Succeed with condensate control

By Cecil L. Smith, Cecil L. Smith, Inc.

ChemicalProcessing.com

Understand the nuances for steam-heated exchangers

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The steam-heated shell-and-tube heat exchanger is the workhorse of the chemical industry. Effective control of such an exchanger’s liquid outlet temperature often is crucial to plant operation. In the first article in this series, we focused on use of a control valve on the steam supply. This article will examine control via a valve on the condensate. Future articles will look at cascade control and liquid bypass with one or two control valves.

In many applications, a control valve on the steam supply poses a major disadvantage for condensate return, especially when the pressure within the shell of the exchanger provides the driving force for the condensate to flow back to the boiler feed-water makeup equipment.

Should the pressure in the shell drop below that required for condensate return, the exchanger fills with condensate, leading to the cycling described in the previous article. Condensate control avoids this.

Figure 1. A change in valve position directly affects the available heat-transfer area.

However, installing the control valve in the condensate return line instead of the steam supply line completely changes the mechanism for varying the heat transfer rate (UAΔT). With a valve on steam supply (previous article), altering the valve position affects the shell pressure, which affects the shell temperature, which affects the heat transfer rate (through the ΔT term). In contrast, with a valve on condensate (Figure 1), changing the valve position affects the level of condensate within the exchanger, which affects the effective heat transfer area, which affects the heat transfer rate (through the A term). The effective area for heat transfer is the heat transfer surface exposed to condensing vapors; the submerged heat transfer area mainly cools the condensate with little contribution to the total heat transfer rate.

With regard to condensate return, the pressure within the shell always is the steam supply pressure. The process designers must size the condensate return piping so that this pressure is adequate to return the condensate to the boiler house.

Exchanger response

Figure 2. A linear valve can be a good choice for control of a condensate line.

The configuration in Figure 1 often raises an issue of dynamics. The exchanger responds more slowly to control actions than does an exchanger with the control valve on the steam supply. Changes in the condensate valve position must first affect the level within the exchanger — causing a slower response. However, for most process applications, the response of the exchanger is sufficiently rapid to provide acceptable performance.

Although equal-percentage valves are generally installed for condensate control, an argument can be made for linear valve characteristics. The driving force for fluid flow is provided by the steam supply pressure. The hydrostatic head of the condensate within the exchanger is negligible in comparison. The pressure drop in the condensate return system also usually is small. (Oversizing isn’t limited to control valves.) Under these conditions, the pressure drop across the control valve in the condensate line is essentially constant. Most guidelines suggest linear valve characteristics for installations where the pressure drop across the control valve is constant. However, these guidelines aren’t always followed.

Figure 2 presents the process operating lines for both the linear valve and the equal-percentage valve. With a linear valve, the liquid outlet temperature is linearly related to the valve position. The process operating line for the equal-percentage valve clearly reflects the inherent characteristics of such a valve.

Heat transfer
Theoretically, the minimum heat transfer rate is zero. With the control valve closed, the exchanger completely fills with condensate, resulting in no heat transfer.

The maximum heat transfer occurs when the exchanger is completely drained of condensate. The value for the maximum heat transfer rate is the same, regardless of whether the control valve is on the steam supply or the condensate.

However, the consequences of attempts to exceed the maximum are very different. The exchanger in Figure 1 has the potential to “blow steam” into the condensate return system. As the control valve opens, the condensate level within the exchanger drops. If the valve is opened too much, the level drops entirely out of the exchanger and steam flows into the condensate return system, a somewhat unpleasant situation.