Who is Responsible for Thermal Design of Heat Exchanger?
Lakshmi Venkatesh, Dy GM - Process, Petrofac Engineering India (Pvt) Ltd

Heat Exchanger is one of the most crucial equipment in any plant, and its design is crucial for its performance. The article highlights why heat exchanger is different from other equipment, and illustrates some of the critical features during mechanical design, which may result in change in a shell & tube exchanger thermal design. The author was one of the winners in the Leadership Excellence awards for women (Women of Achievement) in the Oil & Gas sector by MEPEC 2013 in Bahrain.

When we ask - "Who is responsible for the thermal design of the heat exchanger?" - to professionals engaged in different operations in chemical industry, the responses vary as all of them - be it an end user, an EPC contractor, a consultant or an equipment manufacturer - may have different perspectives. Similarly, when a question arises if a process engineer or a mechanical engineers is best suited for the job, responses may vary again. Regardless of the responses, what matters the most is the performance of the equipment within the process unit and the fact that they deliver as per the intended design.

Project Lifecycle
To analyse this issue let us look at the various phases of a project lifecycle
  1. Select phase: This is the initial phase where the process is selected based on an evaluation of the technologies available and the market requirements.
  2. Define phase: In this phase generally a basic engineering or front-end engineering package is developed that defines the project, establishes the design requirements and standards.
  3. Execute phase: This phase consists of a number of steps such as:
    1. Design: Where the detailed design of the plant including the equipment, piping, instrument, electrical, civil and structural is done
    2. Procure: Where tagged equipment/instrument and bulks are procured. This involves identifying the vendors, making requisitions, performing technical bid evaluations, managing vendor documentation, concession and deviations, delivery and quality
    3. Construct and Install: Where the action shifts to the actual site and equipment, instrumentation, electricals are installed and piping and civils are constructed as per intent of the design phase and mechanically accepted
    4. Commission: This involves preparing commissioning and handover procedures, witnessing factory acceptance tests, performing leak tests, functional tests of systems, powering up the plant and introducing the process fluids into the system.
  4. Operate: This phase takes place after commissioning when the owner/operator takes over the day-to-day operation and maintenance of the plant.
When we analyse these phases (See the Lifecycle of Heat Transfer Equipment table on next page), it is obvious that the major controversies and differences of opinion would arise in the "execute" phase due to the large number of parties involved that have different priorities. However, if in the execution of the project, the basic interest ie, to have an efficient, safe and operable plant gets faded, the conflicts will escalate.

Why is Heat Exchanger Different from Other Equipment?
There are a number of equipment within the plant like vessels, columns, pumps, compressors, where there is no disagreement regarding the responsibilities of the various parties.

Vendor Specific: Pumps and compressors are vendor specific and the plant design contractor has very little to do with the actual design of the equipment. So once the major parameters are defined, like flow, head, seal requirements etc., the vendor takes over with his expertise. This is also true to a large extent with column and equipment internals, and also to vendor proprietary heat exchanger equipment like plate, spiral, brazed aluminium exchangers. In all the aforesaid equipment, the vendor, in addition to mechanical guarantee, assures the performance.

Fabricated Equipment: The fabricated items like vessels and columns can be mechanically designed once the basic parameters like design conditions and size are defined. The vendor only provides mechanical guarantee.

Heat Exchangers: For exchangers, however, there is disagreement as it falls between the above two options; some treat it like a fabricated piece of equipment, where the vendor is responsible only for the mechanical guarantee, whereas others require the vendor to provide thermal guarantee as well.

Major Steps in Exchanger Design
Process Data Sheet: The process data sheet specifies the duty requirement, fluid conditions and properties, fouling tendency, need for cleaning. Additional information like heating/cooling curve is provided in case of phase change within the exchanger.

Thermal Design: During this stage, the exchanger is designed either manually or more often using specialised software packages. This defines the hardware configuration of the exchanger.

Mechanical Design: In this stage, the mechanical design of the various parts within the exchanger is defined.

This interaction between the mechanical design and the thermal design makes the exchanger design complex. (See the table below)

Project Considerations
Let us look at a case, where a new project is being executed by an EPC contractor for a client.

The process data is supplied as part of the tender documents prepared by the FEED contractor and given to the EPC contractor. The EPC contractor verifies the process data and has a number of options available.

  1. It can send the process data sheet to a manufacturer/fabricator and ask for a total design including thermal and mechanical
  2. If it has in-house capability, he can get the thermal design done in -house. This document is then sent to the manufacturer/fabricator.
  3. If the EPC contractor does not have in-house capability or availability of resources, it engages a thermal design specialist or agency to carry out the thermal design. This document is then sent to the manufacturer / fabricator.
Generally major EPC organisations adopt option b or c, due to a number of reasons. Consider a project with 10 exchangers; if the enquiries are sent to 5 vendors, it is theoretically possible to come up with 50 different designs . During the vendor evaluation, the EPC contractor faces a challenge of bringing the various vendors at par for price comparison. The price will include not only the equipment in itself but also all the associated costs like foundation, instrumentation, intermediate piping etc.

Additionally, there is always a schedule constraint. In the absence of a design, further downstream activities like piping, layout, civil and structural cannot be finalised and will need to wait for the vendor selection which may be months away.

To overcome these, a middle path is chosen as in options b & c. Here a preferred configuration is provided to the vendors by the EPC contractor. This is the base design and the vendor is expected to either endorse the design or redesign if he considers the design unsuitable to deliver the required performance.

This effectively simplifies the bid evaluation process and also enables the downstream activities to proceed without waiting for the final vendor selection and design. Changes from vendorís side are expected to be minor and not lead to major rework in the downstream activities.

Managing Uncertainties
In addition to the project consideration, another question is of 'uncertainty' in the input; this uncertainty is managed generally through the addition of a 'margin.' The issue is - who is responsible for the margin?

The EPC contractor (maybe in-line with the contract, or in order to cover his own risks) may add a flow margin or duty margin of 10 per cent. Also, in the case of an air cooler, he may specify maximum potential air temperature of say 55 degree celsius. Fouling is another factor, which may or may not eventually occur. But this data is crucial and defines the 'requirements' to the vendor. That is the input for which the vendor is required to guarantee a thermal performance.

The vendor in order to guarantee performance may add more margins to ensure that it can achieve the performance guarantee, since vendorís information is limited to what is requested via the data sheet.

When the plant starts up, the flows are low, the temperature is normal, the tubes are clean, etc. and the performance test is satisfied, even though the actual design requirements are not seen or experienced. Hence the exchanger is not tested for the actual performance it was supposed to guarantee.

The vendors are also aware of this fact and know that the order is awarded to the lowest cost provider who is willing to take the most risk in terms of margins, ie, lowest margins.

Pitfalls
The dilution of thermal design activities makes vendors think that they are not responsible for it. As we have seen earlier, the close interaction between thermal and mechanical design does not allow for this interpretation .

Another area of concern is that the EPC contractor is tempted to go to a fabricator who has little or no thermal design capability.

At a later stage, there are disputes since the fabricated exchanger may be inadequate for the required performance due to the impact of the mechanical design factors on the thermal performance. In such a scenario, the question for thermal guarantee becomes more pronounced.

What is the Way Forward?
In my opinion, the client needs to monitor the FEED closely to ensure that the 'margins' in terms of overdesign and fouling factors are adequately specified based on their past experience and/or licensor inputs. This sets the base design which is further validated by the EPC contractor.

The fabricator needs to stand guarantee for the thermal performance of the exchanger since it is responsible for the mechanical design, fabrication to the required tolerances, and the quality of the final product.

The fabricator needs to improve its capability in the thermal design area to ensure it understands the implication of the mechanical design on the thermal performance and hence takes the required corrective action at the early design stage. In the present scenario, there are limited fabricators which have this expertise. The thermal engineers and mechanical designers need to work very closely and talk frequently when collaborating on design projects. If the mechanical engineer needs to alter anything or modify the design to suit mechanical design requirements, it is imperative that the thermal design engineer is updated about it immediately. The enabling factor has been the continued improvement in the availability of specialised software. Thermal design software like HTRI, HTFS are becoming more user friendly and the results becoming easier to analyse with a number of graphical outputs that simplify the task of the user. Quick analysis of the impact of various changes on the thermal performance is easily possible.

It is essential that the fabricator invests and develops an in-house thermal group or engages services of a good thermal design engineer and involves him or her during periodic reviews.

In the event of performance failure, the liability of the EPC contractor is higher than the vendorís. Therefore it is in the interest of the EPC contractor and the owner to do their own checks, have regular audits and inspections during the engineering and fabrication phases to ensure thermal performance is achieved with the design.