Vent Condensers

1. Table of Contents

2. Purpose

This procedure describes the guidelines to be followed for the design and specification of vent condensers.

3. General Discussion

There is no official definition of the term “vent condenser”. However at Inflection Point Engineering this term has normally been applied to three general types of heat exchangers. All three types partially or totally condense an all vapor feed stream.

3.1 Refluxed Condenser

The product liquid exits the exchanger through the same line that provides feed to the exchanger.

3.2 Flow-Through Condenser

The product liquid exits the exchanger through the same line as the product vapor. A down stream separation device is then used to produce a vapor product, a hydrocarbon liquid product and, if present, a liquid water product.

3.3 Exchanger-Separator Condenser

The product liquid exits the exchanger through a line that is not used by the feed or the vapor product.

For most vent condensers the liquid product is routed back to an upstream receiver that is providing the vapor that is feeding the vent condenser. Some of the condensed liquid will normally re-vaporize and hence return to the vent condenser. This recirculation will build to a rate determined by the compositions and the temperature difference across the exchanger. A large temperature difference across the vent condenser can result in an unacceptably large recirculation rate. For wet (water containing) systems water can be the major cause of a high recirculation rate. This can be avoided if a water product is produced from the upstream receiver or if liquid water produced from the exchanger is not returned to the upstream receiver.

Often a vent condenser is immediately upstream of a vacuum system and the performance of the vent condenser is critical for the performance of the vacuum system. The vacuum system may be an ejector based vacuum system or a vacuum pump system. Regardless of the type of vacuum system, a Inflection Point Engineering heat exchanger specification is produced for the vent condenser. For an ejector based system the vent condenser supply is normally in the scope for the ejector system supplier. This is to be indicated in both the Inflection Point Engineering heat exchanger specification and the Inflection Point Engineering vacuum system specification.

4. Condensed Liquid Exits through Vapor Feed Line (Refluxed Condenser)

This type of vent condenser can be built with a horizontal exchanger above the receiver or with a vertical exchanger mounted directly to the receiver. A horizontal exchanger will have the cooling medium on the tube side while the vertical exchanger will have the cooling medium on the shell side. If the cooling medium is relatively dirty a horizontal configuration is advantageous because the vertical arrangement does not have a removable bundle to facilitate cleaning of the shellside.

These configurations normally can not be used for wet systems with the receiver being too hot to form a liquid water phase. The recycle rate of the water between the receiver and the vent condenser would be unacceptably high.

The inlet vapor velocity must be low enough so that the condensed liquid can gravity flow against the inlet vapor. This requires that the exchanger and inlet line are sized for very low pressure drops.

4.1 Horizontal Vent Condenser Draining to Upstream Receiver

Figure 4.1 is a section of an old PID module. This shows a horizontal shell and tube vent condenser located above a column receiver.

Figure 4.1

Horizontal Vent Condenser above a Receiver

a. Some key features of this design are:

• The exchanger is located directly above the receiver. The line from the receiver to the exchanger needs to be free draining from the exchanger into the receiver. It is expected that this line will be no longer than 15 ft. This expectation should not normally be specified.
• The exchanger inlet line is sized for a vapor velocity that is less than the smaller of the two following equations.

Procedure has more information on these equations. The first equation is based on ’s law and is likely applicable to systems with atmospheric and higher pressures. The second equation is based on the Intermediate Law and is likely applicable for vacuum systems.

• Liquid that is produced in the exchanger is to drain out of the vapor inlet nozzle and through the inlet line back into the receiver. This line should be marked on PFD and P&ID to show flow in both directions. In general sloping of this exchanger is expected to be desirable. A standard exchanger icon should be shown on the PID with the words “SLOPE FOR DRAINING” written close to it with an arrow pointing to the exchanger.
• The heat exchanger specification should call for a slope of 0.5”/ft (40mm/m) toward the shellside inlet nozzle. This specification should also indicate that the baffles should not block the flow of liquid to the vapor inlet nozzle. This can be done by preventing the use of horizontal baffles or by requiring the use of notches or cut outs in the bottom of the baffles.
• The heat exchanger specification should also indicate that the shellside design should permit liquid-vapor separation, and liquid should not be entrained with the vapor through the vapor outlet nozzle.

b. Advantages:

• Arrangement allows for large vent condensers
• Both the hot side and cold side are mechanically cleanable
• No external separator vessel is required

c. Disadvantages:

• Vaporizing a refrigerant on the tubeside of horizontal tubes causes design challenges and is typically avoided.
• It can be difficult to separate liquid and vapor within the heat exchanger. The objectives of good heat transfer coefficients and good phase separation compete for high vs. low shellside velocities.
• The configuration is not good for wet systems where water recycle between the receiver and the vent condenser would be high.

4.2 Horizontal Vent Condenser Draining to Location Other Than the Upstream Receiver

In some cases, some or all of the liquid from the vent condenser goes to a destination other than the receiver. In these cases, the vapor from the receiver goes into the branch connection of a tee and then up into the inlet of the vent condenser. The bottom connection of the tee (straight connection) is the liquid drain. The liquid drain line will maintain the sizing of the bidirectional flow line per the equations in section 4.1 for at least 3 ft (1000 mm) below the tee to allow vapors to disengage. The liquid lines below the swage should be sized for self-venting flow.

Figure 4.2 is an example of a vent condenser where the liquid drains to a location other than the upstream receiver. This arrangement has been used in the Fractionation Section of Phenol Units.

Figure 4.2

Horizontal Vent Condenser above a Receiver

where liquid drains to another location

The ultimate destination of the liquid is typically at a higher pressure than the vent condenser so the elevation of the tee must be sufficient to allow the system to balance hydraulically. Any instruments in this line (flow meters and/or control valves) must be located below the elevation of the final destination to ensure that there is only liquid in them.

Liquid lines are self-venting if the liquid velocity is less than the following:

4.3 Vertical Vent Condenser Directly Attached to Receiver

Figure 4.3 is a section of the PID module . This shows a vertical vent condenser that is directly attached to a column receiver.

Figure 4.3

Vertical Vent Condenser Directly Attached to Receiver

a. Some key features of this design are:

• A heat exchanger specialist should be consulted to determine the vessel nozzle size for the exchanger. While this size is not included in the Inflection Point Engineering specification it is needed to ensure that a reasonable exchanger design can be obtained.
• This type of arrangement has an upper vapor flow rate limit that is less than the horizontal vent condenser arrangement that is discussed in section 4.1.
• A clean cold fluid (e.g. refrigerant, tempered water) is required for this configuration as the refrigerant is on the shell side with two fixed tube sheets. Normally cooling water would not be acceptable. A closed chilled water system or a vaporizing refrigerant system are often used.
• No process side piping is required between the receiver and the exchanger. This reduces cost and eliminates the possibility that such piping would have a low point loop that would be a problem.

b. Advantages:

• Arrangement works with a vaporizing refrigerant as the coolant
• Requires no additional plot space if located on top of receiver
• No external separator is required

c. Disadvantages:

• Hot stream temperature profile is based on the changing compositions of the liquid and vapor, which can be difficult to model to obtain local temperature differences and proper MTD
• Size is limited to reasonable shell diameters and tube lengths depending on installation location
• Liquid holdup and entrainment limits require a minimum tubeside flow area which may affect required size and cause low heat transfer coefficients
• Shellside cannot be mechanically cleaned, so coolant must be a clean fluid such as refrigerant or tempered water.

5. Condensed Liquid Exits with Vapor (Flow-Through Condenser)

Figure 5.1 is a section of PID module This shows a vent condenser with the liquid exiting through the vapor exit line. The exiting material goes to a three phase separator. The water is removed from the system and the hydrocarbon liquid is returned to the upstream receiver.

Figure 5.1

Liquid Exits with Vapor

Having the condensed liquid exit with the product vapor has several advantages over the condensed liquid exiting through the vapor inlet line.

• The recycle of water to the upstream receiver can be stopped by the use of a three phase separator.
• The inlet line size can be much smaller.
• A higher exchanger pressure drop can be allowed because the liquid-vapor separation does not occur in the condenser. This typically leads to smaller condenser designs as compared to a refluxed condenser design. However a low exchanger pressure drop is normally desired to minimize the separator elevation requirement. Elevation is needed allow the separator liquid to flow to its destination.

The liquid return line from the separator has sometimes had a high loop to attempt to hold a liquid level in the separator. An open ended pipe distributor in the receiver should be used to keep a liquid seal and avoid receiver vapor from by-passing the vent condenser. The vent condenser rundown line should be sized like the rundown line from a standard condenser.

a. Advantages:

• Arrangement allows for large vent condensers
• Both the hot side and cold side are mechanically cleanable
• Better heat transfer coefficients can be obtained because liquid – vapor separation is not done in the exchanger
• The recycle of water to the upstream receiver can be avoided.
• The inlet line does not need to be sized for counter-current flow

b. Disadvantages:

• Vaporizing a refrigerant on the tubeside of horizontal tubes causes design challenges and is typically avoided. Condensing on the tubeside of a “Kettle” can be considered when the coolant is a vaporizing refrigerant.
• External liquid – vapor separator vessel is required

6. Condensed Liquid Exits through Separate Line (Exchanger-Separator Condenser)

This type of vent condenser functions both as an exchanger and a vapor-liquid separator. Normally no liquid level control is used for this system. Sometimes a high loop has been used on the liquid outlet line in an effort to hold a liquid level in the exchanger-separator. The main use of this configuration is when the outlet vapor goes to vacuum producing equipment. The liquid static head is normally used to allow the liquid to flow to a destination pressure that is significantly higher than the vapor inlet pressure.

a. Advantages:

• Arrangement allows for large vent condensers
• Both the hot side and cold side are mechanically cleanable
• No external separator vessel is required
• The inlet line does not need to be sized for counter-current flow

b. Disadvantages:

• Vaporizing a refrigerant on the tubeside of horizontal tubes causes design challenges and is typically avoided
• It can be difficult to separate liquid and vapor within the heat exchanger. The objectives of good heat transfer coefficients and good phase separation compete for high vs. low shellside velocities.

6.1 Exchanger-Separator with Boot

Figure 6.1 is a section of a PID that shows an exchanger-separator with a boot. While such systems have been specified in the past, there is presently no justified use of this configuration. The condenser elevation must be sufficient to allow the liquid to flow to a destination with a pressure that is higher than the source pressure of the inlet vapor.

Figure 6.1

Liquid Boot on Vent Gas Condenser

6.2 Exchanger-Separator without Boot

Figure 6.2 is a section of the PID that shows an exchanger-separator without a boot. This configuration is used when the vapor goes to vacuum producing equipment. The condenser elevation must be sufficient to allow the liquid to flow to a destination with a pressure that is higher than the source pressure of the inlet vapor.

Figure 6.2

Vent Gas Condenser with Liquid from Bottom of Shell