An air vent (air release valve) is a device that automatically discharges air accumulated inside steam or hot-water piping and equipment, preventing heating deficiencies caused by air lock. In short, when air stagnates at the inlet or high points of equipment, the inflow of steam and heat transfer are obstructed, and heating performance drops. By installing an air vent at such locations to release the trapped air, you can reduce start-up delays and temperature unevenness, making it easier to draw out the equipment’s inherent heat-transfer performance. This article organizes, from a practical standpoint, how air lock occurs, the role of the air vent, and the criteria for considering its installation.
Key points of this article
When air stagnates inside piping and equipment, it obstructs the inflow of steam and heat transfer, becoming a cause of insufficient heating and temperature unevenness. An air vent automatically discharges the trapped air, contributing to improved start-up and stable temperatures. If you are unsure about the installation location or equipment configuration, please consult Yoshitake.
How air lock and the air vent work
In equipment that uses steam, air enters the piping and equipment at start-up or while operation is stopped. When this air accumulates in a location where it cannot easily escape, it interferes with heating.
How air lock causes insufficient heating
The state in which air stagnates at the inlet or upper part of equipment is generally called air lock. Air has a lower thermal conductivity than steam, and when it covers a heat-transfer surface it obstructs the transfer of heat. Furthermore, the stagnant air partially blocks the inflow of steam, making it harder for steam to reach throughout the equipment. As a result, start-up is delayed or temperature unevenness occurs within the equipment, and heating performance drops. Rather than air acting as a complete “plug” that stops the flow, it is closer to reality to view it as degrading performance by obstructing both heat transfer and inflow.
How the air vent discharges air
An air vent is a mechanism that, through the action of an internal temperature-sensing element, float, or similar component, distinguishes air (low-temperature gas) from steam and opens the valve to discharge the air when it has accumulated. When the equipment fills with steam and the temperature rises, the valve closes, suppressing the wasteful release of steam. By venting early the air that tends to accumulate at start-up, it tends to lead to shorter start-up times and more stable temperatures.
An example of improvement through installation (the reasoning)
Suppose, for example, there is an inquiry about a tank with a heating jacket where “even when steam is supplied, the rise to the set temperature is slow, and only the upper part of the equipment is slow to heat up.” As a result of inspection, air was found stagnating in the upper part of the equipment, so an air vent was added at that location; the start-up delay and the temperature unevenness in the upper part were eased, and the stability of heating improved—this is a typical flow of improvement. Since the effect varies with the equipment structure and operating conditions, please understand this as no more than one example.
Criteria for considering an air vent
An air vent is not something that is simply “good to install”; correctly assessing the installation location and equipment configuration is important.
Thinking about installation location and equipment configuration
Because air tends to accumulate in the upper part of equipment or at high points in piping, the basic principle is to install the air vent where air tends to gather. Also, in steam-using equipment the discharge of condensate (drain) is likewise a concern, but how the air discharge and the condensate discharge are assigned depends on the configuration. In some cases a steam trap with an air-discharge function is used, and in other cases an air vent and a trap are combined according to the application. It is not necessarily the case that they must be provided separately; it is reasonable to consider this according to the equipment configuration and operating conditions.
Conditions to check when selecting
When selecting, check points such as the fluid used (steam or hot water), the pressure and temperature conditions, where in the equipment air tends to accumulate, and whether the discharge capacity matches the amount of air generated. All figures are guidelines, and the actual effect changes with the conditions. We recommend that you grasp the existing issues (slow start-up, temperature unevenness, etc.) specifically and then consider the location and the model.
Frequently asked questions
Q1. What is the difference between an air vent and a steam trap?
A1. An air vent is a device that mainly discharges stagnant air, while a steam trap is a device that mainly discharges condensate. There are also traps equipped with an air-discharge function, so their roles can partly overlap.
Q2. Does an air vent always have to be installed separately from a trap?
A2. Not necessarily separately. Depending on the equipment configuration, a trap with an air-discharge function may handle the job. The combination is considered according to the application and operating conditions.
Q3. Where is the appropriate place to install it?
A3. Because air tends to accumulate in the upper part or at high points, the basic principle is a location where air tends to gather. Since the optimal position varies with the equipment structure, it is considered based on drawings and the situation.
Q4. Is it safe to conclude that slow start-up is caused by air?
A4. It may be one factor, but it cannot be concluded definitively. Because multiple factors are conceivable—such as a faulty trap or conditions on the steam-supply side—we recommend checking across the entire system.
Q5. It looks as if steam is leaking from the air vent.
A5. Gas may be discharged temporarily at start-up, but if it is discharged continuously, deterioration of internal parts or a malfunction is suspected. If you let us know the situation, we will check it.
Q6. Can it be used in hot-water piping as well?
A6. Air pockets can also occur in hot-water systems, so there are models suited to that application. Please check the conditions of fluid, temperature, and pressure and then consult us.
Q7. How much of an effect can be expected?
A7. Improvement in start-up and temperature stability can be expected, but the effect varies with the equipment structure and operating conditions. Please regard the figures as guidelines.
Summary
The stagnation of air inside piping and equipment obstructs the inflow of steam and heat transfer, becoming a cause of insufficient heating and temperature unevenness. An air vent automatically discharges stagnant air and contributes to improved start-up and stable temperatures, but the effect depends on correctly assessing the installation location and equipment configuration. How the air discharge and the condensate discharge are assigned should reasonably be considered according to the configuration of the steam-using equipment.
If you have concerns such as “heating start-up is slow and we want to determine whether air is the cause,” “we cannot judge where in the equipment to install an air vent,” “we are unsure whether a trap with an air-discharge function or an air vent is more suitable,” or “we want to improve the temperature unevenness of existing equipment,” please make use of Yoshitake’s technical and selection consultation. If you let us know the fluid used, pressure, temperature, equipment structure, and current issues, we will propose considerations for the installation location and model.