REPLACING FLUIDS IN HEAT TRANSFERS

The fluids in heat transfers are crucial to many of its processes. Ensuring the safe, ongoing operation of these systems depend to a great extent on the fluid, during the regular operation and particularly during the systems startups and shutdowns. To ensure a smooth shutdown every time, as well as a reliable startup without damage, follow these guidelines.  

Fluids in Heat Transfers, the Samples and Analysis

Fluid samples should always be sent to qualified laboratories, which specialize in analyzing the fluids in heat transfers. It is advisable to take samples both after draining and filling a system to provide constant monitoring for the fluid of the heat transfer system. A quick-disconnect valve can speed up the unloading and loading process.  
 

Fluid Flow Characteristics

The two fundamental characteristics of industrial fluid flow through a pipe is turbulent and laminar flow. Turbulent flow is characterized as tumultuous and choppy without a specific pattern. Laminar flow on the other hand can be characterized as a layered, smooth flow and the fastest flow occurs in the pipe’s center. The fluid’s flow transitions to turbulent from laminar can be determined by a Reynolds number, which is a number that is dimensionless, but is characterized by the pipe’s length, which the fluid is traveling through, plus the fluid’s velocity, viscosity, and density.    

Within a heating system, as the heater and pump are turned to on, the fluid’s velocity increases and in turn, the viscosity will decrease. The change in viscosity and velocity produces an increase in the Reynolds number. When a Reynolds number exceeds 2000, the fluid’s flow is recognized as turbulent.

During startups, the laminar to turbulent flow is crucial due to the laminar flow’s layered characteristics. Remember we stated that when laminar flow takes place, the fluid within the pipe’s center flows faster than that of the side fluids. Those outside fluids form a stagnant, protective layer adjacent to the walls of that pipe. When too quickly heated, the heat transfer is inhibited and the bulk film temperatures of the fluid are exceeded, which causes sludge and carbon buildup and additionally, thermal cracking.

Turbulent flow of the fluids in heat transfers permit superior transfer of heat, as well as the optimum thermal efficiency adjacent to the pipe’s wall. In line with this rationale, the ultimate consideration at startup of the system is to start fluid circulation prior to raising system temperatures too quickly.   
 

Industrial Procedures for Startup of a Heat Transfer System

There are multiple requirements for proper startup of a heat transfer systems. Regardless of whether a system has previously been used or brand new, every system must be flushed prior to startup to eliminate residue or debris that may be trapped in the pipes. Subsequent to flushing, the fluids must be (see system fill below) charged into the heat transfer systems. Lastly, start slowly and with prudently to avert damaging the fluids in the heat transfers. The following is a more precise guide to each key step in the process:
 

Step #1 System Flush:

To accomplish a required flush of the fluids in heat transfers, fill the system to 80% capacity with the flushing fluid. See below for the fill instructions, and then run at 204°C or 400°F for an eight-hour period. It is critical to utilize the right flushing fluid in your system, whether that system is setup for synthetic or hot oil, your supplier can assist in choosing the right flushing fluid for the system you are flushing. Then, flush the fluid and proceed to Step #2.

Step #2 System Fill:

Charge the systemthrough system low points with fluids in heat transfers. The principle objective in startup of the system should be removing additional trapped light-end (fluids that have a lower boiling point than that of fluids in heat transfers) fluids or moisture. To remove residues, during startup run the fluids through the system expansion tank. Light-ends or moisture escaping could cause a spike in the system pressure or small puffs of steam or vapor as it leaves the vent from the expansion tank.

Continuously, fill and drain the expansion tank to assist in the venting of nitrogen and replenishing the system with new fluids. After replenishing the system and it is full, any residue of light-ends or moisture should be vented through the bleeder valve at the high points of the system use. With the system’s fluids refilled and nitrogen expelled, you can turn on the circulation pump.
 

Step #3 System Start:

Always start the pump before turning on the heater, this ensures fluid circulation and a turbulent fluid flow prior to heating. At the onset of fluid circulation, follow these steps to bring up the system to desired temperature for operation:

Ø  Increase heat at the rate of 1° Fahrenheit or less per minute up to 200°F or 930°C. For one cycle.

Ø  Increase heat at the rate of 1° Fahrenheit or less per minute up to 230°F or 110°C. For one cycle.
Ø  Increase heat at the rate of 1° Fahrenheit or less per minute up to 260°F or 127°C. For one cycle.
Ø  Increase heat at the rate of 1° Fahrenheit or less per minute up to 300°F or 149°C. For one cycle.
Ø  Increase the heat at the rate of 50° Fahrenheit per minute until desired operation temperature is attained for one cycle under close observation.

NOTE: 

Ensure that you allow a full cycle for each step before moving on. One complete cycle is determined by pump output and the system’s total volume. When excessive amounts of vapor are generated at a specific temperature, continue at that temperature until vapor venting declines. Since water’s boiling point is 212°F or 100°C, this is where you can expect the heaviest venting to occur and the temperatures in step #two should be slowly increased until it has passed 212°F or 100°C, that will permit excess moisture to vent.

Shutting Down the System

As was previously mentioned, the critical element is that the fluids in heat transfers are circulating while the system operates at a high temperature. Accordingly, during a shutdown of the system, heat must be decreased proportionately, in the reverse of the startup procedures. Keep the fluids circulating until system temperatures drop below the 200°F or 93°C mark, so that the residual heat is removed. You may now directly shutdown the system pump.

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