Quick answer: Solenoid valves are electrically operated valves that open or close a pipe when their coil is switched on or off. The main types are direct-acting and pilot-operated, and normally closed and normally open. To choose one, match the fluid, pipe size, flow (Kv), pressure difference, coil voltage and type, and temperature rating. In the UAE, also check the coil’s heat rating and choose an enclosure or IP rating that suits the site.
What Is a Solenoid Valve and How Does It Work?
A solenoid valve controls the flow of liquid, gas or refrigerant using an electric coil. Power to the coil creates a magnetic field. In a normally closed direct-acting valve, the field lifts a plunger and opens the port, and a spring pushes the plunger back when power stops. In a pilot-operated valve, the coil opens a pilot passage and the pressure differential helps operate the main valve. Normally open valves work in the opposite fail position.
In refrigeration, a solenoid valve usually sits in the liquid line. In a pump-down system, the liquid-line solenoid closes first and the compressor keeps running until the low-pressure control stops it. Its main purpose is to reduce off-cycle refrigerant migration and flooded starts, rather than flood-back during normal running. Solenoid valves may also be used in dedicated hot-gas or defrost circuits, which use their own valves rather than the liquid-line valve.
What Are the Main Types of Solenoid Valves?
| Type | How it works | Best for | Watch out for |
|---|---|---|---|
| Direct-acting | Coil lifts the seat directly | Small lines, low or zero pressure difference | Limited flow size |
| Pilot-operated | Line pressure helps open the main seat | Larger pipes, higher flow | Needs a minimum pressure difference to open |
| Normally closed (NC) | Closed without power | Most refrigeration liquid lines | Closes on power loss |
| Normally open (NO) | Open without power | Bypass lines | Uses power while closed |
A 3-way valve switches flow between two paths and is common in defrost circuits.
How Do You Size a Solenoid Valve?
Step 1: Confirm the fluid. Check that the body, seals and coil suit your refrigerant, oil, water or air. Not every valve works with every refrigerant.
Step 2: Find the flow you need. For water, use Q (m³/h) = Kv × √Δp, where Δp is the pressure drop in bar. For refrigerants, use the manufacturer’s capacity table. It needs the refrigerant type, evaporating temperature, condensing temperature and system capacity in kW.
Step 3: Check the pressure difference. Every valve has a maximum operating pressure differential (MOPD). MOPD and the minimum opening differential both refer to the pressure difference across the valve, not the total system or line pressure. For pilot-operated or servo-operated valves, confirm the minimum opening differential before you choose.
Step 4: Match the coil. Confirm the exact coil voltage, AC or DC type, and frequency where applicable. Common supplies include 24 V and 230 V, but a 24 V AC coil and a 24 V DC coil are different products and should not be treated as interchangeable.
Step 5: Do not oversize. Select a Kv suitable for the required flow and allowable pressure drop. An oversized pilot or servo valve may not see enough pressure differential to operate reliably. Liquid hammer is mainly related to rapid changes in liquid velocity when a valve opens or closes quickly, so consider the line layout and valve speed as well.
Worked example: A chilled water line needs 4 m³/h with an allowed pressure drop of 0.5 bar.
- Kv = 4 ÷ √0.5 = 4 ÷ 0.707 = 5.66
The required Kv here is about 5.66, so a valve around Kv 6 can be suitable. Then check the valve’s MOPD against the pressure differential across it, not the line pressure, and confirm its minimum opening differential if it is pilot or servo operated.
Why Does UAE Climate Change Solenoid Valve Selection?
- Heat: Coils warm themselves when energised. In plant rooms and on roofs above 45 °C, check the coil’s ambient rating.
- Dust: Do not assume one IP rating fits every site. Select the coil enclosure rating according to the actual environment and the manufacturer’s approved coil options.
- Coastal air: Salt and moisture speed up corrosion, so look for brass or stainless steel bodies.
- Power supply: Check the coil’s voltage tolerance if your site has fluctuations.
For refrigerant system safety rules, see the ASHRAE Standard 15 overview.
What Mistakes Cause Solenoid Valve Failure?
- Ignoring MOPD and the minimum opening differential. If the differential pressure exceeds MOPD, the valve may fail to open. Pilot or servo-operated valves may also operate unreliably if the differential pressure is below their minimum opening differential.
- Fitting the valve against the flow arrow.
- Skipping the filter drier or strainer, so debris jams the seat.
- Energising a coil that is not fitted on the valve, which can burn it out.
- Using a solenoid valve as a safety device. It is a control valve, not a relief valve.
Which Solenoid Valves Does Haste Offer?
Haste supplies solenoid valves for refrigeration and HVAC across the UAE. Send us your refrigerant, line size, pressure and coil voltage and type, and we will help match the right valve.
Frequently Asked Questions
What is a solenoid valve used for? It starts, stops or redirects flow in refrigeration, HVAC, water and air systems. In refrigeration, it is most common in liquid lines.
What is the difference between direct-acting and pilot-operated solenoid valves? A direct-acting valve opens using the coil alone. In a pilot-operated valve, the coil opens a pilot path and line pressure helps open the main valve, so it handles bigger flows but needs a minimum pressure difference.
Should I choose a normally closed or normally open valve? Normally closed is the usual choice for refrigeration liquid lines, because the line shuts when power is lost.
What does Kv mean? Kv is the flow in m³/h of water that passes through a valve at a 1 bar pressure drop. A higher Kv means more flow.
Can one solenoid valve be used for any refrigerant? No. Seals, body material and capacity vary, so check the manufacturer’s refrigerant list.
Summary
- A solenoid valve uses a coil to open or close a line.
- Pick direct-acting for small lines and pilot-operated for larger flow.
- Size by Kv for the required flow and pressure drop, and check MOPD against the pressure differential across the valve, not pipe size alone.
- Confirm the coil voltage, AC or DC type and frequency.
- UAE heat, dust and coastal air affect coil rating, IP protection and body material.
Haste Trading has supplied HVAC/R products in the UAE since 1983. If you are unsure which valve fits, get in touch with our team.
This guide draws on general refrigeration practice and ASHRAE 15. Always confirm requirements against the project specification and local authority rules.