Heat Exchanger Input Control

1. Purpose

This procedure describes and compares two methods of controlling the heat input to the steam side of a heat exchanger. In addition it establishes standard design for condensate pots, when required.

2. General

The two methods to consider are:

- Control valve in the condensate outlet line.

- Control valve in the steam inlet line.

Inflection Point Engineering uses both methods in process designs, depending on the particular circumstances. In many cases both methods are acceptable. In this situation Inflection Point Engineering would recommend the method with the control valve in the condensate line.

3. Description of Systems Using the Two Methods

3.1 Control Valve in Condensate Outlet Line (Condensate Control)

This method is shown in Attachment 1, Figure 1 and will be referred to as "condensate control" in this procedure. The exchanger is operating at full steam header pressure with a level of condensate in the exchanger. A reduction in heat input is achieved by reducing the condensate flow rate and allowing condensate to back up and flood heat exchanger surface. Flooded surface has a much lower heat transfer coefficient than condensing surface. Therefore, heat input control with condensate control is accomplished by changing the flow rate of condensate and by the resultant variation of the effective heat transfer surface of the exchanger.

This type of control system can also be used for a fractionator overhead condensing system, e.g., where xylene splitter overhead is used to reboil a Raffinate Column.

3.2 Control Valve in Steam Inlet Line (Steam Control)

This method is shown in Attachment 1, Figure 2 and will be referred to as "steam control" in this procedure. The exchanger is operating at a pressure equivalent to the full steam header pressure less the pressure drop across the control valve. The steam trap discharges condensate as fast as it forms. A change in the flow rate of steam to the exchanger causes a change in the steam pressure in the exchanger. This in turn causes a change in the condensing temperature of the steam and in the LMTD of the exchanger. The FRC adjusts the control valve to control the flow rate at the new downstream exchanger pressure. Therefore, heat input control with steam control is accomplished by changing the flow rate of steam and by the resultant variation in the LMTD of the exchanger.

4. Advantages and Disadvantages of the Two Methods

4.1 Operation at Turndown

Using steam control under turndown conditions could result in a low and variable steam side pressure if the process side temperature is low. This could make steady state return of the condensate impossible and would result in unsteady heat input control. As a guideline, steam control is not appropriate if the process side outlet temperature is lower than the saturation temperature of steam at the condensate return line pressure plus 30 psi. This is shown in the table below. Note that this is a general guideline. The guideline may be violated if detailed calculations show that the desired operating cases can be achieved. See Attachment 2 for a sample calculation.

Condensate Return Minimum Process Side

System Pressure * Outlet Temperature **

(psig) (F)

0 274

10 287

20 298

30 307

40 316

50 324

60 331

70 338

80 344

90 350

100 356

110 361

120 366

130 371

* Obtained from Basic Engineering Design Questionnaire

**Based on saturation temperature of steam at 30 psi higher than condensate return line pressure

This type of pressure surging at turndown should not occur with condensate control, since the exchanger is always at full steam header pressure. During condensate control at lower turndown, subcooling of the condensate may occur. To avoid hammering in condensate return systems, design condensate piping to avoid pocketing and be free draining.

4.2 Operation at Start-up

At start-up on steam control, clean heat exchangers can result in low steam side pressures. This could lead to similar problems at start-up as described in Section 4.1 for turndown. Check this by estimating the clean exchanger steam side condensing temperature at start-up, allowing for some turndown. Check to ensure that this gives sufficient pressure to enable condensate to return to the condensate return system. If the return pressure is too low, specify either condensate control or a pumped condensate return system. See Attachment 1 for a sample calculation.

4.3 Capital and Operating Cost

Since the steam condensing temperature is higher for condensate control, less surface is required. Capital cost will be lower for condensate control. Steam control requires less steam because the latent heat is higher at lower pressures. Operating cost will be lower for steam control. Neither cost effect is particularly significant.

4.4 Heat Sensitive Materials

The steam condensing temperature is lower for steam control. This can be used to an advantage if there are heat sensitive materials on the process side, all other factors being equal.

4.5 High Pressure Steam for Reboilers

Consider steam control when using the high pressure steam as the heating medium for hydrocarbon reboilers or the saturation steam temperature at header pressure exceeds 125ºF over the process temperature. The high steam temperature could result in film boiling and unstable reboiler operation.

4.6 Thermal Stresses

If the temperature difference between the process and the saturated steam temperature at the steam header pressure is high, use steam control to advantage to minimize thermal stresses in the reboiler.

4.7 Condensate Control Steam Blow Through

If a condensate control system tries to remove more condensate than the exchanger can condense, then steam, rather than condensate, will pass through the FRC. This may cause the condensate control valve to open and close rapidly.

4.8 Condensate Corrosion

During condensate control a condensate level is always blanketing some of the tubes. This condensate may cause corrosion and fouling of the tubes.

5. Pressure Drop Across Control Valve in Steam Control System

Use the following guideline to set the pressure drop across the control valve in a steam control system. Choose the operating pressure in the exchanger so that the saturated steam temperature in the exchanger (Tes) gives a LMTD that is 85 per cent of the LMTD based on the saturated steam temperature in the steam header. The saturated steam temperature in the exchanger is given by formula 1 below.

1) Tes = (Tro - Tri)/(K - 1) + Tro

where

K = ((Ths - Tri)/(Ths - Tro))1/0.85

2) For isothermal reboilers, where Tri = Tro, equation 1 simplifies to the equations below. Isothermal behavior is normally assumed when the exchanger pressure drop is less than 2 psi.

Tes = 0.85*(Ths - Tro) + Tro

Tes = 0.85*Ths + 0.15*Tro

where

Tes = saturated steam temperature in reboiler or exchanger

Ths = saturated steam temperature in header

Tri = reboiler or exchanger inlet temperature

Tro = reboiler or exchanger outlet temperature

After the temperature is calculated, find the steam pressure using the steam tables.

6. Standard Condensate Pot Design

As a default Inflection Point Engineering does not use a condensate pot for either condensate control or steam side control. However many of our customers prefer to have condensate pots. The use of condensate pots will prevent steam from blowing into the condensate header. Figure 3 and 4 show P&IDs for condensate pots for condensate control and steam side control, respectively. These are expected to be appropriate for most jobs but should be modified when required. Some customers may have their own standards and these should be used if possible.

For both systems the pot pressure floats on the steam inlet pressure to the exchanger. The elevation difference between the liquid level in the pot and the exchanger outlet nozzle is the head that is available for the frictional loss through the exchanger and the rundown line. Steam heaters are normally designed to have a steam side pressure drop that is between 0.1 (nil) and 0.5 psi. Having the bottom of the exchanger elevated 900 mm above the condensate pot bottom tangent line should be sufficient to supply the required head for steam/condensate control. The condensate line from the exchanger to the pot is expected to be short with a minimum number of elbows and sized for a maximum delta pressure per 100 ft of 0.15 psi.

The pot should be designed for a surge time (0 to 100% of LI range) of 30 seconds. This specification will determine the required vessel diameter. The Inflection Point Engineering Vessel Design Program Win254 is normally used to design the pot. The Service Type is “Surge Drum” and the Sub-Service is “Normal Surge”.

The LT should have top/side connections. The bottom level nozzle should be 6 inches above the bottom tangent line. The condensate inlet nozzle should connect to the side of the pot close to the bottom tangent line.

6.1 Condensate Control

A 32 inch (810 mm) float is normally used for this system. The condensate control valve is normally controlled by the upstream FIC. The LIC on the condensate pot can take over control if the level falls below the set point so as to prevent steam blowing out the line to the condensate header. The level control should normally have a low level alarm point of 10% of the range. Inflection Point Engineering does not provide a set point but it is expected to be at the low level alarm point. The observed normal liquid level will be higher than this set point and is a result of the hydraulics. Partial flooding of the exchanger is normal operation. There is no need for a high level alarm.

The P&ID identifies the minimum elevation of the condensate drum above the condensate control valve. This is normally based on providing at least 50 inches of water head (50 inches/SG of the condensate). IPE-TM-400-11 attachment 3 has some discussion on a similar criterion.

The P&ID identifies the minimum control valve elevation to allow maintenance. This is normally set at 1.5 ft (450 mm) plus half the pipe diameter. IPE-TM-400-11, Attachment 3 also documents this criterion.

6.2 Steam Side Control

A 14 inch (360 mm) float is normally used for this system. The condensate control valve is normally controlled by the LIC on the condensate pot. The minimum float range is used here as this level is normally tightly controlled. A low level is desirable as this minimizes the elevation requirement for the exchanger.

Attachment 1 Figures

Figure 1 Condensate Control

Figure 2 Steam Control

Figure 3 Condensate Side Control Standard

Figure 4 Steam Side Control Standard

Attachment 2 Calculation Example of Steam Control Reboiler Under Clean Start-up Condition

The equation relating overall and clean heat transfer coefficients is:

U = 1 (1)

1/Uc + Ro + Ri

where U = overall heat transfer coefficient (Btu/h-ft2-ºF)

Uc = clean heat transfer coefficient (Btu/h-ft2-ºF)

Ro = fouling resistance outside tubes (h-ft2-ºF/Btu)

Ri = fouling resistance inside tubes (h-ft2-ºF/Btu)

The general equation for heat transfer in an exchanger is:

Q = U*A*LMTD and it follows that Qs = Uc*A*LMTDs

LMTDs = U*LMTD*Qs (2)

Uc*Q

where Q = exchanger duty (Btu/h)

Qs = exchanger duty at start-up (Btu/h)

A = exchanger area (ft2)

LMTD = exchanger log mean temperature difference at design conditions (Fº)

LMTDs = exchanger log mean temperature difference at start-up conditions (Fº)

Consider a simplified example of a C3/C4 splitter with a steam control reboiler under the following conditions:

Steam pressure at reboiler = 44 psig

Condensate return pressure = 30 psig

*Steam condensing temperature = 290ºF

*Reboiler inlet and outlet temperature = 240ºF

U = 100.0 Btu/h-ft2-ºF LMTD = 50Fº

Ro = 0.001 h-ft2-ºF /Btu Qs = 15.0*106 Btu/h

Ri = 0.001 h-ft2-ºF /Btu Q = 20.0*106 Btu/h

From equation (1) Uc = 1

1/U - Ro - Ri

= 1

1/100 - 0.001 - 0.001

= 125.0 Btu/h-ft2-ºF

From equation (2) LMTDs = 100*50*15 = 30.0 Fº

125*20

Steam temperature at start-up = Reboiler Temperature + LMTDs = 240 + 30 = 270ºF

Steam pressure at startup = 27 psig (from steam tables)

This is insufficient to get into the condensate return header at 30 psig.

*Note that this example violates the guidelines of Section 4.1, which, if followed, would avoid the need for this calculation. Section 4.1 indicates that for a condensate return system pressure of 30 psig the minimum process side outlet temperature is 307F. In this example the reboiler outlet temperature is only 240F.