Heating and cooling FAQ
Below you will find a number of common questions along with answers from Alfa Laval’s experts. Don’t see your question here? Contact us to get a personal response from our experienced team.
Questions about brazed and fusion-bonded plate heat exchangers
Questions about gasketed plate heat exchangers
Questions about gasketed plate heat exchangers
What is the minimum temperature difference you can have between the two sides of a gasketed plate heat exchanger?
Customers today are focused on saving more energy, and the trend in heat recovery duties is for minimal additional cooling/heating to recover more energy. That means that the units will work with a smaller temperature difference.
For liquid/liquid duties, Alfa Laval offers plate heat exchangers that can work down to a temperature difference of about 1°C. Compare this to shell-and-tubes, where the limit is around 5°C. The reason why we can work with such a low-temperature difference is that our units have an arrangement that is almost counter-current.
What is the maximum temperature difference you can have between the two sides of a gasketed plate heat exchanger?
If looking at liquid/liquid duties from a thermal point of view, there is no maximum limit. If you have very high-temperature differences, which also includes very high temperatures, you should take into consideration the radiation effect. However, since Alfa Laval has a maximum design temperature of around 180°C for gasketed plate heat exchangers, this is not an issue.
When you have a large temperature difference, it can be a large temperature difference between the bulk temperature and the wall temperature. You should check the wall temperature to avoid boiling or freezing as well as excessive fouling.
What gasket material should I use?
Depending on the media, different gasket materials are more or less suitable. In HVAC applications where the media tends to be water/water, the recommended gasket material is NBR, which is available in several different qualities.
What is the gasket lifetime?
Gasket lifetime depends on many different factors, most importantly the operating temperature and pressure, and the media in question. However, the lifetime is also determined by the cleaning agents used, how often the plate heat exchanger has been opened and closed, if the heat exchanger is used in continuous operation or not, and how the gaskets have been stored.
If the plate heat exchanger is running at the maximum recommended temperature, the gasket lifetime is one year. The lifetime will be longer at a lower temperature, but all of the factors listed above must be considered in order to reach a rough lifetime prediction.
How do I know which type of gasket I have in my heat exchanger?
There are three ways to identify the gaskets. Start by finding the documentation that was supplied with the unit at the time of purchase. Next, look for a tag on the gasket with the article number, which you can find on our new T-range. If your gaskets do not have this tag, you can look for a color code, which is typically located on the clip of the gasket. In some cases, the color code is on the inside of the gasket, and this can sometimes even be seen from the outside. Contact Alfa Laval with this information and the information located on your unit’s nameplate, and our representatives can help you determine your model and gasket type.
Questions about semi-welded plate heat exchangers
What is the minimum temperature difference you can have between the brine/water side and the evaporation temperature in a SWPHE flooded evaporator?
We normally say that an approach could be minimum 2K, if the liquid separator is mounted above the unit. Then the liquid column will be more or less optimal (c/c + 0.5 m) and the pressure losses be reasonable. A good oil separation system will also improve the possibilities to achieve a close approach. Please ask your Alfa Laval representative for an advice if you are in doubt.
What is the largest temperature difference I can have between the inlet and outlet brine/water in a SWPHE evaporator?
To give a general answer we normally say 15K. The reason is that if we have a higher Δt we will have a low pressure drop in the channels. That means a lower turbulence in the channels which will reduce the heat transfer efficiency and make the self-cleaning effect lower. A clean heat transfer area is always important for the performance.
Using a multi-pass heat exchanger will increase the heat transfer length of the SWPHE and allow for a higher temperature difference.
Can I use any type of media in a SWPHE?
No, you should always ask the Alfa Laval representative if you are going to use an unknown fluid. That can e.g. be a brine type on the brine/water side or a compressor oil on the refrigerant side. If you are in doubt your Alfa Laval representative will be able to advice both in metal and in gasket material selection.
A SWPHE allows for use of a wide range of heat transfer plate material such as titanium and SMO (ALLOY254) enabling use of corrosive material such as sea water or chloride-based brines.
What level of design pressure shall I select for a SWPHE?
First, you must check what your locally used pressure vessel code is saying. We generally say that the minimum design pressure should be the operating pressure times 1.3. If you use this factor you can assure the high-pressure safety switch-off device will work properly. It is then allowed to have some small pressure variations in the refrigeration system without a lot of false alarms and unwanted shut offs. Alfa Laval offers SWPHE with design pressures up to 63 bar (900 psi).
Why is the refrigerant side pressure drop so important in a SWPHE thermosiphon evaporator?
As the system is only using the natural laws, with help from the gravity, the density and the height of a liquid column, every pressure drop will affect the performance. An increased pressure drop can always be converted to a higher compressor load and therefore a higher need of input power. It will always cost more to be imprudent with the pressure drop on the refrigeration side compared to the water side. The energy needed in the compressor motor is always much higher compared to the need in the circulating pump motor.
Why is it important to drain oil in a refrigeration system with a SWPHE evaporator?
Compressor oil contamination is common in refrigeration systems. In industrial ammonia refrigeration, using non soluble oil, it is expected that this oil will gather in the coldest place in the system; the evaporator. In the evaporator the oil will build up a film on the heat transfer surfaces, effectively creating a fouling which reduces the evaporator performance.
In order to reduce the oil fouling, Alfa Laval recommends any user of a non-soluble oil to install an efficient oil separation before the refrigerant enters the evaporator.
How often do I need to change the gaskets in a SWPHE?
The time between service of any gasket plate heat exchanger or SWPHE is dependent on the duty. A unit running with sea or river water might need cleaning of the heat transfer surface more often than a heat exchanger running with a closed loop brine circuit.
For a normal closed loop glycol circuit evaporator or a CO2/ammonia cascade condenser normal time between gasket changes will be 15 years or more. Please contact your local Alfa Laval representative for advice.
Why is the heat transfer area so small in a SWPHE compare to a shell-and-plate or shell-and-tube type of heat exchanger?
The plates of a SWPHE are designed to always maintain a turbulence flow in the channels.
The turbulent flow secures a very high efficient heat transfer which drastically reduces the amount of heat transfer area needed compared to other technologies.
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