Heat Exchanger: Type, Maintenance and Efficiency Calculation

In the series of my blogs of engineering knowledge, here a small introduction to the heat exchanger. So let’s starts with this basic question.

What are the heat exchangers?

In general, simply we can say that the heat exchanger uses to exchange heat or heat energy from one medium to another. This medium may be heat transfer from liquid to liquid, liquid to gas or gas to liquid and this will depend on application and requirements. For example, we use a heat exchanger in refrigerator gas by cold water or air in a refrigeration system.

There are various system and types of machinery where we use the heat exchanger on the ship and in industries. On the ship, we use an air cooler in main engine, lube oil cooler of the generator, F.O.  &  D.O. cooler in fuel oil line system, in fresh water generator and many more places. In some cases, we use the heat exchanger to reduce the temperature in minimum time with maximum rate or efficiency. And with the help of heat exchangers simply we can reduce the temperature of liquid or gas and change the phase of medium i.e. from gas to liquid or liquid to gas by reducing the temperature of one medium and transferring to another medium.

What are the different types of the heat exchanger?

Basically heat exchangers are classified on the basis of their flow of liquid or their method of contact of liquid. On the basis of flow there is two type of heat exchangers:

  • Parallel flows
  • Counterflow

Parallel flow heat exchangers are those where both coolant (or heating fluid) and the other substances(which is to be cooled or heated) will flow in the same direction. This system is used when the range of temperature exchange is less, in fact, both fluids while exiting the exchanger body will have the same temperature(if efficiency is high). Whereas in counterflow heat exchangers the flow of fluids is in opposite directions. This gives an advantage over the other heat exchanger. There will always be a temperature difference between fluids. The heat exchange takes place throughout the exchanger.

And on the basis of contact, we can divide them  as:

  • Direct contact
  • Indirect contact

Those heat exchangers where cooling medium is in direct contact of substance to cool. For example cooling of hot iron in metal industries, barometric condensers throughout the petroleum industry etc. Please note that the medium of both coolant and other substance should be of different state mostly. Indirect contact heat exchangers are commonly used in the marine industry. In this type, there is always a boundary between fluids(always fluids). Same as in case of plate type or shell and tube type heat exchangers.

But here we are going to discuss heat exchangers which are used in marine industry, they are indirect contact type means both the liquid is separated by a tube or wall. On the basis of application, design, and requirement there are basically three types of heat exchanger we are using in the marine industry. Let’s discuss one by one.

heat exchanger shell & tube type
Basic structure of Shell & Tube Type Heat Exchanger

Shell and tube type of heat exchanger:-This is one of the common type of heat exchanger, a combination of shell and tube as shown in the figure, this shell and tube exchanger consists of a number of tubes mounted inside a cylindrical shell in which tubes runs parallel to shell. The transfer of heat takes place through the surface of the tubes. Tube side liquid which flows in tubes(is commonly called coolant for the cooler) and shell side liquid flows outside and at the surface of tubes inside the shell. This type of heat exchanger consists of basically six elements:

  • Bonnet
  • Tub sheet
  • Shell
  • Tubes
  • Baffles or support plates
  • Tie rods
  • Expansion tube plate
Heat exchanger shell & tube actual
LO Cooler, Sea water side cleaning

Here in this diagram flow passage of liquid and different parts are shown. The tubes of this cooler( or better we call it heat exchanger) are aluminum brass (76% copper; 22% zinc; 2% aluminum), because of its good conductivity of heat.
The tube stacks are made up to have a fixed tube plate at one end and a tube plate at the other end (Figure above ) which is free to move when the tubes expand or contract. The tube stack is constructed with baffles of the disc and ring, single or double segmental types. The fixed end tube plate is sandwiched between the shell and water box, with jointing material, Synthetic rubber ‘O’ rings for the sliding tube plate permit free expansion.

Shell and tube type heat exchanger is extremely economical to install and easy to clean. However, the frequency of maintenance is higher than other types. Manufacturers recommend that coolers be arranged vertically. Where horizontal installation is necessary, the sea water should enter at the bottom and leave at the top, to prevent any airlock. Air in the cooling system will encourage corrosion and it will reduce the cooling area and cause overheating. Vent cocks should be fitted for purging air and cocks or a plug are required at the bottom, for draining. Clearance is required at the cooler fixed end for removal of the tube stack.

heat exchanger plate type
Basic Structure of Plate Type Heat Exchanger

Plate type heat exchanger:– In this plate type heat exchanger two corrugated plates joined together and creates the cavities between plates in which liquids flow. Two different liquids flow alternate sides of the plate, the transfer of heat between this two liquid take places through this corrugated plate. The installation of the plate type heat exchanger is expensive than shell and tube type heat exchange but the efficiency of plate type heat exchanger is higher than shell and tube heat exchangers.

Plat type heat exchanger consists of five basic elements:

  • Cover
  • Carrier rail
  • Heat transfer plates (corrugated plat)
  • Support column
  • And the tie bolts
Heat exchanger fresh water generator
Condensor and Evaporator of Fresh Water Generator

Numbers of corrugated plats are sandwiched between a fixed cover and a movable cover with the help of tie bolts. Two top and bottom carrier rails are provided to keep align the plates to each other. Each plate contains a gasket at the boundary and in the nozzle ports of the plate. This gasket prevents the two fluids from mixing and leakage from the plates.

“Corrugation in the plates gives strength to them which reduces the size of the heat exchanger by thining plates and it also increases the surface area. Both the factors lead to increase the efficiency of the heat exchanger.”

Air-cooled type heat exchanger:– It is also called fin-fan heat exchanger. It consists of a bundle of tubes, a fan system, and a supporting structure. The tube can have various types of fins in order to provide an additional surface area on the air side. Air is either sucked up through the tubes by a fan mounted above the bundle (induced draught) or blown through the tubes by a fan mounted under the bundle (forced draught). They tend to be used in locations where there are problems in obtaining an adequate supply of cooling water. For example the radiator of the emergency generator.

Maintenance in Heat Exchangers:

All the heat exchangers required routine maintenance for best performance which is affected by scale formation and deposits. To avoid scale formation and minimize fouling we need to do.

  • Backflush to remove deposits
  • Chemical treatments at regular intervals
  • Dismantle the component to clean properly after a certain time interval as mentioned in manual. In tube-type, we need to clean the tube and in plate type, we have to clean plates.
  • Leakage test with help of N2/He testing of Soap Testing. In case of any leakage, we have to repair that leakage.
  • Pressure testing of body

Note: – In the case of the defect in one plate in the plate type heat exchanger, we need to change with another plate or we can remove. We have to take care while removing defective plate we have to remove adjacent of that plate too. So, therefore with one damaged plate replace two plates the damaged one and one of the adjacent plates with two new plates.

Performance and Efficiency of the Heat Exchangers:-

To understand or assess the performance of a real component or system, we need to calculate the efficiency of the system or component. The term efficiency means the comparison between the actual and ideal performance of the component or system. There are various factors which affect the efficiency of that system, with time the efficiency of the system decreases, if proper maintenance not carried out. By calculating the efficiency we can alter the possible factor to achieve the best performance.

In heat exchangers, there are two of the widely used methods to judge the performance.

LMTD (Logarithmic Mean Temperature Difference):– In any heat exchanger there is two end one end for entry of fluid and another for the exit. The LMTD is the logarithmic average of the temperature difference between the hot and cold feed at each end of the heat exchanger. It can be assumed as the driven force for the heat exchangers.

If the LMTD value increases then the amount of heat transfer between the two fluids will increase vise-versa. This means by calculating LMTD we can get the idea of performance and effectiveness of the heat exchanger.

LMTD method is used when the inlet and the exit temperature of the liquids are known.
To calculate LMTD value –

Q = U * A * LMTD

Where
Q – Amount of heat exchanged
U – Heat transfer coefficient (in watts/Kelvin)
A – Area of heat transfer

*** Here it is clear from the above equation that LMTD is directly proportional to the amount of heat exchange.

   LMTD = (ΔTa-ΔTb)/(ln(ΔTa/ΔTb)) = (ΔTa- ΔTb)/(lnΔTa-lnΔTb)

ΔTa= Temperature difference at the end ‘a’ of the exchanger
ΔTb= Temperature difference at end ‘b’ of the exchanger

Here graph represents the comparison of the amount of heat exchange between parallel flow and counter flow.

Q – Amount of heat flow
Tci – Inlet temp. of cold liquid
Tco – Outlet temp. of cold liquid
Thi – Inlet temp. of hot liquid
Tho – Outlet temp. of hot liquid

NTU (Number of Transfer Unit) method: This method is mostly used in situations where the size of the heat exchanger and inlet temperature are known. It is based on a dimensionless parameter known as the heat transfer effectiveness, which is defined as –

Efficiency (ℇ) = (Actual heat transfer rate)/(maximum heat transfer rate) = q/(q(max))

To find NTU we have to make assumptions that the heat exchanger has infinite length and the rate of heat transfer is maximum (Qmax). Then it is given by –

Effectiveness (ℇ) = q/(q(max))

Qmax = Cm,minimum (Thi – Tci)

Where –
Thi – Inlet temp. of hot liquid
Tci – Inlet temp. of cold liquid
Cm – (Mass flow rate * Specific heat of hot or cold liquid)

We will take the least value of Cm for Cm cold or Cm hot.

Actual heat transfer rate q = Ch(Thi – Tho) = Cc(Tco – Tci)

Where
Thi – Inlet temp. of hot liquid
Tci – Inlet temp. of cold liquid
Tho – Outlet temp. of hot liquid
Tco – Outlet temp. of cold liquid

heat exchanger scale formation
Scale formation on plate of Heat Exchanger

The value of efficiency we can get and compare.

The most important maintenance work carried out on any cooler or heater (heat exchanger) is periodic cleaning of them. Normally all sea water coolers are cleaned onboard once in a month, seawater side. Other heat exchanger cleaning includes air side of air cooler cleaning, fresh water generator condenser, and evaporator cleaning, LO heater LO side cleaning etc. The heat exchanger efficiency will be maximum if its surface is free of any deposits/scale/sludge.

Please do read our blogs on Ballast water management and Boiler water treatment.

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Abhay Kumar

He is working as engineer in Shipping Corporation Of India, member of Nielsons Educational Forum. He is alumni of Marine Engineering & Research Institute(MERI) Mumbai. He loves driving and writing blog in his free time. Specialty : Tanker ship

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