Fouling in heat exchangers

Fouling is commonly known as the accumulation of unwanted material on surfaces as, for example, tubes and pipes. Fouling phenomena are common in different industrial environments, ranging from ship hulls, natural surfaces in the marine environment (marine fouling) and fouling of heat-transfer components through chemicals contained in the cooling water.

This article contains excerpts from the paper, "Influence of inter-coolers technologies on the performance of isotherm centrifugal compressors" by Luca Porreca of MAN Diesel & Turbo Schweiz at the 2018 Turbomachinery Symposium.

Fouling in heat exchanger surfaces increases the resistance to heat transfer and therefore a loss in performances (efficiency) which is associated with a significant increase of operating and maintenance costs. Design of heat exchangers usually takes into account the decrease of heat transfer and compensate for it by implementing a larger heat transfer area.

This method, although commonly used in all industry areas, is not very convenient since it increases the capital costs of the coolers, overdesign of a number of components as water pumps, piping etc as well as the casing of isotherm compressors.In addition, increasing the heat transfer area is no measure to prevent fouling but only to account for it.

Therefore a number of preventive measures must be implied as periodic (online or offline) cleaning, water treatments, use of antifouling chemicals in the cooling water etc.All these measures have a high cost and, in some cases, require interruptions in the production which might be very expensive. Therefore a lot of effort is done in several research areas in order to minimize (or even cancel) the detrimental effect of fouling.

The difficulty of having a correct estimation of the fouling effect lies in the diversity of fouling behaviour and growth mechanisms linked with several parameters as flow velocity, wall temperature, temperature difference between water and wall, heat flux but also chemical factors as composition, concentration, corrosion and reaction behaviour, growth of micro/macro organisms etc.

In literature a classification is done where fouling is categorized in mainly 5 phases (initiation, transport, attachment, removal, ageing) and in 5 different groups (crystallization, particulate, chemical reaction, corrosion and biological).

Another important subject with fouling is the quantification and its modelling, which allows prediction of its behaviour in industrial components, in particular heat exchangers. Normally, when the primary concern is the effect on heat transfer, fouling can be quantified by the increase of the resistance to the flow of heat (m²K/W) due to fouling (termed "fouling resistance"), or by development of heat transfer coefficient (W/m²K) with time.

Fouling modelisation can be of numerical or experimental nature and is very different if considered, for example, on gas turbine/compressor blades or in non-rotating components as heat exchangers. TEMA Standards states different fouling factors (in terms of m2 K/W) according to different kind of water used as coolant media, such as river water, seawater, cooling tower water etc. However these values are only an indication and the real fouling resistance found on industrial site is, most of the time, unknown.

Typically EPC contractors specify the fouling factor.In isotherm compressors, fouling has a detrimental effect because, as explained, decreases the intercooler performances and increases capital and maintenance costs. Normally in tube/fins cooler the fouling on the gas side (fins) is much less important and detrimental compared on the fouling on the water side (inside the tubes).

Fouling on the fins is normally neglected and has much lower impact on the heat transfer since the gas wetted surface is much larger than the water wetted surface inside the tubes. For these reasons, the following analysis will consider only fouling factors on the water side. Specifying too high fouling factor in the design phase of isotherm compressors can lead to negative effects.

In fact, since in the first period of compressor operation the tubes are almost clean (no fouling), the water flow should be then reduced by throttling the inlet coolant valve.This leads to a (unexpected) very low water speed, which is actually promoting even more biological and organic fouling. Accurate quantification of the effect of fouling factor on isotherm compressor in operation is, as explained, a quite challenging task and very rare since precise site data (coolant and flow temperature at the inlet/outlet of the coolers) are seldom available.