Fixed Metering Device Superheat Calculation

Understanding the Purpose First

Before learning calculations, it’s important to understand why we measure superheat. The main purpose of superheat is to prevent liquid refrigerant from entering the compressor. Liquid refrigerant is not compressible, and if it reaches the compressor, it can cause serious mechanical damage (floodback or slugging).

We want only 100% vapor refrigerant entering the compressor.

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What Is Superheat?

A refrigerant is said to be superheated when its temperature is above its boiling (saturation) temperature at a given pressure.

Superheat = Actual Temperature – Boiling point

Example:

• Saturation temperature (boiling point) = 40°F

• Actual suction line temperature = 55°F

Superheat=55°F−40°F=15°F

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What Happens Inside the Evaporator?

To understand superheat, you must first understand the refrigerant process:

1. Liquid to Vapor

• Refrigerant enters the evaporator as a low-pressure, low-temperature liquid.

• It absorbs heat from the air and begins to boil.

2. Saturation State (Boiling)

• While boiling, the refrigerant stays at a constant temperature.

• It stays at a constant temperature because the heat added to the system is not used to increase temperature; instead, it is used to break intermolecular bonds, causing a phase change from liquid to gas (latent heat of vaporization).This is called the saturation temperature.

3. Superheating State

• Once all the liquid has turned into vapor (100%), any additional heat increases the temperature of the vapor.

• This temperature rise above the boiling point is called superheat.

As we stated earlier, we want superheated vapor leaving the evaporator so that no liquid enters the compressor. This ensures that Only vapor is compressed, the compressor is protected and e system operates efficiently

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Fixed Metering Device Behavior

A fixed metering device (capillary tube or fixed orifice) cannot adjust refrigerant flow. Like a TXV because of this with a fixed orifice, superheat will change as system conditions change.

Changes in indoor load, changes in airflow and changes in outdoor temperature all cause superheat to rise or fall

Since superheat is always changing in a fixed orifice system, we compare: Actual superheat (measured) with Target superheat (calculated from conditions) to ensure the evaporator is fully boiling off refrigerant without sending liquid to the compressor.

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How to Find Measured Superheat (Step-by-Step)

Step 1: Measure Suction Pressure

• Use a gauge on the suction line

Step 2: Find Saturation Temperature

• Use a pressure-temperature (P-T) chart

Step 3: Measure Actual Temperature

• Measure suction line temperature near the evaporator outlet

Step 4: Calculate Superheat

Measured Superheat=Measured Temp−Saturation Temp

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How to Find Target Superheat (Step-by-Step)

Target superheat is based on Indoor Wet Bulb (IWB) → indicates indoor heat load and Outdoor Dry Bulb (ODB) → indicates outdoor conditions. The recommended way is to use a manufacturer superheat chart as this gives the most accurate target value.

In the absence of a chart, use this approximation formula instead:

Target Superheat = [(3 × IWB) − 80 − ODB] ÷ 2

Step 1: Measure Indoor Condition (Heat Load)

• Measure Indoor Wet Bulb Temperature (IWB)

• Take reading in the return air

• This tells you how much heat the evaporator needs to remove

Step 2: Measure Outdoor Condition

• Measure Outdoor Dry Bulb Temperature (ODB)

• Take reading near the condenser air inlet

• Avoid direct sunlight on your thermometer

Step 3: Calculate Target Superheat

Given:

• IWB = 63°F

• ODB = 90°F

Target SH= [(3×63) −80−90] ÷2

= (189−80−90) ÷2

=19÷2 =9.5°F

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Evaluating the Reading

we have to compare Actual Superheat to Target Superheat.

• Actual Superheat>Target Superheat: Not enough refrigerant, Evaporator starved (Add Refrigerant)

• Actual Superheat

• Within ±3°F of Target Superheat: Proper charge