1.2 Ton of Refrigeration and Temperature Ranges

Understanding the Ton of Refrigeration

In industrial refrigeration, system capacity is rarely expressed in British Thermal Units (Btus) because the numbers would be impractically large. Instead, the industry uses a unit called the Ton of Refrigeration (TR). This term originated in the late 19th century when ice harvesting was transitioning to mechanical refrigeration. To convince ice-house owners to purchase refrigeration machines, manufacturers rated their equipment based on how many tons of ice it could replace.

The Physics and Math of a Standard Ton

A standard ton of refrigeration is defined as the amount of heat transfer required to melt one ton (2,000 lbs) of pure ice at 32°F to liquid water at 32°F during a 24-hour period.

To calculate this value, we use the latent heat of fusion of ice, which is 144 Btu/lb:

$\text{Total Heat removed} = 2,000\text{ lbs of ice} \times 144\text{ Btu/lb} = 288,000\text{ Btus}$

Since this process takes place over 24 hours, we can calculate the hourly and minute heat transfer rates:

• Daily Capacity: $288,000\text{ Btu/day}$

• Hourly Capacity:

  $\frac{288,000\text{ Btu}}{24\text{ hours}} = 12,000\text{ Btu/hr}$

• Minute Capacity:

  $\frac{12,000\text{ Btu}}{60\text{ minutes}} = 200\text{ Btu/min}$

These three numbers are critical for the CARO exam:

• 1 Ton of Refrigeration = 288,000 Btu/24hr
• 1 Ton of Refrigeration = 12,000 Btu/hr
• 1 Ton of Refrigeration = 200 Btu/min

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Calculations: Heat Load to Refrigeration Capacity

An operator must be able to convert heat loads (heat that must be removed) to refrigeration capacity to evaluate if system components (like compressors and evaporators) are operating within their design parameters.

Conversion Formulas

To convert a heat load in Btus per hour to tons of refrigeration:

$\text{Tons of Refrigeration} = \frac{\text{Heat Load (Btu/hr)}}{12,000\text{ Btu/hr/ton}}$

To convert a heat load in Btus per minute to tons of refrigeration:

$\text{Tons of Refrigeration} = \frac{\text{Heat Load (Btu/min)}}{200\text{ Btu/min/ton}}$

Worked Example 1: Calculating System Capacity from Hourly Heat Load

An industrial glycol chiller is removing heat from a food processing line at a rate of 780,000 Btu/hr. What is the equivalent refrigeration capacity in tons?

1. Identify the formula: $\text{Tons} = \frac{\text{Heat Load (Btu/hr)}}{12,000}$
2. Substitute the values: $\text{Tons} = \frac{780,000}{12,000}$
3. Solve: $\text{Tons} = 65\text{ TR}$

The chiller is operating at a capacity of 65 tons of refrigeration.

Worked Example 2: Calculating System Capacity from Minute Heat Load

A low-temperature ammonia evaporator is absorbing heat at a rate of 4,800 Btu/min. What is the refrigeration capacity?

1. Identify the formula: $\text{Tons} = \frac{\text{Heat Load (Btu/min)}}{200}$
2. Substitute the values: $\text{Tons} = \frac{4,800}{200}$
3. Solve: $\text{Tons} = 24\text{ TR}$

The evaporator is absorbing 24 tons of refrigeration.

Worked Example 3: Finding Required Ice Production

If a processing plant has an ammonia system rated at 150 tons, how many pounds of 32°F ice could this system theoretically produce from 32°F water in 24 hours?

1. Recall that 1 ton of refrigeration = melting/freezing 2,000 lbs of ice per day.

2. Apply the ratio:

   $\text{Pounds of ice per day} = \text{Refrigeration Capacity (Tons)} \times 2,000\text{ lbs/ton}$ $\text{Pounds of ice per day} = 150 \times 2,000 = 300,000\text{ lbs of ice}$

The system can freeze 300,000 lbs (150 tons) of ice in 24 hours.

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Industrial Temperature Ranges

Ammonia systems are designed differently depending on the temperature range of the space or process they serve. In industrial facilities, these are typically grouped into three main categories: Coolers, Freezers, and Blast Freezers.

1. Coolers (High-Temperature Refrigeration)

• Operating Range: 35°F to 45°F (1.7°C to 7.2°C)
• Typical Evaporator Temperature: 20°F to 30°F (to maintain temperature difference for heat transfer)
• Common Applications: Produce storage rooms, loading docks, meat cutting rooms, and glycol jacket cooling.
• Operator Considerations: Since the room temperature is above freezing (32°F), moisture does not freeze on the product, but frost will still form on evaporator coils if the coil temperature is below 32°F. Defrost cycles are still required, though they can often be accomplished using room air (air defrost).

2. Freezers (Medium/Low-Temperature Refrigeration)

• Operating Range: 0°F to -20°F (-17.8°C to -28.9°C)
• Typical Evaporator Temperature: -10°F to -30°F
• Common Applications: Frozen food storage warehouses, ice cream storage facilities, and frozen distribution centers.
• Operator Considerations: At these temperatures, moisture in the air will freeze on contact with the evaporator coils, forming ice that acts as an insulator and restricts airflow. Active defrost (such as hot gas defrost or water defrost) is critical to maintain heat transfer efficiency.

3. Blast Freezers (Ultra-Low-Temperature Refrigeration)

• Operating Range: -30°F to -50°F (-34.4°C to -45.6°C)
• Typical Evaporator Temperature: -40°F to -60°F
• Common Applications: Quick-freezing of poultry, seafood, beef, or prepared meals immediately after processing. Fast freezing forms small ice crystals, preserving product texture and quality.
• Operator Considerations: These systems operate at very low pressures, often in a vacuum (e.g., at -40°F, ammonia saturates at -10.9 in. Hg vacuum). Air and moisture infiltration are major concerns. Blast freezers require multi-stage compression systems (booster systems) to manage the high compression ratios efficiently.

Summary of Temperature Ranges

Two-Stage Compression in Low-Temperature Applications

For systems operating in freezer and blast freezer ranges (suction temperatures below -10°F), the compression ratio becomes very high if compressing directly to standard condensing pressure (typically 120 psig to 180 psig). High compression ratios lead to low volumetric efficiency and extremely high discharge temperatures that break down lubrication oil. To solve this, industrial systems use two-stage compression. A booster compressor handles the low-stage compression (from low evaporator pressure to an intermediate pressure), and a high-stage compressor handles the final compression (from intermediate pressure to condensing pressure). An intercooler cooling vessel is placed between the stages to desuperheat the booster discharge gas and cool the liquid going to the low-temperature evaporators, which significantly increases overall system efficiency.