Heat Exchanger Technology

As the price of electricity, natural gas and other fossil fuels continues to climb, chemical processors are more closely examining high-temperature operations and heat-transfer systems to see if more efficiency can be had. In many cases

it can, and, as a result, heat-transfer projects are not only justifiable, but downright attractive.

There was a dialog among a few heat exchanger specialist from Alfa Laval, Paul Muller, Exergy LLC, etc. The dialog mainly discussed on strategy to improve heat transfer efficiency, heat recovery and efficient process control during this high energy price arena.

Tubular Exchanger Manufacturers Association (TEMA) has developed a series of fouling factor for Shell and Tube (S&T). These fouling factor are generally higher than Plate Heat Exchanger (PHE), why not use S&T fouling factors to size PHE ?

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Product fluid in reactor involve exothermic process is commonly hot. It is then sent to distillation and separation system for catalyst and raw material recovery. Separated product is then cooled by plant wide Cooling Water (CW) before it is sent to storage tank. The product temperature will have to be maintained.

How this temperature is controlled ?


Temperature Control Methods
There are several ways to maintain the product temperature :

1. Provide a product bypass across the Cooler, control valves on Product bypass line and Outlet line (Split range control) with fixed CW flowrate
2. Provide a CW bypass across the Cooler, control valves on CW bypass line and CW inlet to Cooler with full product flow across cooler
3. Provide a CW bypass across the Cooler, control valves on CW bypass line and CW outlet to Cooler with full product flow across cooler
4. Provide a Control valve at the Cooler inlet with full product flow across cooler
5. Provide a Control valve at the Cooler outlet with full product flow across cooler
   

Generally the product flow is fixed by operator based on production plan and the product flow shall not be controlled. Thus, it is always not recommended to provide a control valve at the inlet and outlet of product line for product temperature control.

Disturbance of CW Network Balance
Cooling water is in a network supplying to many heat exchanger through out the plant for cooling purpose. It is normally supplied by a set of centrifugal pump. As centrifugal pump head will be affected flow across, any changes in the CW demand will affect the CW balance in network. This will further affect the pressure in the network and hence the CW flow into other heat exchangers. Thus, it is always recommended not to throttle the CW flow as much as possible to avoid CW balance.

Scaling
Throttling CW flow into heat exchanger would potential lead to low CW flow into heat exchanger, high film temperature at on CW side and promote scaling. The option (ii) and (iii) are always recommended IF throttling on CW side is chosen.

Potential affecting Production
Controlling product fluid temperature with product bypass across the Cooler, control valves on Product bypass line and Outlet line (Split range control) and fixed CW flowrate (option i) is one of the common way in temperature control for product cooling. As it minimize the impact to CW network. Nevertheless, there is still concern about manipulating product fluid or mal-operation (controller failure) of control valves would potentially lead to production lost, the option (ii) and (iii) are always the recommended option.

CW Pressurise or Non-Pressurise
Option (ii) and (iv) compare to option (iii) and (v), the difference is the location of main CW line control valve (either at the inlet or the outlet). Providing a control valve at the outlet will have the following advantages :

a) Maintain high pressure in the heat exchanger and higher pressure will results higher heat transfer

b) CW at high pressure will minimise potential of boiling

c) CW at high pressure will minimise potential release of dissolved gases in CW , trap in heat exchanger and reduce heat transfer

d) In event of Control valve failure (failed to full close position), not further Cooling. CW in the heat exchanger will be heated and potentially lead to heat exchanger overpresure due to thermal expansion and/or boiling. The CW will be relieved via Pressure Relief Device provided on the heat exchanger. Providing control valve at the outlet would allow continue CW feeding into the heat exchanger, this minimise the potential of sudden temperature increase and cause heat exchanger due to thermal shock. The downside is release CW into disposal network.

Considering above advantages, it is always recommended to provide control valve on CW line at the outlet IF throttling CW side is chooses.

CONTROLLING SHELL AND TUBE EXCHANGERS
"Controlling Shell & Tube Heat Exchanger", an excellent article by Walter Driedger discussed about all type of control schemes around heat exchanger. Check out.

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Steel picklers have traditionally used carbon block heat exchanger to heat their shallow tank high turbulence pickle baths. The multi-gasketed designs and fragile nature of graphite heat exchangers still require continuous maintenance and repair which results in expensive downtime and spare parts. This has forced the industry to look for alternatives. The use of metal shell and tube heat exchangers virtually eliminates all of the problems associated with carbon block heat exchangers.

As claimed, some of the benefits of metal shell and tube heat exchangers are :

• Cost competitive with Carbon Block Heat Exchangers
• Easily retrofittable into existing equipment footprint
• Elimination of downtime due to equipment failure
• No spare parts to keep in inventory
• Superior corrosion resistance
• High heat transfer
• High steam pressures to reduce required surface area
• Fully welded metal design eliminates breakage during handling, installation and operation
• Elimination of acid leaks into steam condensate

Typical comparison of a Carbon block HEX to a tantalum S&T HEX

Description	Carbon block HEX	Tantalum S&T HEX
Heat Input(BTU)	1,000,000	1,000,000
Steam Pressure (PSI)	75	75
Typical Overall U (BTU/hrft2.F)	250	650
Surface Area Required (Sq. Ft.)	29*	11.2*
Inventory of Spare Parts Required	YES	NO
Fully Welded Metal Design	NO	YES

Basic heat transfer equation used to calculate required surface area.
*Tantalum heat exchanger surface area required does not take into consideration using a higher pressure steam.

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As the price of electricity, natural gas and other fossil fuels continues to climb, chemical processors are more closely examining high-temperature operations and heat-transfer systems to see if more efficiency can be had. In many cases
it can, and, as a result, heat-transfer projects are not only justifiable, but downright attractive.

There was a dialog among a few heat exchanger specialist from Alfa Laval, Paul Muller, Exergy LLC, etc. The dialog mainly discussed on strategy to improve heat transfer efficiency, heat recovery and efficient process control during this high energy price arena.

A few tips have been present :

• The energy crisis results high prices all of the time shorten paybacks. Energy efficient is one of the way to minimize cost
• Energy efficient heat transfer equipment such as Plate Heat exchanger, Gasketed Heat exchanger, Bonded Heat exchanger, etc is one of the option.
• For a service using Shell & Tube (S&T), the overall heat transfer coefficient (HTC) is around 300 Btu/h ft2°F. However, the overall heat transfer coefficient (HTC) for a compact heat exchanger can be improved 3-4 times (~1000 to 12000 Btu/h ft2°F).
• With lower overall heat transfer coefficient, this may translate into less space, smaller installation and handling cost. Capital cost may not be low as the fabrication cost for compact heat exchanger is high.
• Gasketed Heat exchanger good for maintenance. However shall take additional attention on the compatibility between gasket and fluid.
• All welded or Bonded heat exchanger may be considered if there is gasket & fluid compatible problem
• For laminar flow, heat transfer rate is only the function of fluid thermal conductivity. Operate heat transfer equipment at lamina flow during turndown could significantly reduce it heat transfer rate
  
• Compact heat exchanger promote turbulence. High turbulence increase heat transfer rate and reduce fouling (read more)
  
• Thus plant releasing hot exhaust gas from burner, boiler, gas turbine, etc to atmosphere may take the opportunity to recover heat
• Improve temperature control in process system would reduce energy usage

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Self Cleaning heat exchanger operating concept is mainly base on fluidization concept. It commonly used for severe fouling service. It has been proposed in 1970s and used for seawater application. Now Self-Cleaning Heat Exchange principle has been extended to evaporation in the tubes.

Operating Principle


More excellent article and application of Self Cleaning heat exchanger :

• Zero-Fouling’ Self-Cleaning Heat Exchanger
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• Developments and Achievements in Self-Cleaning Fluidized Bed Heat Exchangers
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Heat Exchangers are a significant part of many industrial processes that involve energy exchange. Most of these heat exchangers become fouled with use. The United Engineering Foundation, which hosts a conference every three years on the fouling problem, estimates that the cost of heat exchanger fouling is 0.4 % of global Gross Domestic Product (UEF, 2001). This high cost has lead to frequent study of the fouling problem, including numerous books and conferences on the subject (Somerscales and Knudsen, 1981; Melo et al., 1988; Bott, 1995). Much of this work has focused on particular industries. Crude oil processing, dairy and food processing, and nuclear reactor cooling are all industries that have conducted a large amount of research aimed at understanding and mitigating fouling.

Particulate fouling of HVAC heat exchangers can lead to negative energy and indoor air quality impacts. This work investigated the mechanisms and consequences of particle deposition on fin-and-tube heat exchangers.

Above combination may end-up with different heat exchanger which best suit their own need. Thus, whenever specify solid in fluid, concentration (wt% or ppmw) and particle size (micron or mm) are required to be spelt out so that other (vendor , consultant) can provide the most cost effective solution whilst meeting process demands.

PHE is having a very unique feature where narrow channel and proper engineered cross-sectional areas promote turbulence and high heat transfer (convective) capabilities. High turbulence has the potential of removing scale stick on the plate. However, present of solids in fluid promote pluggage in the narrow channel and ultimately results NO flow.

(Click image for better view)

Fouling in heat exchanger is one of most challenging operation problem. It is the buildup of sediments and debris on the surface area of a heat exchanger and reduce heat transfer.

Fouling will reduce heat transfer, impede fluid flow and increase the pressure drop across the heat exchanger.

Plate heat exchanger as compare to Shell & Tube Heat Exchanger showed lower fouling factor. Have you ever thought of the reasoning behind ?

This article has briefly discussed types of fouling, fouling factor used in Shell and Tube Heat Exchangers for common fluids, why lower fouling factor in plate heat exchanger (PHE) and, design consideration to reduce effects of fouling....

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Heat exchanger sleeving, a money-saving technology generally associated with the electric power industry, is drawing increasing interest from engineers in the chemical process industries (CPI), as well.

Sleeving consists of expanding thin tubes (sleeves) into the tubes of a heat exchanger. The expanding process produces a residual, interfacial fit pressure between the outside surface of the sleeve and the inside surface of the tube. The sleeves may be short, for instance about 6 to 16 in., or may extend for the full straight length of the tubes. Typical sleeve thicknesses are 0.01 to 0.03 in., depending upon material of construction and thickness of the original tubes. In addition to the expansion step, sometimes the inner end of the sleeves is welded to the inside wall of the tube.

In tubular heat transfer equipment in power plants, sleeving has long been used for one or more of these purposes:

• To reduce the prospect of inlet-end tube erosion (short sleeves for this purpose are also called ferrules, and their use is called ferruling)
• To restore tubes to service that had been plugged by plant personnel because of known perforations in discrete, identifiable locations
• To restore tubes to service that had been plugged because their walls had become excessively thin
• To bridge failures in discrete locations of tubes that are otherwise intact; for example, if a tube has a circular crack just beyond the inner face of the tubesheet
  

Before applying sleeving to similar problems at CPI plants, it is useful to be aware of the sleeving methods and equipment available, be able to determine how sleeves affect the heat transfer performance of heat exchangers, and be able to calculate the changes in pressure drop through the tubes.

Related Topics

• Practical Design Tips for Heat Exchanger
  
• Should Forced Draft or Induced Draft Air Cooler be employed on offshore platform ?
• Typical Heat Transfer Coefficient For Air-Cooled Heat
• COLLECTION of Typical Overall Heat Transfer
• COLLECTION of Fouling Factor (FF) use in Heat Exchanger Design
• FAYF - Heat Transfer - Useful Heat Transfer Equation

Heat Exchanger design is one of the common activities in plant design. However, although heat exchanger is designed in proper manner and follow most of the good engineering & design procedure, yet you might fails to achieve the desired performance by a wide margin. With an understanding of some common reasons why this might happen, designers can avoid these problems in the first place, and troubleshooters can recognize the root causes quickly.


Exchangers for single-phase operation, condensing and boiling are considered in that order here; but as we shall see, exchangers often handle a combination of these, and it is not always obvious which process is causing the problem. In fact, some of these problems are quite unexpected and can even take experienced designers by surprise. It must be recognized that the most important cause of problems in exchangers is excessive fouling. Other articles, books, and conferences have been dedicated to this problem, so fouling will not be addressed here. Instead we consider those exchangers that have failed for some reason other than fouling.

Related Topics

• Sleeving Extends Heat Exchanger Life
  
• Should Forced Draft or Induced Draft Air Cooler be employed on offshore platform ?
• Typical Heat Transfer Coefficient For Air-Cooled Heat
• COLLECTION of Typical Overall Heat Transfer
• COLLECTION of Fouling Factor (FF) use in Heat Exchanger Design
• FAYF - Useful Heat Transfer Equation
  

Basic heat transfer relationships is apply to Air-Cooled Heat Exchanger (ACHE). The fundamental heat ransfer equation :

where

A = Heat transfer area

LMTD = Log mean temperature difference

F = Correction factor

Typical heat transfer coefficient for Air-Cooled Heat Exchangers

Fouling factor is generally used in the design to cater for heat exchanger deficiency resulted by fouling. TEMA has based on many years of experiences and experiments list out fouling factor for Shell & Tube Heat Exchanger for many services. Only TEMA subscribers are eligible to use this information. However, there are several reputable researchers & manufacturers have shared this information to the public. Following are the collection of fouling factors for different service.