Prospective users of steam injection heating equipment often ask how to select an appropriate inline heater or tank jet sparger for an application. The answer to that question will vary depending on the type of fluid and the process heating requirements for the application. The fluids to be heated can range from clear water to high solids slurries or high viscosity wastewater and sludge. This article will discuss heating options for both inline steam injection heating and tank jet spargers with a focus on water heating applications.
Both inline steam injection heaters and tank jet spargers can be used in fluid heating applications. Depending on the heating application, one heater design might be a better choice than the other based upon heater design requirements, process heating goals and project economics. Taking a look at common water heating applications can help illustrate when an inline steam injection heater or tank jet sparger can be used.
Industrial Inline Water and Steam Heating
Methods of inline heating using steam injection include single-pass heating and inline recirculating heating (figure 1).
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Single-Pass Heating. In applications requiring single-pass heating, the water is heated as it passes through the heater. Then, it is sent to a downstream process or user before being discharged outside the water-heating loop. Typical applications include:
- Industrial boiler water conditioning for feedwater preheating, deaerator water heating, condensate heating or reverse osmosis systems.
- Meat and poultry processing such as carcass washing, chicken paw scalders and clean-in-place (CIP) hot water for plant sanitation.
- Vegetable and potato processing such as hot water can topping, hot water blanching and pasteurization.
When selecting a steam injection heater for single-pass heating, consider the following:
- The temperature rise increment can vary. For instance, some applications may only require increasing the inlet water temperature by 10 to 15°F (5.5 to 8.3°C). Other applications may require a 200°F (111.1°C) temperature rise increment in a single pass.
- Applications may have a high flow rate or high pressure requirements, and these requirements must be taken into account. For instance, one application may involve complex, engineered, high flow water heating in a range from 100 to 15,000 gal/min with 8:1 turndown requirements. Another may have steam and temperature ratings up 600 psig at 600°F (315°C). Still another may require simple water heating for low to moderate water flow of 1 to 500 gal/min with up to 3:1 turndown and steam pressure from 50 to 150 psig.
In addition, when evaluating your application for single-pass heating, ask yourself: Do you require continuous or batch water heating? What are the water turndown requirements?
Inline Recirculation Heating. In this type of application, water is heated and sent to a downstream use point. The post-process water is returned to the heater to be reheated and recirculated. Heater requirements can include:
- High steam flow and water temperature rise for the initial system cold water startup.
- Low steam flow addition for the return water heating. The temperature rise in such uses typically is 10 to 25°F (5.5 to 13.9°C).
- Heater turndown requirements of up 8:1 on liquid flow and 100 percent on steam.
- Precise and stable water temperature control to meet process or sanitation requirements.
Common applications for inline recirculation heating are jacketed vessel heating, CIP hot water for plant sanitation and paper mill whitewater heating.
Industrial Tank Water and Steam Heating
Typical tank heating uses within a plant include intermittent or batch hot water tank supply, heating jacketed reactors or kettles, or multi-user hot water systems.
Manual Tank Fill. For these types of applications, a tank is filled with hot water in a batch or intermittent water flow mode. Manual tank filling often is used for chemical-mix batch processing or staged hot water supply where 1,000 to 5,000 gal of hot water is required for end-of-shift cleaning or washdown. Heater options can include either a simple inline water heater or a tank-mount jet sparger.
Heater requirements include:
- An inline heater setup with a pneumatic temperature controller for steam pressures of 50 to 300 psig. The inline unit heats water going into the tank, improving batch cycle times (figure 2).
- A tank-mount jet sparger for 15 to 300 psig steam pressure. The jet sparger will heat water on-demand while the tank is filling. The volume of water level above the jet sparger is an important design consideration.
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Either approach provides a simple, reliable and cost-effective means of localized, manual hot water tank supply.
Intermittent or Batch Hot Water Supply Tank. In these applications, a hot water tank is required to be maintained at the hot water setpoint temperature. Typical uses can include:
- A paper mill tissue machine and hydropulper supply, where hot water needs to be added to the recycled paper for improved fiber/water mixing to support hydropulper function.
- Slurry heating applications with solids. Filter-cake presses dewater the solids, and hot water is required to clean the filter presses.
- Plants with multiple plant sanitation users for washdown. Multiple users can create temperature control issues for an inline heater. A surge tank can be added to allow the heater to maintain a constant hot water supply at the target temperature, regardless of the number of downstream users or a rapid variation of water flow demand.
- Vegetable can-topping with hot water in food processing. This is a good example of small-batch delivery of hot water.
When selecting a heater for intermittent hot water heating, there are keep several points in mind.
- An inline heater can be used directly to supply short-run batches of hot water. A more practical approach, however, might be a hot water surge tank with a recirculation heater on the tank. This ensures that a constant hot water supply can be maintained for all downstream users.
- In intermittent hot water heating applications, the hot water run time typically can range from 2 to 30 minutes.
- The temperature control loop should be designed to ramp up to the target setpoint, and the control logic should be designed to monitor temperatures and avoid overheating.
- Adding a surge tank can improve the target temperature setpoint control by using an inline heater to maintain stable tank temperatures.
Heating Jacketed Reactors or Jacketed Kettles
In jacketed reactors, the jackets or coils provide heating to the vessel contents or allow heat to allow cooling for heat removal during exothermic reactions. The jackets also provide the heat required to produce desired endothermic reactions.
In food processing, jacketed kettles are used to heat and cook soups and sauces, which are scorch sensitive. Introducing hot water to the heating jacket can reduce hot spots, improve temperature control and provide fast changeover from heating to cooling mode.
In these heating vessels, the fluid ingredient mix is ramped up to the desired temperature and then held at that temperature for a period to complete the chemical reaction or desired cook. A line steam injection heater is a good choice.
When sizing these systems, keep these points in mind:
- Identify the time required to heat the vessel to warm the structure and avoid thermal shock.
- To determine the heat load required for the heater, determine the starting fluid temperature, the volume of fluid to be heated and the setpoint temperature.
- Identify the water flow rate to determine the heater liquid-connection size.
- Establish a cascade heating control cycle (ramp up the heating, hold, then ramp down).
Jacketed reactors are common in the petrochemical, chemical and bio-pharm industries.
Multi-User Hot Water System
Good examples of multi-user hot water systems can be found in food, beverage, vegetable or meat processing plants. Water heating for plant sanitation or other processes requires a hot water supply system that can support multiple users with changing demands. Daytime use might be a small heat-load requirement while nighttime cleaning requires high flow water temperatures typically at 140°F (60°C), for instance. Multiple plant sanitation hot water users can be washdown hose stations, the CIP hot water system and spray balls for clean-out-of-place (COP) hot water cleaning (figure 3).
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When sizing a system for a multi-user hot water heating application, keep these considerations in mind:
- What are the normal daytime process operation heating needs?
- What are the plant sanitation hot water flow and temperature needs by shift? How many users are on each shift?
- What is the temperature loss on the return water for daytime use?
- What is the temperature loss on the return water for nighttime cleaning?
- Is there a cold system startup time limit that will impact mass flow steam requirements?
- What is the flow and temperature of the makeup fresh water into the system?
- Is there a limit on boiler steam supply?
- What is the size of the hot water surge tank?
- What is the expected tank turnover time based on the minimum and maximum downstream hot water users?
Beyond the CIP example already mentioned, another common application for a multi-user hot water heating system is a pulp and paper mill’s water recollection tank. It is used for water reheating and has water return feeds of whitewater, felt shower spray system water and re-pulper. Other users within the mill can include the hot water supply for pulp stock prep and sheet formation.
Most paper mills work on a closed-loop water heating system, reclaiming and returning hot water into the system after use. The collection tank will receive multiple streams and various hot water streams at lowered temperatures. They are mixed in the collection tank and sent out at the required use setpoint for downstream users. Because of the multiple water streams into and out of the tank, and the various temperatures of the return water streams, an inline heater on each return water line may not be practical. A tank-mount jet sparger with variable steam flow control is a good choice to heat this hot water system design. The dynamic nature of the multiple fluid streams entering the tank creates a mixing effect for the steam and water, thereby producing a more uniform tank temperature.
Many hot water tanks are well suited for a tank jet sparger. It is a simple, reliable method of heating water in a tank. The tank jet sparger (figure 4) can use steam pressure as low as 15 psig. This can reduce the cost of the heating system because boiler and personnel qualifications are different for low pressure steam.
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Tank jet sparger selection considerations include:
- What is the expected tank water volume?
- How often will tank supply be turned over?
- Will a cold startup mode be required along with a temperature-maintenance mode?
- What is the available steam pressure?
The tank heat-load calculations would be similar to a multi-user hot water system described earlier.
In conclusion, a reliable hot water supply is critical in many plants, whether it is for process plant operations to achieve optimal process heating, or simply to meet plant sanitation requirements.
Before selecting the heating solution, take these steps to help identify which hot water supply will best meet your needs:
- Determine if there is one primary user, or if there are multiple users downstream with variable heating needs.
- Identify water turndown based on process requirements.
- Ascertain startup conditions and the normal process temperature setpoint to determine the range of steam flow requirements.
- Determine whether a hot water tank will be needed in the system for hot water staging, or if surge tank heating is needed to support multiple downstream users.
- Establish whether there will be freshwater makeup additions to the water heating loop. If present, freshwater additions will affect heat load requirements.
- Ascertain the steam pressure. Inline heaters typically require steam pressure greater than or equal to 50 psig. Tank jet spargers can use steam pressure as low as 15 psig.
The above applications and fluid heating approaches are not just for water. They also can be applied when heating challenging fluid applications such as high solids and high viscosity fluids, wastewater and sludges, and other difficult-to-pump fluids.
Depending on your process needs, water heating applications and systems can be simple or complex. Taking the time to accurately assess your operating conditions and process needs leads to a better performing and more reliable hot water system. Incorporating the recommendations into your planning process can help you to develop a solid plan to design and optimize your hot water tank and steam injection heating system.