PROCESS COOLING

Temperature control units are relatively simple in their design and operation, but the impact they have on industrial processes can be significant. To better understand this, it is important to review:

• The components of construction.
• How the units operate.
• Factors to consider when selecting a design for a specific application.

A temperature control unit, or TCU as it is commonly known, has two major components: the pump and the heater. Both are managed by a programmable logic controller (PLC).

During operation, a temperature setpoint is input into the controller. The pump continually circulates a fluid — most commonly water — from the TCU to the process and then back to the TCU. This is referred to as the temperature control loop or process loop. While the fluid is circulated through this loop, the controller will monitor input from a temperature sensor on the TCU’s return line. If the fluid is returning to the TCU lower than the setpoint, this means energy is being transferred from the fluid into the process. Therefore, the TCU is heating the system. The controller will activate the heater in the TCU to add more energy to the fluid. Conversely, if the fluid is returning to the TCU warmer than the setpoint, this means energy is being transferred from the process into the fluid. In this case, the TCU is cooling the system. The controller reacts to this situation by actuating the cooling valve.

Indirect or Direct Cooling Operation

The TCU does not generate the cooling itself. Instead, it utilizes a cooling source to lower the process loop’s fluid temperature. Common cooling sources are a chiller system, a cooling tower system or city water.

Two configurations are possible for the cooling operation: direct and indirect. The name refers to how the cooling source fluid interacts with the process loop fluid.

For direct-injection cooling, when the cooling valve is opened, it allows the warm fluid from the process loop to escape from the system. This warm fluid can go directly to a drain or into a return line going back to a cooling source such as a chiller. The cooling source supply replaces the volume that released from the process loop. The cooler fluid is mixed directly into the warmer fluid of the process loop, which reduces the temperature of the system, achieving the desired cooling.

This typical TCU shows the pump and heater tube.

Indirect designs still use a cooling valve, but the process loop’s fluid is isolated from the cooling source fluid with a heat exchanger. The cooling valve controls the flow of the cooling source fluid through the heat exchanger. The warmer fluid transfers energy into the cooling source fluid, thus achieving the required cooling.

There are pros and cons to each cooling style. Direct injection mixes the two fluids, so contamination is a concern. The energy transfer rate is not limited by a physical heat exchanger, however. Indirect cooling maintains separation of the fluids, but it is limited by the capacity of the heat exchanger. There also are implications for the maximum setpoint of the system because of pressurization requirements that make high temperatures possible.

Indirect cooling maintains separation of the fluids, but this method has a more limited flow based on the capacity of the heat exchanger. 

Solenoid vs. Modulating Valves

In both designs, the type and size of the cooling valves are critical for accurate, reliable control. Solenoid valves use simple open or closed control. By contrast, modulating valves use a control signal to actuate the valve’s orifice over a range of values. The additional level of control with a modulating valve increases the precision of the cooling; in turn, this improves temperature stability in the system.

Selecting the right size valve based on its Cv (valve characteristic) also influences the precision of the cooling operation. If a valve is too small, it cannot achieve enough cooling, and the TCU will not be able to maintain its setpoint. Heat will build up, eventually forcing the system to shut down. When the cooling valve is oversized, an excess of warm fluid is released from the process loop and replaced by an influx of cold fluid. This drives the temperature far below the setpoint and reduces the TCU’s setpoint accuracy. Understanding your particular process cooling requirements can help you navigate these various design options.

Direct injection allows more flow, but there is concern with contamination of the two fluids. 

Heating Function

The heating function of the TCU is a more straightforward process. An inline heater is installed as part of the process loop inside the TCU. These heaters often are high watt density, which refers to the heating element’s wattage compared to its surface area. High density heaters can impart more energy, more quickly than standard density heaters. Most commonly, these heaters are made of Incoloy or sheathed in Incoloy. Incoloy is a superalloy or high performance alloy. It is known for excellent resistance to corrosion and the ability to transfer heat. For TCUs, Incoloy is superior to copper, glass-lined or even ceramic heating elements because of its performance and long life.

TCU heaters are selected based upon their kilowatt (kW) rating. When determining what kilowatt heater is required for an application, the required time to heat up a process is one of the most important factors. Kilowatt is a rate, which means that the higher the kilowatt value, the shorter the heatup time. (Remember, when using a TCU for a heating application, the process loop transfers heat to your application and returns to the TCU colder than the setpoint.)

One of the most important considerations for the design of the TCU is the type of contactor used to actuate the heater. The controller on the TCU will cycle the heater on and off to maintain an accurate temperature. Traditional electromechanical relays (EMRs) are rated for a limited number of cycles before failure. These can fail in the on position, which would force the heater to stay on and drive the temperature beyond maximum safe levels. For this reason, some TCU manufacturers use solid-state relays (SSR) in place of electromechanical relays. The solid-state relays typically are rated for 100 times more cycles than the electromechanical relays. They are more expensive, but increased performance, reliability and safety justify the design.

Two types of flow can occur inside a pipe or heat exchanger: laminar and turbulent. The higher the flow, the more likely heat transfer will occur.

Pump Size Selection

The last key component of a TCU is the pump used to circulate the fluid through the process loop. It is crucial to select the right pump for each application. Many pump sizes are available to provide the correct amount of flow at a required pressure. Different pump manufacturers have designs that achieve better efficiencies than others and generate higher flows with their specific grouping of the motor, impeller and volute. For this reason, simply assuming that all 1-hp pumps are the same can create scenarios where either the required flow rate is not achieved, or energy (and operating costs) are wasted because a 2-hp pump was installed when another manufacturer’s 1-hp design could provide the same performance. It is important to review the published pump curves for the various sizes available in TCUs.

The installation design also plays a role in the pump’s ability to achieve the required flow. The piping type, material and routing all affect the pump’s operation by creating pressure losses in the system. The process-loop design should minimize these losses when possible. This ensures the flow through the system is high enough and, therefore, turbulent. Flows generally are described two ways: turbulent or laminar. Lower flows are more likely to be laminar while higher flows are turbulent. Relating these types only to flow rate is a significant simplification of the concept. Laminar or turbulent flows are governed by a ratio known as the Reynolds number, which considers the density of the fluid, flow speed and viscosity over a defined length.

For the purposes of this article, we do not need to take a deep dive into the physics. A good example to help grasp the concept of laminar and turbulent flow is found with a faucet. When the faucet it barely turned on, the water coming out appears to move in smooth paths and is visually clear — this is laminar flow. As the faucet is opened further, the water becomes less clear and has irregular fluctuations — this demonstrates turbulent flow.

These concepts are important for TCUs because laminar versus turbulent flow directly affects the ability to transfer energy. The relationship is exponential, so turbulent flow can have an order of magnitude improvement in energy transfer. This means that the TCU is more effective at either heating or cooling as needed. A higher flow rate through the system also reduces the temperature differential from supply to return. This helps to create a more uniform temperature gradient through the process, which ultimately promotes consistent operation. The ability to achieve these flow rates is directly related to the pump. For this reason, the pump is an important aspect of the design. It is the heart of the system and crucial for efficient and effective performance.