Heat Exchangers

Maximize Plant Throughput by Optimizing Wash Frequency

Fouling and scale buildup is the bane of any processing plant. It restricts capacity, reduces heat transfer, increases cleaning requirements, fuel consumption, and total costs.

In some high fouling industries pumps and pipelines can require cleaning and washing on extremely high frequencies, and the ability to identify when these become a ‘bottleneck’ can increase total plant throughput.

The type of equipment which can foul and require cleaning is vast, and can include anything and everything:

But how can you identify when a piece of equipment’s throughput or performance is degrading?

Every piece of equipment is different, but the key identifier is the same - historic key performance indicator degradation. The Key Performance Indicator (KPI) should be identified for every critical piece of equipment, but can be looked at for individual pipelines as well.

For example, take a heat exchanger with the purpose of recovering as much heat as possible (that is, it is not controlling to a set point):

In terms of fouling and wash requirements, the KPI would be total heat transfer (MW). If the heat transfer is continually reducing over time then the economic breakeven point can be used.

Another example is a simple pump. Looking at the pumping efficiency, or another common identifier is pumping throughput at maximum speed. If 6 months ago a pump was able to achieve 300 kL/hr at 100% speed, but is now only able to achieve 250 kL/hr, then the possibility of a wash should be considered (as well as pumping maintenance). This could be a result of solids buildup on either the pump suction or discharge or both. Looking at line pressures is often enough to identify the location of any restrictions.

Another tip to identify solid buildups within pipes where the fluid is hot is a thermal imaging camera - the buildup will act as an insulation and will result in a cold spot, further proof of washing requirements.

All different pieces of equipment have different KPIs which can be used to identify performance degradation. This degradation can then be tracked and measured to estimate the rate of scale build up, or fouling rate, leading to an improvement in planning and wash frequency.

“Success is simple. Do what’s right, the right way, at the right time”

Arnold Glasow

Washing frequency is often one of the simplest ways to maximize plant performance, throughput, and efficiency. The rule of thumb is to complete the wash before the restriction becomes a restriction.

Heat Exchangers - Optimizing Operations through Cleaning Frequency

Heat exchangers are one of the most common piece of equipment and are present in almost all processing plants. Their design is very well understood, however their operation often has opportunities for improvement.

Deutsch: Industrielle Hochdruckreinigung von W...

Photo credit: Wikipedia

Depending on the circumstances and fluid conditions heat exchanger condition may degrade rapidly due to tube leaks, or fouling buildup. This results in poor heat transfer, which can impact downstream processes or lessen heat recovery leading to increased costs.

Q = UA * LMTD

Once installed the only way to improve heat exchanger performance is through the temperature differential or by increasing the heat transfer coefficient, usually through optimizing cleaning schedules. Without replacing the heat exchanger or expensive modifications the heat exchange area is difficult to increase.

There is a big difference between calculating the rate of degradation in performance and scheduling and organizing heat exchanger cleaning. Graphing the change in UA over time through several cycles vs the required outlet temperatures the cleaning schedule can be identified more accurately. A minimum required performance can be targeted based on required temperature pickup or desired heat recovery, allowing an appropriate level of cleaning to be schedules ahead of time.

This data can also be used to be converted into dollars, which is always more justifiable. The reduction in performance and temperature vs the heat recovery and cost of energy will result in a breakeven point around which cleaning may be based. As can be seen in the graph below, an increased frequency of cleaning may or may not be economically justifiable. Naturally this will depend on the circumstances.

Before optimizing a heat exchanger the question must be asked of the purpose of the exercise - what are your trying to achieve?

  • Is the heat exchanger fouling more quickly than expected and requires additional cleaning just to maintain required outlet temperatures?
  • Is there potential to recover more heat to reduce overall fuel consumption?
  • Is the heat exchanger currently cleaned far too frequently, and the required performance can be obtained by reducing the schedule frequency?
  • Is the cleaning not as effective as historically, leading to further concerns about equipment condition?

There are large opportunities in reducing costs and improving fuel efficiency through optimizing heat exchanger performance, but only if the correct equipment indicator is targeted.