When preparing to design or specify a heat exchanger, do you ever wonder where to start? You've done it before, but you hate that feeling of getting half way through the design and realizing that you forgot to consider one important element. All too often end-users specify equipment poorly because they fail to understand how the equipment works, or what is involved in correctly selecting it. The thought process behind the original specification can be just as important as the detailed design calculations. The following steps should help give you a better insight and understanding into the methodology behind heat exchanger design.
The design of a heat exchanger usually proceeds through the following stages.
1. Specifying the required process conditions and ensuring that a heat or energy balance is achieved. This involves listing the flow rates, temperatures, pressures and fluid compositions for both hot and cold sides respectively. Operating pressures are very important for gases as their properties vary greatly with pressure. From this information the heat load can be determined and the energy balance checked. The hot side must equal the cold side, and if any term is unknown (e.g. the cold side flow rate) then it can be easily evaluated.
(m . cp . dT)Cold = Q = (m . cp . dT)Hot or (m . dH)Cold = Q = (m . dH)Hot
where, m = mass flow rate, kg/s
Cp = Specific heat of the fluid, kJ/kgK
dT = Difference in temperature between inlet and outlet, °C.
Q = Heat Load, kW
dH = Enthalpy, kJ/kg (generally used for multi-phase fluid)
2. Determining the fluid physical properties over the operating temperature and pressure range. For most applications these would include specific heat, density, thermal conductivity, viscosity and latent heat (enthalpy) for phase changes. For most common fluids these properties are readily available.
3. Determining the allowable pressure drops in the exchanger? End-users often don’t recognise the importance of specifying an appropriate pressure drop for the application. All too often a very low pressure drop is specified as it means a few dollars can be saved on a pump, not recognizing that the heat exchanger sized/ selected will need to be much bigger and hence more expensive, often far outweighing the dollars saved. Lower pressure drops also mean lower velocities, reduced heat transfer and efficiency and can also lead to increased fouling problems. Each application needs to be assessed individually, but generally for water systems 100kPa is a good starting figure to use, refrigerant oil coolers ≈ 70 kPa, and for process gas systems approximately 2% of the operating pressure. For refrigerants you need to refer back to the pressure/enthalpy chart and determine effects of the pressure loss to the system.
4. Type of shell and tube exchanger required? Shell and tube HX’s can come in many different configurations and it is important that the best configuration is chosen. The different styles are outlined in TEMA, and include fixed tubesheet, U-tubes, floating head type and kettle reboilers to name a few. Each has its advantage for different applications. For applications without large temperature differences (as in most refrigeration applications) fixed tubesheet designs provide the most economical solution. When thermal stresses become an issue (e.g. steam fired CIP heaters for the food process and dairy industries) then U-tube or floating head designs should be employed as they allow for differential thermal expansion. It is normal to put the fluid that fouls or is the most corrosive on the tubeside as it is generally easier to inspect and clean the tube side than the shell side.
5. Preliminary sizing of the heat exchanger. This is where the fun begins! The standard heat transfer equation is Q = U . A . LMTD, where Q = the known heat load, LMTD is the log mean temperature difference which is just a simple function of the temperatures in and out, U is the overall heat transfer coefficient and A is the surface area of the exchanger required. Normally we are trying to determine the surface area required, so it is just a matter of calculating the overall heat transfer coefficient, plugging it into the above equation and solving for the area required.
The overall heat transfer coefficient is made up from a number of terms. It is a function of the exchanger geometry and physical properties of the fluids involved. Because the geometry of the exchanger affects the heat transfer coefficients, it is normal to take an iterative approach to sizing. First we assume some ‘typical’ heat transfer values to determine an estimate of the surface area required. Then we can nominate a unit to give this estimated surface area, and using this geometry calculate the exact heat transfer coefficient. This can then be compared to the original estimate and further iterations made if required. Typical overall HT coefficients for oil coolers are around 600-900 W/m2K, ammonia condensers 1200-1800 W/m2K, ammonia flooded evaporators 800-1400 W/m2K.
6. Calculating the overall heat transfer coefficient U. The million-dollar question…! There are literally thousands of different correlations and formulas, many very complex for calculating this. The overall heat transfer coefficient is made up from a number of terms. They are added similar to electrical resistances in parallel.
U = 1 / (1/hs + 1/ht + 1/hw +fs + ft)
where hs,ht = shell and tube side film coefficients
hw = coefficient due to tube wall (function of tube conductivity and wall thickness)
fs, ft = fouling factors for shell and tube side fluids.
These latter 3 terms are known or easily obtained, hence the real problem is to evaluate the film coefficients. Some text books show Nu = 0.27(Re)0.8(Pr)0.33 as the primary equation for turbulent flow heat transfer. What they often fail to mention is that exponents can vary markedly for different situations (e.g. evaporating in the tubes has different exponents than that in the shell). It is generally recommended at this stage to approach the experts for detailed design - who will utilise expensive high-powered computer programs with complex algorithms for optimising the heat exchanger both thermally and mechanically. Flotech is a licensee of Aspentech BJAC, in addition to utilising extensive in-house thermodynamic and mechanical design software.
Hopefully the above will enable you to better appreciate heat exchanger sizing and the basic principles behind the thermal design of shell and tube units. For more information, please contact one of our offices listed on the "contacts" page.
- Steve Rowntree, BE Mech (Hons)