It is often desirable to utilise vent or waste steam rather than simply release it to the atmosphere or condense it into water to eliminate the steam. The difficulty has been finding a way to safely and stably condense the steam into a process fluid so the energy can be used in a productive manner. Vent steam is often variable in pressure and availability. Traditional heat exchangers often have trouble functioning in this manner making them problematic.
Some issues associated with heat exchangers
Sizing the exchangers to handle the liquid and steam flow results in the heat exchangers overly large, adding expense and pressure drop to the system
Sizing the trap system for the heat exchanger is difficult to accomplish without the potential of overloading the system with live steam
ProSonix Solution
The ProSonix J-Series Jet Sparger allows the vent steam to be utilised in a safe and simple manner. The flexibility in mounting the heater allows the steam to be condensed in a wide range of pipe sizes, or into a tank or other vessel. Control of the steam addition can be accomplished in a simple manner. Often, a pressure switch is utilised on the vent steam line, activating the PSX heater to quickly condense the steam. An air cylinder on the ProSonix heater quickly opens to allow the steam to disperse quickly and quietly into the flowing fluid. When the steam vent closes, or the pressure drops, the air cylinder on the ProSonix heater closes.
Features & Benefits
It reduces the cost of waste steam that is vented or flashed and sent to drain
Flexible installation as it can be installed inline in the pipe or on a tank or vessel wall
Can be installed directly in the pipe of a tank, thus eliminating the need for floor space or expensive re-working of piping.
Single or multiple unit configurations for tank heating can be installed to allow for large volume heating and then trim heating with a single unit open
Inline Process Water Heating
Inline process heating has many applications in a variety of process and utility applications. Many industries, such as food, chemical and pulp and paper make use of inline process heating for water, slurry, sludge and aggressive fluid heating applications. Direct steam injection for inline heating is a very good choice for a variety of applications. One of the fundamental principals for efficient and reliable steam injection is the ability to produce and deliver high velocity steam. High velocity steam is what assures rapid and complete condensation and mixing of the steam in the fluid. Often people will look at all forms of steam injection as the same and then make a investment decision based on price.
Internally Modulated vs. Externally Modulated Steam Control
There are two distinct design types of Direct Steam Injection, relative to steam control and they can have a significant impact on the steam injection heater’s performance.
Externally Modulated Steam Control
Externally modulated steam control has been a common approach for direct steam injection heating. This approach uses a Remote Steam Control Valve (PRV) to throttle (reduce) the steam pressure prior to a fixed opening steam injection point. Typically the steam pressure needs to be reduced at least 50% to control the amount of steam for temperature control. As the steam injection point has a fixed opening area, the reduced steam pressure also reduces the velocity (sub-sonic flow) of the injected steam.
Issues
Effective steam condensation rates are dramatically reduced as steam velocity is goes down. When low velocity steam injection occurs, sub sonic steam conditions exist. This results in:
Eliminate Steam Hammer
Low velocity steam leads to uncondensed steam bubbles, which tend to collapse against the pipe walls. This is what leads to the vibration and steam hammering
Uncondensed steam can also travel past the temperature sensor which results in temperature control issues such as over heating
Process upsets are common and damage to equipment can occur from the steam hammer
Maintenance Issues result when steam collapses on the surface or in the steam injector, which leads to excessive wear which increases maintenance costs and reduces reliability
Ultimately problems arise when the steam pressure has dropped to a point where the steam flow is no longer choked and is sub-sonic. This can be seen most often at start-up and shut-down of externally modulated steam injectors in the form of noise, vibration and hammering. With an external steam valve, there is always a pressure drop before the steam reaches the injector. Some sparger devices use passive springs and tube assemblies that tend to stick, resulting from scale build-up, causing temperature control problems. The use of external steam control devices to control the steam flow by modulating the steam pressure can lead to excessive steam hammering and vibration. Steam hammering and vibration often result from poor mixing and condensing of the steam.
As stated above, efficient and reliable condensation of steam is directly dependent on steam velocity. By using a steam pressure reducing valve for control, steam flow will become sub-sonic. The way to overcome this problem is to inject steam at choked flow conditions. Choked flow conditions allow steam to be injected at sonic flow velocities.
ProSonix’s unique method of steam injection utilises an internal steam control to precisely deliver the appropriate mass flow of steam, and not the pressure, for the required heating. This is achieved via and integral pneumatic actuator, and a variable position stem plug in the steam jet diffuser. We do not throttle or regulate steam pressure. This design offers a precise method of steam control through a choked flow control delivery of the steam.
Choked flow is the phenomenon of accelerating a vapor to maximum velocity by creating a pressure differential through an engineered nozzle. By establishing choked flow, the steam mass flow can be metered to precisely control the heating of the liquid. This produces predictable results based on position of the stem plug. Through a variable area steam diffuser, steam flow is metered at the point where steam and liquid first contact and mix. Internally Modulated DSI heating controls the mass flow of the steam and not the pressure.
Key Benefits of Internally Modulated Steam
No process upsets. High velocity steam flow optimizses the steam mixing and condensation with the liquid and eliminates problems with vibration/steam hammering
No steam control valve required asthis method eliminates the need for an external steam control valve or downstream mechanical mixing devices
Rapid and complete condensation of the steam allows for temperature reliable control of +/- 1°F (-17°F).
High velocity steam also is self cleaning and eliminates debris along scale and mineral build-up on the steam diffuser
Lower maintenance costs as proper condensation of the steam eliminates excessive wear
Low pressure drop of typically 1-2 psig reduces pump demand and energy consumption
The PSX inline heater can be controlled either locally or remotely via the plant DCS or PLC control system. Remote control inputs typically used are 3-15 psig, 4-20 mA, or other Fieldbus devices. The PSX heater can be installed on the incoming water or slurry line or in a re-circ heating loop to the tank, re-circulating through the heater until the tank reaches a steady state temperature. The PSX heater has a high heating capacity and can achieve a temperature rise from 1°F to 250°F (-17°C to 121°C) in a single pass through the heater.
Tank Heating
Jet Sparger Optimisation
Jet Sparger Optimisation Heating of water tanks or vessels in many applications is done to stage hot water for use in production, cleaning, line and filter flushing and a number of other uses. Common methods of tank heating are through the use of heat exchangers, eductors or spargers. The incoming water is usually around 45°F – 60°F (7°C – 16°C) and is typically heated anywhere from 100°F – 150°F (38°C – 66°C) for applications involving sanitizing or cleaning.
Tanks and vessel systems typically heated by the use of heat exchangers, eductors, and steam spargers can experience the following issues:
Eliminate Steam Hammer
Spargers and eductors may damage the tank because of the sometimes violent nature of the condensation process
Using an external steam control valve to control steam pressure produces low velocity steam which leads to inefficient steam mixing and condensation. Uncondensed steam will either produce steam hammering or escape to atmosphere leading to energy losses
Heat exchangers require a condensate return system. Plug and ineffective steam traps along with flash releases can lead to energy losses and maintenance issues
Heating of water can leave mineral and scale build-up that causes maintenance and performance problems for heat exchangers and spargers
Some sparger devices use passive springs and tube assemblies that tend to stick, resulting from scale build-up, causing temperature control problems
Jet Diffuser Efficiency
High velocity steam delivery assures complete mixing of steam, reducing occurrences of vibration and saves energy losses from uncondensed steam escaping. Integral stem plug eliminates need for steam pressure control valve.
Single or Multiple Jet Sparger Configurations
A single Jet Sparger can be used for small tanks. When heating larger tanks, or if a higher temperature rise is required, multiple PSX Spargers (2-4) can be placed around the tank. Multiple Jet Sparger configurations also allow for rapid heat-up, and when desired temperature is achieved, (1) PSX Jet Sparger can be operated for trim temperature control.
PSX Heater Solution
Install the PSX Jet Diffuser Tank Mount Sparger on the tank wall to the heat the tank until the tank reaches a steady state temperature. Various size tanks can be accommodated with single or multiple Jet Sparger arrangements.
The PSX heater has a high heating capacity which accelerates tank heat-up time over the time it takes conventional steam sparging. Once the desired tank temperature is achieved, a single the PSX heater steam injector can be operated to allow for trim temperature control of the tank until the desired tank temperature achieved. Injection of high velocity steam minimises improves condensation and mixing, thus structural damage to the tank from the sparger can be eliminated. As the PSX heater utilises sonic velocity steam injection, scale and mineral build issues in the steam injector is eliminated.
The tank temperature can be controlled via a tank sensor, a discharge sensor, a cascade control loop or in a variety of configurations allowing consistent precise temperature control. The internal steam control design of the PSX heater controls the steam mass flow and not the steam pressure, thus eliminating steam hammering and vibration.
Note: Use of tank sparging for applications above 150°F (66°C) may be inefficient and lead to extended heating times. For tank heating applications above 150°F (66°C), the PSX Inline heater is recommended.
Key Direct Steam Injection Benefits
Energy savings resulting from more efficient steam condensation, faster tank heat-up and reduced heat loss to atmosphere
Lower maintenance due to the PSX Jet Spargers self cleaning design
Improved safety due to better steam injection heating methods (elimination of steam hammering)
Better temperature control allows for a more reliable heating process
Reduced maintenance costs from the elimination of condensate return system
Application: Waste & Vent Steam Condensation
Jet Sparger Optimisation