Ventilation and Safety in Refrigerated Spaces

Sub-topic 2E lists "ventilation" alongside refrigeration processes for a reason. Two requirements recur:

• Machinery room ventilation and refrigerant detection under ASHRAE Standard 15, developed in Section 8.3. Industrial cold storage overwhelmingly uses R-717 (ammonia), a Class B2L refrigerant whose toxicity drives the detection and emergency ventilation design.
• Controlled-atmosphere storage for long-term fruit storage deliberately holds oxygen at 1% to 3% and carbon dioxide at 1% to 5% to suppress respiration. Those atmospheres are lethal, so CA rooms require entry procedures, lockable doors, and gas monitoring that a normal cooler does not.