The Air Compressor Thermostatic Valve Element (also referred to as a Thermal Valve Core or Temperature Control Core) is a crucial control component in the oil circuit system of rotary screw air compressors. Its primary function is to automatically regulate the flow of lubricating oil based on its temperature, ensuring the compressor unit operates within its optimal temperature range.
Below is a detailed technical guide covering the working principle, common brand specifications, failure symptoms, selection, and maintenance of thermostatic valve elements:
1. Working Principle and Function
During operation, the friction within the compressor air-end and the process of gas compression generate a significant amount of heat, which requires lubricating oil for cooling and lubrication.
Low-Temperature State (Cold Start / Below Set Temperature):
When the oil temperature is below the opening threshold of the valve element (e.g., below 68°C or 71°C), the thermal wax inside the element remains contracted. The valve stays closed or in a bypass position, allowing the oil to enter the air-end directly via a "short loop" without passing through the oil cooler. This allows the compressor to warm up quickly after startup, preventing excessive wear or moisture condensation caused by low oil temperatures.
High-Temperature State (Reaching Set Temperature):
As the oil temperature rises to the opening threshold, the expansion medium (thermal wax) inside the element expands, pushing the valve rod to move against spring tension. The bypass gradually closes, switching the oil circuit to the "main loop," forcing the lubricating oil through the oil cooler before injecting it back into the air-end.
Dynamic Regulation:
The valve element automatically adjusts its opening position between the short loop and main loop based on real-time oil temperature, keeping the injection temperature stably maintained within the ideal range of 75°C to 85°C.
2. Common Brand Specifications and Opening Temperatures
Opening temperatures and structural dimensions vary across different air compressor brands and models. Common temperature ratings include:
38°C / 45°C: Typically used in oil-free air compressors or specialized equipment requiring lower oil temperatures.
68°C / 70°C / 71°C: The standard setting for most oil-injected rotary screw air compressors (e.g., Atlas Copco, Ingersoll Rand).
75°C / 83°C: Commonly used in high-humidity environments or heavy-duty units with strict anti-condensation requirements (e.g., FuSheng, Sullair).
Representative Brands and Applications:
Atlas Copco: Common opening temperatures include 40°C, 60°C, and 71°C.
Ingersoll Rand: Common element settings include 60°C, 71°C, and 80°C.
Boge / Kaeser / Kobelco: Utilize specific thermal valve cores with pre-set temperatures (frequently sourcing universal elements from manufacturers like Danfoss or AMOT).
3. Common Failure Symptoms
The thermostatic valve element is a wearable service item. Subjected to oil quality degradation and thermal cycling, internal thermal elements can gradually age or fail.
Symptom | Possible Cause | Resulting Consequences |
High Discharge Temperature / High-Temp Shutdown | The element is stuck in the "short loop" position, or the thermal wax has lost elasticity, preventing oil from passing through the cooler. | Lubricating oil fails to cool, causing frequent compressor shutdowns due to high-temperature protection (> 100°C). |
Low Oil Temperature / Moisture Condensation in System | The element is stuck in the "main loop" (normally open) position, or the return spring is broken. | Oil temperature fails to rise after startup. Water vapor in the air condenses inside the oil-gas receiver, leading to oil emulsification and bearing corrosion. |
Severe Temperature Fluctuations | Internal jamming or a bent valve rod causes sluggish movement. | Unstable oil injection temperature, impacting overall compressor efficiency. |
4. Selection and Maintenance Recommendations
Procurement by Part Number (PN) or Specifications:
Always prioritize providing the Compressor Brand + Part Number (PN) when ordering.
If the part number is unavailable, measure the overall length, outer diameter, rod stroke, and the stamped opening temperature (e.g., 71°C) on the valve element.
Replacement Interval:
It is recommended to inspect or replace the thermostatic valve element during major overhauls or every 8,000 to 12,000 operating hours.
If oil varnishing or severe degradation occurs, clean or replace the element early to prevent seizing.
Simple Testing Method:
Submerge the removed valve element in hot water heated to 5°C to 10°C above its stamped opening temperature. Observe whether the valve rod extends smoothly. Remove it, let it cool, and check if it resets via spring force. If it fails to move or the stroke is insufficient, the element has failed and needs replacement.
