Technical Tips Volume 1 Number 5

Tips on Specifying and Operating Heat Exchangers

Many heat exchangers fail because the user’s specification does not fully disclose to the manufacturer all of the conditions to which the exchangers will be exposed and how they will be operated. Specifying excess surface can lead to unanticipated results. Failure to vent non-condensable gases can make well-designed heat exchangers perform erratically or appear to be undersized. Some exchangers fail because they are subjected to process and flow changes for which they were not designed. Following are some tips for specifying and operating tubular heat exchangers.

Technical Tip 1: Fully Disclose All Conditions of Operation

Cooling water temperature range The temperature of cooling water pumped from rivers and wells may range from more than 90 °F (32 °C) in the summer to just above freezing. An exchanger designed for cooling with summer water temperature will have considerably more heat transfer capability than is required when the cooling water is just above freezing.

As the first diagram shows, this can result in far lower outlet temperatures of the fluid being cooled than are acceptable. In the diagram Ti to To is the desired temperature path of the temperature drop of the hot fluid and ti to to is the anticipated path of the temperature rise of the cooling water. When the cooling water inlet temperature is lower than the specified design basis, the temperature drop of the hot fluid is from Ti to T'o and the temperature rise of the cooling water outlet temperature will be from ti to t'o.

 

Technical Tip 2: When specifying heat exchangers be very wary of specifying excess surface

Effects of excess heat transfer surface Excess heat transfer surface has an effect similar to that of colder-than-design cooling water.  However the heat transfer path is as shown in the second diagram.

Technical Tip 3: Be aware of the process effects of colder-than-design cooling water and excess surface

Following are some of the ways that colder-than-design cooling water and excess heat transfer surface affect the process fluid.

  1. If the cooling water temperature is below the melting point of the process fluid a layer of solid material deposits on the heat transfer surfaces.  The solid layer insulates the surface.  The hot fluid outlet temperature changes from below the desired outlet temperature to a higher-than-desired outlet temperature.  The heat exchanger appears to be under-surfaced.   The only cure is to shut down and melt the solidified process material.
  2. If the function of the heat exchanger is to heat up the cold fluid with hot fluid, when the exchanger is over-surfaced, the outlet temperature of the cold fluid will be higher than desired.  If the cold fluid is temperature sensitive it may degrade.
  3. At low loads the excess surface in the Desuperheating zone of a closed feedwater heater can absorb all of the superheat from the superheated steam causing it to condense prematurely near the Desuperheater outlet.  Droplets of condensate entrained in the steam exiting the Desuperheater will erode the cross-flow baffles at the exit end of the zone and the tube supports in the condensing section.  The erosion enlarges the tube holes and makes the tubes vulnerable to vibration damage.  Consequently, when specifying feedwater heaters be sure to specify a minimum of 2 F° dry wall margin.

Technical Tip 4: Dealing with colder-than-design cooling water and over-surface

The most common mistake that operators make when the cooling water is too cold or there is excessive surface is to choke the cooling water supply, which reduces the flow velocity in the cold fluid side. 

Effects of throttling cooling water in the tubes. Turning down tube side cooling water flow reduces the flow velocity.  This allows silt from river or lake water to deposit on the heat transfer surfaces.  If the cooling water is drawn from a well, the low velocity will increase the tendency for scale to deposit on the surfaces. Subsequently increasing the tube side flow velocity will not remove deposits of silt or scale.  When the cooling water comes to the summer temperature, the fouled surfaces are ineffective and the exchanger appears to undersized.

Effects of throttling cooling water in the shell.  Turning down shell side cooling water reduces the mass velocity.  Silt deposits between the cross flow baffles.  Turning down the flow too far can lead to unexpected results.  The author of these tips witnessed a costly example of the effect of specifying excess surface with which operators dealt by throttling the cooling water.  Looking toward a future increase in plant capacity, a chemical plant specified 35% excess surface in a condenser in which the process fluid was to be condensed inside 2” Nickel 200 tubes.  Because the excess surface sub-cooled the condensate unacceptably, the operators turned down the cooling water so much that it evaporated into steam.  When the increased plant capacity was achieved, the condenser could not perform as designed.  Investigation showed that the shell side was so badly scaled that the condenser had to be scrapped.

Alternatively use a bypass loop shown in the following sketch that re-circulates some of the cooling water exiting the exchanger and controlling the recirculation rate to temper the cooling water to the design point temperature.  In either case, maintain the design point cooling water flow.

Similar schemes can be applied if the cooling water flows in the shell.

Technical Tip 5: Always vent non-condensable gases from heat transfer surfaces

Unvented non-condensable gases blanket heat transfer surfaces.  The effects may appear as an erratic or underperforming exchanger.  Feedwater heaters are typically provided with separate start-up and operating vents.  The operating vent lines are fitted with orifices that meter approximately ˝ of 1% of the extraction steam to the surface condenser which has an air removal section.

Technical Tip 6: Make sure to specify how the heat exchanger will be used.

Normal operating conditions cover starting up, shutting down and operations over the course of production.  Always describe the start-up and shut-down procedures in your specification.  If the heat exchanger will be subject to cyclical temperature and pressure conditions you must make this fact known to the Manufacturer along with an estimate of the number of cycles anticipated over a given time period. Specify that the Manufacturer shall design the heat exchanger for the most severe conditions of all normal operations.  The most severe condition may occur during start-up, during shut-down or during changes in process conditions, some of which can change fluid characteristics.  This means that the specification writer must disclose how the exchanger will be started up and shut down and all other normal conditions of operation.  Specify start-up and shut-down procedures in supplements to the standard TEMA or HEI specification sheets.  Specify low load and overload conditions and any other conditions other than design point conditions by providing the Manufacturer with separate specification sheets that show these conditions.

Specifying conditions for fixed tubesheet exchangers with shell expansion joints The differential between the metal temperature of the shell and the tubes varies during start-up, shut-down and when there are changes in flow, pressure and temperature conditions in the course of operation.  The thermal designer of an exchanger must apprise the heat exchanger Manufacturer of the coincident tube and shell mean metal temperatures to allow the expansion joint designer to set the amplitude and direction of the deflections that it will undergo for all of these operations.  Expansion joint design requires not only knowing this information but also the anticipated number of cycles of deflection.  In addition, the expansion joint designer must know of any imposed bending or twisting that the joint may undergo.

Section VIII Appendix 5 details the ASME Code requirements for flanged or flanged only (thick walled) expansions joints.  Appendix 26 details the requirement for bellows type (thin-walled) expansion joints.

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