Background: Combustion air preheating by recovering heat from flue gases is a cost-effective method of increasing the overall thermal efficiency of the refining and petrochemical processes. Consequently, the use of Air PreHeaters (APH) in oil refineries has become well established and widespread in the last 4 decades or so.
Refinery operators have, however, over the years experienced well known problems with conventional APH types, these include:
- Reliability and maintenance.
- Premature failure & replacement.
- Inability to use in cold weather when internal acid condensation occurs.
- Excessive air leakage.
This article describes how Heat Pipe Air PreHeaters (HP-APH) address the limitations of conventional APH types and offer a proven and effective, alternative solution.
Conventional APH Technology
Many types of APH are utilised within refineries the most common types being plate & frame, DEKA and rotary wheel. Operators have reported a range of problems with APH equipment primarily caused by flue gas acid condensation which leads to premature equipment failure and air leakage. The problem is well understood by operators and equipment vendors who have developed a range of solutions to avoid acid condensation by maintaining minimum metal temperatures above the acid dewpoint of the flue gas. These solutions all require careful operation to avoid the occurrence of “condensation events” in the APH.
The effect of uncontrolled acid-condensation in a conventional APH is profound and usually results in corrosion breakthrough giving rise to leakage and cross contamination requiring service limitation and eventual bypassing.
Most conventional APH equipment is notoriously difficult to repair and often follows a path of reducing performance and eventual shut down.
The available acid condensation avoidance solutions are however limited in effectiveness and have a number of drawbacks including reduction of the APH performance resulting in lower energy recovery.
Acid Condensation Avoidance Solutions
Common acid condensation avoidance solutions include:
- Cold Air Bypass (CABP) – This involves by-passing part of the cold air stream around the APH to reduce the air volume being heated. This has the effect of elevating the lower air mass to an exit Temp higher than that required and by doing so increasing the minimum metal Temp in the heat exchanger. The bypass damper is controlled to keep the average of the flue gas exit Temp and ambient air Temp above the known acid dew point by a safety margin of 20F – 30F.
- Hot Air Recirculation (HAR) – This is often used in conjunction with the CABP and entails recirculating a proportion of the hot air exiting the APH back to the cold air inlet. This has the effect of elevating the temperatures in the APH in an attempt to avoid acid condensation.
The above solutions are only moderately effective at acid condensation avoidance but do reduce the overall effectiveness of the APH through reduction of the Log Meant Temperature Difference (LMTD). Furthermore, even with the use of a CABP and/or HAR, most operators lose a number of operational days per year where cold conditions dictate that the APH needs to be fully bypassed to avoid acid condensation damage. The costs of lost days of recovery is often not calculated and comes as an unforeseen additional production cost.
Heat Pipe Air Pre-Heater Characteristics
The HP-APH utilises as its heat transfer mechanism an array of heat pipes arranged in the hot and cold stream flows.
Each individual heat pipe within the HPAPH effectively acts as an independent selfcontained heat exchanger in its own right, this characteristic is extremely advantageous in that it creates significant multiple redundancy in the unit and addresses a key vulnerability in conventional APH equipment.
The table on the following page outlines the known issues with conventional APH equipment and how they are addressed in the HP-APH:
|Conventional Air Pre-Heater||Heat Pipe Air Pre-Heater|
|Acid condensation formation||Acid condensation formation Difficult to achieve even temperature distribution in large APH equipment. This invariably leads to “cold corners” where localised acid condensation can occur.||By the laws of thermodynamics Heat Pipes are inherently Iso-Thermal. This property eliminates the possibility of “cold corners” and hence unexpected acid condensation formation.||By the laws of thermodynamics Heat Pipes are inherently Iso-Thermal. This property eliminates the possibility of “cold corners” and hence unexpected acid condensation formation.|
|Low minimum metal temperatures||The minimum metal temperatures in the conventional APH are largely a function of the ambient air temperature and the flue gas exit temperature. Whilst this can be calculated in practice because of the uneven temperature distribution throughout the unit it is unlikely to be accurate.||The heat pipe working temperature is not a simple average of ambient temperature and flue gas exit temperature. Through design this can be elevated significantly above the average of ambient and flue gas exit temperatures.||An elevated and known minimum metal temperature is advantageous in avoiding flue gas acid condensation. It also enables increased heat recovery by permitting lower flue gas exit temperatures.
Use of CABP can be reduced or even avoided.
|Vulnerability||Conventional APH equipment relies on thin metal surfaces to effect good heat transfer. There is an inverse relationship between heat transfer surface thickness and heat transfer coefficient. This creates conflicting requirements in that increased metal thickness is required for corrosion allowance but this has the effect of reducing heat transfer effectiveness.||The heat transfer coefficient of the heat pipe is less affected by the tube wall thickness than in conventional heat transfer.||Generally thicker material thicknesses can be used in the heat pipes Thus providing increased corrosion allowance.|
|Material Selection||Conventional APH construction does not readily accommodate mixed materials. The option of utilising Stainless Steel in the areas where acid condensation is most likely is generally not available. To build a large APH wholly in stainless steel is cost prohibitive.||As each heat pipe is independent it is possible to utilise SS material in only the colder end of the APH and non alloy steel grades elsewhere.||The mixed material approach combines cost effectiveness and additional protection.|
|Resilience||A corrosion failure in a conventional APH will result in immediate leakage and eventual withdrawal from service.||Each heat pipe is independent and self-contained. In the event of a pipe wall failure there will be no leakage as the pipe will still remain sealed in the tube plate.||The impact of individual heat pipe failure is minimised.|
|Maintenance||Conventional APH equipment is notoriously difficult to repair once corrosion damage has occurred. At end of life a whole unit replacement will be required||Heat pipes are individually replaceable.||Heat pipes are individually replaceable. Whole unit replacement is avoided as individual heat pipes can be changed on site without the need to uninstall the unit.|
|Control||It is not practical to measure the true minimum metal temperature in a conventional APH due to uneven temperature distribution.
The protection against acid dew point condensation therefore relies on calculations of ambient air temperature and flue gas exit temperature rather than on real time measurements.
|Within the HP-APH the precise locations of coldest spots are known through design. This enables real time temperature measurement of selected heat pipes in the coldest row of the APH.||The coldest pipe real time temperature can be used to control the CABP to ensure that flue gas exit is targeted to minimum acceptable temperature hence maximising heat recovery duty whilst avoiding acid condensation..|
By way of simple comparison with conventional APH types it is the case that the HP-APH offers the benefit of REDUCED RISK of failure and REDUCED CONSEQUENCE of failure. Evidence of fully operational HPAPH equipment installed in the 1980’s and 90’s confirm the longevity of the HP-APH in even the most aggressive and demanding conditions.
The HP-APH is available in a variety of configurations and can be custom designed to accommodate minimum ducting changes as a retro-fit replacement for a failed or end of life conventional APH equipment.
All units incorporate temperature sensing heat pipes in key locations and also feature up to 4 rows of stainless-steel heat pipes in the cold end of the units to accommodate any transient condensation during start-up and shut down conditions.
For further information or equiries, please contact:
Econotherm (UK) Ltd
Tel: +44 (0)1656 658640