IPE-TM-400 Heat Exchangers
Steam as Heating Medium
IPE-TM-400-05
1. Purpose
This procedure describes Inflection Point Engineering practice for specifying the steam side of tubular type heat exchangers when steam is used as the heating medium.
2. General
In general the temperature and pressure conditions of the inlet steam, used to specify the exchanger, are the minimum (MIN) conditions indicated in the BEDQ. One exception to this is when an exchanger uses steam side control. For this system calculate the temperature and pressure conditions for the inlet steam using the method given in Procedure . A note is added to the exchanger specification defining the maximum conditions in the BEDQ for the vendor to check the performance. Tool , can be used to provide the necessary calculations as described below.
Use the following guidelines whenever steam is used as a heating medium.
3. Guidelines
3.1 Desuperheating
General
The high heat flux in the superheated steam area of a reboiler can lead to unstable boiling on the process side. Superheated steam can also be prone to cycling temperatures and thermal shocking of the exchanger. Hydraulic calculations should be based on the steam being completely desuperheated. The controls for the desuperheater should be set to have a small amount (~10ºF (~5ºC)) of superheat remaining. Since achieving saturation requires a more expensive desuperheater and since the temperature controller cannot work in the saturated region of constant temperature, this is a more economical arrangement. The saturated temperature shall be based on using the maximum steam pressure less the pressure drop losses from the desuperheater and piping (typically 7 psi (0.5 kg/cm2)). Refer to Procedure for more information on desuperheaters.
If there are no process degradation concerns and nucleate boiling can be maintained, the requirement to desuperheat the steam may be eliminated following consultation with a Heat Exchanger Equipment Specialist.
Shell & Tube Heat Exchangers
Superheated steam results in large thermal gradients in the steam side girth flanges of the exchanger, which can lead to leakage. Steam with more than 100ºF (56ºC) of superheat, at either of the minimum, normal or maximum operating conditions, shall be desuperheated to near the saturated temperature prior to entry into a shell & tube type heat exchanger.
Hairpin Heat Exchangers
Steam with more than 200ºF (111ºC) of superheat, at either of the minimum, normal or maximum operating conditions, shall be desuperheated to near the saturated temperature prior to entry into a multitube hairpin or double pipe heat exchanger.
3.2 Steam Conditions
The steam conditions shown in the 401 Specification vary depending on the controls that exist between the steam manifold and the exchanger. The pressure drop within the exchanger itself is assumed to be zero. A minimum condition is shown in the data sheet portion of the specification while the maximum condition is shown as a note for that item. The seven probable configurations (Figure 1) are listed below:
a. Case 1: Condensate Control - No desuperheater used
The inlet steam temperature to the exchanger is shown as the BEDQ minimum operating temperature value. The pressure in and out of the exchanger is the BEDQ minimum operating pressure value.
- The conditions in the note for alternate steam conditions shall reflect the maximum operating values for exchanger pressure and temperature. The saturated temperature at the maximum operating pressure conditions is also provided.
b. Case 2: Condensate Control - Desuperheater used
The inlet steam temperature to the exchanger is shown as the desuperheater outlet temperature. This is determined by calculating the saturation temperature at all three operating pressure levels from the BEDQ less the pressure drop losses from the desuperheater and piping (7 psi typical) and then adding 10°F to the highest value. If the minimum steam operating pressure level’s temperature is greater than this calculated value, the calculated value is used. If the minimum steam operating pressure level’s temperature is less than this calculated value, the value in the BEDQ is used. The pressure in and out of the exchanger is the minimum operating pressure value less the pressure drop across the desuperheater and piping.
- The conditions in the note for alternate steam conditions shall reflect the maximum operating pressure level considering a failure of the desuperheater. The value for exchanger pressure will be the maximum operating pressure level from the BEDQ less the pressure drop across the desuperheater and piping. The temperature listed will be the steam temperature entering the desuperheater, which is the maximum operating temperature from the BEDQ. The saturated temperature at this exchanger pressure is also provided.
c. Case 3: Condensate Control – Steam Pressure Controlled - No desuperheater used
The pressure in and out of the exchanger is the pressure controller’s target pressure value. The inlet steam temperature to the exchanger is calculated based on using the BEDQ minimum operating pressure value less the pressure drop across the control valve. The steam temperature changes across the pressure control valve.
- The conditions in the note for alternate steam conditions shall reflect the maximum operating pressure values for exchanger pressure and temperature as shown in the BEDQ. The basis for this condition is the failure of the pressure control valve to a wide open position. The saturated temperature at the maximum operating pressure condition is also provided.
d. Case 4: Condensate Control – Steam Pressure Control Upstream of Desuperheater
The pressure in and out of the exchanger is the pressure controller’s target pressure value less the pressure drop losses from the desuperheater and piping (7 psi typical). The inlet steam temperature to the exchanger is shown as the desuperheater outlet temperature. This is determined by calculating the saturation temperature at all three operating pressure levels from the BEDQ corrected considering the temperature and pressure change across the pressure control valve and then less the pressure drop losses from the desuperheater and piping (7 psi typical) and then adding 10°F to the highest value. If the minimum steam operating pressure level’s temperature is greater than this calculated value, the calculated value is used. If the minimum steam operating pressure level’s temperature is less than this calculated value, the value in the BEDQ with the pressure valve correction is used.
- The conditions in the note for alternate steam conditions shall reflect the maximum operating pressure level considering a failure of the desuperheater but that the pressure controller is functioning. The temperature will be the steam temperature entering the desuperheater corrected for the pressure drop across the pressure control valve and the desuperheater and piping. The values for exchanger pressure will be the maximum operating pressure level less the reduction across the pressure controller and less the pressure drop losses from the desuperheater and piping. The saturated temperature at this pressure is also provided.
e. Case 5: Condensate Control – Steam Pressure Control Downstream of Desuperheater
The pressure in and out of the exchanger is the pressure controller’s target pressure value. The inlet steam temperature to the exchanger is shown as the desuperheater outlet temperature corrected for the pressure drop across the pressure control valve. This is determined by calculating the saturation temperature at all three operating pressure levels from the BEDQ less the pressure drop losses from the desuperheater and piping (7 psi typical) and then adding 10°F to the highest value. This value is then corrected considering the change in temperature across the pressure control valve. If the minimum steam operating pressure level’s temperature is greater than this desuperheater outlet value, the corrected desuperheater outlet value is used. If the minimum steam operating pressure level’s temperature is less than this calculated desuperheater outlet value, the value in the BEDQ with the pressure valve correction is used.
- The conditions in the note for alternate steam conditions shall reflect the maximum operating pressure level considering a failure of the desuperheater but that the pressure controller is functioning. The values for exchanger pressure will be the maximum operating pressure level less the reduction across the pressure controller. The saturated temperature at this pressure is also provided. The temperature will be the steam temperature entering the desuperheater but corrected for the pressure drop across the pressure control valve.
f. Case 6: Steam Flow Control
Calculate the temperature and pressure conditions for the inlet steam using the method given in .
- The conditions in the note for alternate steam conditions shall reflect the maximum operating values for exchanger pressure and temperature as shown in the BEDQ. The basis for this condition is the failure of the steam control valve to a wide open position. The saturated temperature at the maximum operating pressure condition is also provided.
g. Case 7: Steam Flow Control – Steam Flow Control Downstream of Desuperheater
Case 7 is a very unusual situation that results only if very highly superheated steam is used for flow control. Calculate the temperature and pressure conditions for the inlet steam using a modification of the method given in . The steam is desuperheated as in Case 2 except that the amount of superheat remaining is set by the designer instead of the normal 10°F. The amount of superheat must allow for sufficient margin to control the steam flow rate.
The conditions in the note for alternate steam conditions shall reflect the maximum operating values for exchanger pressure and temperature as shown in the BEDQ. The basis for this condition is the failure of the desuperheater. The saturated temperature at the maximum operating pressure condition is also provided.
3.3 LMTD
Inflection Point Engineering uses saturated steam temperature at both exchanger inlet and outlet to calculate LMTD for sizing purpose. However, vendors may determine the exchanger LMTD based on the superheated steam conditions. This will require calculation of heat transfer rates in desuperheating zone and condensation zone separately.
3.4 Steam Quantity
The enthalpy of the steam is determined at the data sheet inlet condition for the appropriate case above. The enthalpy of the liquid condensate at the exchanger operating pressure is also determined. The difference in the enthalpies and the exchanger’s duty is used to determine the steam flow rate shown on the data sheet.
Likewise, these two conditions are used to determine the density of the steam or condensate shown in the property table.
Tube to Tubesheet Joint
Specify strength welding of the tube to tubesheet joint (reference Procedure ). This is to prevent leakage of the tube joint due to temperature cycling of the superheated steam.
3.6 Design Temperature and Pressure
Specify the steam side design temperature and pressure based upon the mechanical design conditions for the steam system in the BEDQ. Assume that the desuperheater is not functioning.
Figure 1 – Steam Heated Exchanger Configurations
Case 1 - Case 2 -
Condensate Control - No Desuperheater Condensate Control - With Desuperheater
Case 3 - Case 4 -
Condensate Control Condensate Control
Steam Pressure Control – No Desuperheater Steam Pressure Control – Upstream of Desuperheater
Case 5 - Case 6 -
Condensate Control Steam Flow Control
Steam Pressure Control – Downstream of Desuperheater No Desuperheater
Case 7 -
Steam Flow Control
Downstream of Desuperheater
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