10 things that you may have not known about Gasketed Plate and Frame type Heat Exchangers

Suspended or dissolved Solids

Similar to what happens in a flowing river – changes in velocity and direction can caused the solids to drop out of the general flow. If these particles stick together then a blockage can occur at the top / bottom of channels between the plates, or actually inside the channels.
To know when sludge or scale is beginning to accumulate attach pressure gauges to the inlet and outlet pipes so that any increase in pressure drop (which will indicate fouling up of the PHE) can be detected by carrying out periodic inspections.

Insufficient flushing of new pipe work.

Debris such as

• Grit, stones and sand
• weld slag
• metal particles from grinding and cutting
• drink cans
• paper
• cigarette ends
• cloth/rags
• plastics (crisp bags, polythene, cable/zip ties, cable clips, drink bottles)

will all end up in the heat exchanger if not cleared out of the pipe work. The narrow plate channels prevent most debris from passing through and hence the brand-new heat exchanger acts as a filter and blocks up during the very first use. Also, “hard” debris such as stones and weld slag can become trapped at the back of the heat exchanger. Due to the turbulence, the stone/stones can twirl around in the plate port holes and rub against the very last plate in the pack and eventually wear a hole in this plate.

Fibres –

Like hairs down a plug hole – fibres can wrap around the tops of the plates slowing and/or preventing the flow from entering into the plate channel. Although there are wide gaps and FreeFlow types of plates available, if the fibres are long enough then the heat exchanger will suffer the same issue
Attach pressure gauges to the inlet and outlet pipes so that any increase in pressure drop (which will indicate fouling up of the PHE) can be detected by carrying out periodic inspections.

Threaded connections.

To allow:

• the parts to be replaced
• the use of dissimilar metals
• to reduce manufacturing operation/time and cost

many manufacturers use “push in” type threaded connections.

These are basically “top hat” shaped and are fed in from the back of the heat exchanger frame plate. The “rim” of the hat or the flange is what the O-ring on the first heat transfer plate seals against. This tight compression seal also holds the “top hat” in place and prevents it from rotating – PROVIDING that excessive rotational force is not used to attach any unions etc onto the threaded part protruding out of the front of the heat exchanger. In the real world, it is sometimes necessary to tighten up whatever is connected to the point where the friction between the first plate O-ring and the rim of the connection is exceeded and the connection rotates. In doing so, this rotation tears the first plate O-ring, or pulls it out of the gasket groove, and so the heat exchanger leaks at start up.

Pipe work installers need to know of this and to use a wrench / clamping tool to prevent the connection from rotating when tightening fittings to the PHE connections.

Attaching flanges

It is common practice for many plate heat exchanger manufacturers to provide threaded bolts holes around the connection port holes to allow pipe flanges to be bolted to the heat exchanger. Some manufacturers drill these holes all the way through the frame plate and either partially, or completely thread the length of the hole. This means that if a bolt is used that is longer than the combined thickness of the flange plus the frame plate, then there is a chance that the end of the bolt can hit the plate pack. If enough force is used to tighten the bolt then it can drill its way through the first one or two heat transfer plates thus wreaking the heat exchanger before its first use.

Check first to avoid aggravation in the future!

Pressure shocking

Fast acting valves opening and closing, rapid pump starts, poorly primed pumps, poorly drained condensate etc can all create a pressure spike to travel along the pipe work. When this spike enters the PHE, it can literally separate the plates albeit only for a millisecond. However, this is enough to force a gasket out of its groove so that when the pressure has escaped, the “blown” gasket remains stuck out of position between two plates thus a permanent leak is the result. A system of 2.5 bar can result in a pressure spike inside a PHE of 50 bar.
Gasketed plate heat exchangers need to be protected from pressure shocking.
 

High shearing of liquids

As the liquids pass through the plate channels they can be subject to high amounts of turbulence. This effectively “shears” the fluid (similar to whisking ingredients in the kitchen) Some liquids are sensitive to shearing and this can separate out individual components that make up the liquid (such as mayonnaise which contains oil and eggs and carbonated drinks which contain suspended gas and some de-icing fluids). Thus the nature of the liquid can be changed as a result of passing through the heat exchanger.

Secondly, some fluids can alter their viscosity as a result of shearing. These are non-Newtonian fluids. Thixotropic liquids thin when sheared such as tomato sauce and yoghurt and non-thixotropic liquids (rheopectic liquids) become thicker when sheared (such as cream). In many cases the change in viscosity does not have much of a detrimental effect on the liquid as many revert back to their original states relatively quickly once the shearing has stopped, but in some cases it may cause a more permanent change. In addition, the initial selection of the heat exchanger can be complex as during the selection process, the designer needs to take into account these changes in viscosity in order to correctly size pumps and to obtain the correct heat transfer area.

If the process fluids are shear-sensitive then a plate type heat exchanger may not be the best solution for the heat transfer application.

Installation location and space

Similar to an engine that has the potential to leak oil, a gasketed plate heat exchanger has the potential to leak and “sods law” will come into play and at some time in its life, there will be a leak. Also, being a gasketed device, the gaskets are not indestructible and will need to be replaced at some time.

The very act of opening the plate pack will cause some liquid to leak out onto the floor. Therefore, attention needs to be paid to where the heat exchanger is installed thus allowing for ease of maintenance to access the plate pack. Precautions should also be taken to facilitate the removal any potential leakage from the heat exchanger if spillage onto the floor / underneath the heat exchanger is going to cause problems.

Welding pipe work that is in-line / connected to the heat exchanger.

If the heat exchanger is not isolated and becomes part of the welding circuit (i.e. the welding “ground” is attached on or near to the heat exchanger) then it is possible that electrical sparks will jump across the first few plates in the pack. These sparks will be inside the plate pack and not seen from the outside. The sparks will erode the metal plates and burn away the gaskets (most likely the O-rings around the port holes). Even before the first use, the heat exchanger will be damaged to such an extent so as to cause it to leak when initially filled.

When installing a PHE, make sure that the pipe work guys know about this issue.
 

Un-supported O-rings used on steam side

Very few Plate Heat Exchanger Manufacturers produce a dedicated gasket that is to be used on the first heat transfer plate or produce the first heat transfer plate that is different to the others in the pack.

In these cases, the gaskets used on the first plate, and the plate itself, are the same as those used within the rest of the plate pack.
It is common practice to take two flow plate gaskets and cut then “muller” them to pieces to make up two half gaskets that are then glued to the first heat transfer plate.

Usual Method:
Take a gasket and cut it in half length ways
Cut off all of the clip in / snap in / hang on tabs and any other parts that will prevent the gasket from sitting correctly in the gasket grooves

Glue the two halves to the first heat transfer plate

As you can see from the picture of the first plate in the pack, the right-hand side has more gasket (for example, there are the intermediate supports going from the O-ring to the diagonal running portion of the gasket and there are locating tabs still in place) when compared to the left hand side.

The O-rings / circular parts of the gasket on the right-hand side sit into purpose made gasket grooves pressed into the plate whilst the O-rings on the left-hand side sit in an area that is usually open to the liquid flow and hence there is no purpose made gasket groove for this part to locate within. It is therefore effectively “unsupported” and is more reliant upon the adhesive holding it into place than the right- hand side is.

What happens when the steam is switched on?

In some cases, when the steam enters into a cold heat exchanger, the steam volume totally collapses and pulls a full vacuum. As you can see from the picture of the first plate, the O-rings on the left-hand side do not have much to stop them from being sucked into the port hole as the area that they sit in are usually open to the flow of liquid. Hence, the ring can be pulled out of place towards the port hole resulting in a leakage.

To prevent this type of problem, make sure that the plate pack is assembled so that the first heat transfer plate is configured so that the steam circuit is flowing through the side of the plate that has supported O-rings on the first heat transfer plate (in the picture this would be through the right-hand side)