Understanding Thermostatic Valves in Compressor Systems

All about Thermostatic Valves in Compressor Systems

Introduction to thermostatic valves

Thermostatic valves play a crucial role in oil-injected screw compressors by regulating the flow of oil to the cooling jacket around the cylinder. This helps control the temperature of compressed air and prevents oil starvation, ensuring the proper functioning of the compressor. In this article, we will explore the working of thermostat valve and the benefits they offer to compressor systems.

What are thermostatic valves and why are they used?

A thermostatic valve is a device that controls the temperature of compressed air by regulating the oil flow to the cooling jacket around the rotating elements. It works by sensing the temperature of the compressed air and adjusting the oil flow accordingly. This helps to maintain a consistent temperature of the compressed air, which is crucial for the proper functioning of the compressor.

What does a thermostatic valve do?

A thermostatic valve operates using a self-acting temperature-sensitive element that expands or contracts based on the temperature changes of the compressed air (heating System). When the temperature of the compressed air rises, the temperature-sensitive element expands, opening the valve to allow more oil to flow to the cooling jacket. Conversely, when the temperature of the compressed air drops, the temperature-sensitive element (sensor) contracts, reducing the oil flow to the cooling jacket.

These valves are also known as temperature regulating valve (trv) or thermostatic radiator valves due to their principle of working.

Benefits of Using a Thermostatic Valve

There are several benefits to using a thermostatic valve in an oil-injected screw compressor system. Some of the key benefits include:

Maintains Consistent Air Temperature:

A thermostatic valve helps to maintain a consistent temperature of the compressed air, which is essential for the proper functioning of the compressor.

Protects Against Oil Starvation:

By regulating the oil flow to the cooling jacket, a thermostatic valve helps to prevent oil starvation, which can cause damage to the compressor.

Increases Compressor Efficiency:

By maintaining a consistent temperature of the compressed air, a thermostatic valve helps to increase the efficiency of the compressor, leading to lower energy costs.

Improves Compressor Longevity:

By preventing oil starvation, a thermostatic valve helps to improve the longevity of the compressor, reducing the need for repairs and maintenance.

Types of Thermostatic Valves

There are several different types of thermostatic valves available on the market, each designed to meet specific requirements. Some of the most common types include:

Piston Type:

A piston-type thermostatic valve uses a piston to control the oil flow to the cooling jacket. The piston is connected to a temperature-sensitive element that expands or contracts based on the temperature of the compressed air.

Poppet Type:

A poppet-type thermostatic valve uses a spring-loaded poppet to control the oil flow to the cooling jacket. The poppet is connected to a temperature-sensitive element that expands or contracts based on the temperature of the compressed air.

Bellows Type:

A bellows-type thermostatic valve uses a bellows to control the oil flow to the cooling jacket. The bellows is connected to a temperature-sensitive element that expands or contracts based on the temperature of the compressed air.

Installation Methods for thermostatic radiator valve

There are several ways of installing thermostatic valves, but the two most common applications are “mixing” and “diverting”. When diverting the fluid, temperature into the valve is being controlled. When mixing the fluid, temperature from the valve is

controlled. In mixing mode the valve may run at 1-2˚C (1.8-3.6˚F) higher than the normal set temperature.

Thermostatic Valve Selection Flow Chart

Valve Sizing for thermostatic radiator valves

Pressure Drop highest setting

The normal recommendation in sizing Thermostatic valves is to select a pressure drop between 0.14 to 0.5 bar (2 and 7 Psi).

Valve Flow Coefficient for flow of water

A Cv is the valve’s flow coefficient (Cv), it is defined as the number of US gallons per minute of room temperature water which will flow through the valve with a pressure drop of 1 Psi across the valve. The basic formula to find a valve’s Cv is shown below. (Kv is metric coefficient).

Viscosity Correction for thermostatic valve head

For the selection of valves for use with more viscous fluids than water the following must be calculated in addition to using the previously mentioned formulas.

Viscosity:

Find the viscosity of the fluid which shall be used in the valve. This will generally be in centistokes – ISO grade oil is easy to calculate as the grade no. is the viscosity. i.e. ISO VG 46 = 46 centistokes at 40˚C.

Viscosity Correction:

Once the viscosity value has been found, by using the graph below the Flow Coefficient correction factor can be established. The correction value that is produced by the graph should then be multiplied by the original Flow Coefficient. This gives the corrected Flow Coefficient which can then be used in the standard formulas.

Example: 100 CST = correction factor of 0.68

0.68 x Flow Coefficient = corrected Flow Coefficient (Kv or Cv)

Trouble-Shooting

In the event that the cooling system does not operate close to the desired temperature, the following guide may help to identify or locate the problem.

System temperature too cold

1. Insufficient heat transferred to coolant to maintain temperature.
2. Wrong nominal element sensor temperature selected.
3. Thermostatic valve greatly oversized or cooling capacity of system much greater than required.
4. Thermostatic valve installed backwards, thus sending water to cooler at low temperatures.
5. Worn or leaking O-rings allowing leakage to cooler.
6. Excessive pressure drops across the valve.
7. Foreign matter preventing closure of elements.
8. Bi-metallic type thermometers will indicate low if calibrated in oil then in hot water.

System temperature too hot

1. Cooling capacity of system inadequate wrt temperature rises.
2. Thermostatic valve body too small for flow rate causing high pressure drops and possible cavitation problems.
3. Valve installed backwards, reducing flow to cooler as temperature increases.
4. By-pass will not close due to worn or pitted seats, sticking valve, sliding valve, seals, worn valve piston etc.
5. Elements may have suffered sufficient over-temperature to prevent full movement, thus preventing full cooling.
6. Solids building up on element sliding valve preventing correct operation.
7. Foreign matter stuck between sliding valve and seat.

HYSTERESIS in a Thermostatic Valve

The gap found between the upstroke and downstroke curves is the element hysteresis. The hysteresis is caused by element temperature delay and by the friction of parts in motion affecting temperature control.

Conclusion

Thermostatic valves play a crucial role in oil-injected screw compressors, helping to regulate the flow of oil to the cooling jacket and control the temperature of compressed air. By preventing oil starvation, increasing compressor efficiency, and improving compressor longevity, thermostatic valves are a key component in ensuring the proper functioning of the compressor system.