4.1 Saturated Ammonia P-T Tables

4.1 Saturated Ammonia P-T Tables

The Saturation Relationship of Anhydrous Ammonia (R-717)

For industrial refrigeration operators, the saturated pressure-temperature (P-T) table is one of the most critical diagnostic tools. In thermodynamics, saturation describes a state of phase equilibrium where a substance exists as both liquid and vapor simultaneously at a specific temperature and pressure. During this state, the rate of boiling (liquid turning to vapor) is exactly equal to the rate of condensation (vapor turning to liquid).

Under saturated conditions, temperature and pressure are dependent variables. This means that if you change the temperature, the pressure must change correspondingly, and vice versa. For example, if liquid ammonia and ammonia vapor are in equilibrium inside a vessel:

• If heat is added to the vessel, some liquid will boil, increasing the vapor density and raising the pressure. The temperature of both the liquid and vapor will rise to a new saturation point.
• If heat is removed, some vapor will condense, reducing the pressure and lowering the temperature to a new saturation point.

Because temperature and pressure are locked together in a saturated mixture, an operator can determine the temperature inside an evaporator or condenser simply by reading a pressure gauge and referencing a P-T table.

Saturated Liquid vs. Saturated Vapor

In a refrigeration system, we encounter two main saturated states:

1. Saturated Liquid: Liquid refrigerant that is at its boiling point for a given pressure. It is 100% liquid. Any addition of heat will cause some of this liquid to flash or boil into vapor.
2. Saturated Vapor: Vapor refrigerant that is at its condensing point for a given pressure. It is 100% vapor. Any removal of heat will cause some of this vapor to condense back into liquid.

Between these two states lies the wet vapor region (or quality region), where a mixture of saturated liquid and saturated vapor exists together. In an evaporator or condenser, the refrigerant is in this mixture state, meaning its temperature is always at the saturation temperature corresponding to the operating pressure.

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Pressure Scales: Gauge vs. Absolute

To read a P-T table correctly and avoid severe diagnostic errors, operators must distinguish between gauge pressure and absolute pressure:

• Absolute Pressure (psia): Measured relative to a perfect vacuum, which is defined as 0 psia. A perfect vacuum represents the complete absence of air or molecular pressure.
• Gauge Pressure (psig): Measured relative to the local atmospheric pressure. Standard atmospheric pressure at sea level is approximately 14.7 psi. Therefore, a standard pressure gauge open to the atmosphere reads 0 psig, which is equivalent to 14.7 psia.

The formulas to convert between these two scales are:

• psia = psig + 14.7
• psig = psia - 14.7

[!IMPORTANT] Always check the column headers on a P-T table. Some tables are printed in absolute pressure (psia), while others are printed in gauge pressure (psig). Confusing the two introduces a 14.7 psi error. For example, if a table lists pressures in psia and you look up a gauge reading of 15.7 psig directly without adding 14.7, you will determine the temperature to be -28°F (the saturation temp for 14.7 psia/0 psig) instead of the correct temperature of 0°F (the saturation temp for 30.4 psia/15.7 psig).

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Vacuum Conditions: Operating Below 0 psig

When an ammonia refrigeration system operates at a pressure below atmospheric pressure, it is operating in a vacuum. In this regime, gauge pressure is negative, and standard gauges represent this using inches of mercury vacuum (in. Hg vac).

• 0 in. Hg vac corresponds to 0 psig (or 14.7 psia).
• 29.92 in. Hg vac represents a perfect vacuum (or 0 psia).

A common rule of thumb in the field is that 2 inches of mercury vacuum is approximately equal to 1 psi below atmospheric pressure (more precisely, 1 psi = 2.036 in. Hg).

To estimate the absolute pressure (psia) from a vacuum reading in in. Hg:

• psia = 14.7 - (in. Hg vacuum / 2)

For example, if a low-temperature accumulator gauge reads 10 in. Hg vacuum:

• psia = 14.7 - (10 / 2) = 9.7 psia Using the exact conversion factor (1 in. Hg = 0.4912 psi):
• psia = 14.7 - (10 * 0.4912) = 9.79 psia

The Risks of Operating under Vacuum

Many industrial low-temperature applications, such as blast freezers and low-temperature storage rooms, require temperatures below -28°F. Since the boiling point of ammonia at atmospheric pressure is -28°F, achieving lower temperatures requires the system's low-side pressure to drop below 0 psig into a vacuum.

Operating a refrigeration system in a vacuum presents severe operating risks:

1. Air Ingress (Non-Condensable Gases): If a leak occurs in any low-side component (e.g., valve stem packing, flange gaskets, pipe welds, or compressor shaft seals), the lower pressure inside the pipe will draw atmospheric air into the system. Unlike ammonia, air is a non-condensable gas under standard refrigeration operating conditions. Air travels through the system and collects at the top of the condenser. This air blankets the condenser tubes, reducing the available heat transfer surface area. This results in:
   • Abnormally high condensing (discharge) pressures (head pressure).
   • Elevated compressor discharge temperatures.
   • Increased compressor work and electrical power consumption.
   • Reduced overall system cooling capacity.
2. Moisture Ingress: Atmospheric air always contains water vapor. When air is drawn into a vacuum-operating system, moisture enters with it. Ammonia has an extremely high affinity for water. The water dissolves into the ammonia, forming ammonium hydroxide (NH4OH), which is highly corrosive. Moisture ingress leads to:
   • Corrosion of steel piping and valves.
   • Attack of copper-bearing alloys (which may be present in instruments, bearings, or seals).
   • Chemical breakdown (sludging) of compressor lubricating oil.
   • Formation of sticky acid-oil emulsions that clog expansion devices, oil filters, and strainers.
   • Increased evaporator temperatures (water raises the boiling point of the ammonia-water mixture, reducing heat transfer efficiency).

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Reading and Interpreting Saturated Ammonia Tables

A standard saturated ammonia table contains several columns, typically including Temperature, Saturation Pressure (in psig, psia, or in. Hg vacuum), Specific Volume, and Enthalpy. The columns of primary concern for daily operations are Temperature and Saturation Pressure.

Below is a representative sample of a saturated ammonia P-T table showing key operating reference points:

Step-by-Step Table Walkthrough

To locate values using the table:

1. Identify the Given State: Determine if you are starting with a known temperature or a known pressure.
2. Locate the Unit Column: Ensure your gauge readings match the correct column (gauge pressure vs. absolute pressure, or vacuum vs. positive pressure).
3. Find the Value:
   • If given a suction pressure of 15.7 psig, scan down the Gauge Pressure column to find 15.7 psig. Look horizontally to the left column to find the corresponding saturated evaporator temperature of 0°F.
   • If given an evaporator operating temperature of -40°F, find -40°F in the Temperature column. Look horizontally to the right to find the target suction pressure of 8.7 in. Hg vacuum.

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Practical Operator Exercises (Worked Examples)

Worked Example 1: Calculating Superheat

Scenario: An operator is troubleshooting a liquid overfeed evaporator. A pressure gauge on the suction line reads 33.5 psig. A thermocouple strapped and insulated on the same suction line reads 30°F. Is the suction vapor superheated, and if so, by how much?

Step-by-Step Solution:

1. Find Saturation Temperature: Locate the suction pressure of 33.5 psig on the saturated ammonia table. The corresponding saturation temperature is 20°F.
2. Read Actual Temperature: The actual measured temperature of the suction line is 30°F.
3. Calculate Superheat: Superheat is the difference between the actual temperature of the vapor and its saturation temperature at that pressure:
   • Superheat = Actual Temperature - Saturation Temperature
   • Superheat = 30°F - 20°F = 10°F Diagnostic Meaning: The vapor is superheated by 10°F. This indicates that all liquid has boiled off in the evaporator, and the vapor has absorbed an additional 10°F of sensible heat. This is a safe condition for a compressor, as it ensures no liquid is returning to cause compressor slugging.

Worked Example 2: Detecting Non-Condensable Gases

Scenario: A plant has an evaporative condenser operating on a hot summer day. The outdoor wet-bulb temperature is 68°F, which should result in a condensing temperature of 86°F based on the condenser manufacturer's design approach of 18°F (Approach = Saturation Temperature - Wet-Bulb Temperature). However, the compressor discharge pressure gauge reads 181.1 psig. What is the expected saturated temperature, and what does this indicate?

Step-by-Step Solution:

1. Find Saturation Temperature from Discharge Pressure: Locate the discharge pressure of 181.1 psig on the saturated ammonia table. The corresponding saturation temperature is 95°F.
2. Compare with Design Saturation Temperature: The design saturation temperature is 86°F (corresponding to 154.5 psig). The actual saturation temperature is 9°F higher (95°F - 86°F = 9°F).
3. Analyze the Deviation: A saturation temperature higher than design indicates that the condenser is not performing efficiently. Since the wet-bulb temperature is normal, the 9°F elevation in saturation temperature suggests that the condensing pressure is being artificially inflated by non-condensable gases (air) trapped in the condenser, or the condenser tubes are heavily scaled.

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Operating and Safety Precautions for Vacuum Systems

Due to the inherent hazards of air and moisture ingress, operators must observe the following precautions when managing systems that operate below 0 psig:

• Monitor the Purger constantly: Industrial ammonia systems operating in a vacuum must be equipped with an automatic non-condensable gas purger. A sudden increase in purger activity or water-bubbler venting indicates a low-side air leak.
• Oil Analysis: Regularly sample compressor oil. A milky appearance indicates water contamination. Oil analysis showing high water content (above 100-200 ppm) requires immediate oil replacement to prevent bearing failure.
• Maintenance Pumpdown Safety: Before opening any low-side piping for maintenance, it must be pumped down to remove ammonia. However, pulling a deep vacuum on a section to be opened is dangerous, as air will rush in as soon as a flange is loosened. Always break the vacuum by pressurizing the isolated section slightly above atmospheric pressure (1 to 2 psig) using dry nitrogen or warm ammonia vapor before opening the piping. This creates an outward sweep of gas, preventing air and moisture from entering.