Heat Exchanger designing method contains heat Exchanger designing general method
It has following steps
STEP 1 Heat Duty Calculations
STEP 2 Selection of Cooling Medium or Heating
STEP 3 Energy Balance and Heat Transfer Calculations
STEP 4 Mean Temperature Difference
STEP 5 Estimation of Overall Heat Transfer Coefficient
STEP 6 Finding Shell Diameters
STEP 1 Heat Duty Calculations
1. For cooling or heating or for no phase change, heat duty is calculated by the equation
ø = m Cp Δt
where,
ø = Heat duty required, kW
m = Mass flow rate of fluid, kg/s
Cp = Specific heat of fluid, kJ/(kg )
At = Temperature difference to be effected, C
2. For condensation with sub cooling
where,
m = Mass flow rate of vapour, kg/s
CL = Specific heat of condensate, kJ/(kg . °C)
𝜆 = Latent heat of vaporization, kJ/kg
3. For reboilers
where, mv𝜆 = Vaporization rate, kg/s
𝜆 = Latent heat of vaporization, kJ/kg
STEP 2 Selection of Cooling Medium or Heating
Selection of cooling or heating medium mainly depends on the medium temperature (inlet temperature) at which cooling medium or heating medium is required.
Other factors like heat transfer coefficient provided by cooling medium or heating medium, cost of the medium, etc., are also considered in the final selection.
Tables 1 and 2 can be used as a guideline for the selection of heating medium and cooling medium respectively.
Commonly Used Heating Fluids
Heating mediums
If temperature of heating medium is required in between 100° to 180°C, saturated steam is preferably used as heating medium.
Saturated steam condenses in dropwise manner, hence it provides very high heat transfer coefficient [about 6000 W/(m2. °C)].
Superheated steam is not generally selected as heating medium for indirect (through the wall) heat transfer because desuperheating zone provides very low heat transfer coefficient [about 100-150 W/m2. °C).
If the temperature of heating medium required is greater than 180°C, then use of saturated steam as a heating medium is not economical.
From 180°C to 300°C, hot oil (thermic fluid) is preferred as heating medium.
Hot oil system is easily available at comparatively low cost.
Hot oil provides low heat transfer coefficient and also properties of oil are uncertain.
Dowtherm A is an organic fluid of high heat stability, an eutectic mixture containing 73.5 percent diphenyl oxide and 26.5 per cent diphenyl by mass.
Dowtherm A is used as a heating medium in both phases; as a liquid as well as saturated vapour.
Dowtherm E is specially processed orthodichlorobenzene.
For the precise application on a large scale, Dowtherm E can be considered as an alternate to hot oil.
Molten salts are a molten mixtures of inorganic salts; one of which is an eutectic, consisting 40% NaNO2, 7% NaNO3 and 53% KNO3.
Sodium potassium alloys: 56% Na 44% K and 22% Na-78% K, are also used as heating mediums.
Cooling mediums
Cooling mediums In refrigeration terminology a brine is any liquid cooled by a refrigerant and circulated as a heat transfer fluid.
Operating temperature range is 68°C to 5°C.
For any fluid to be cooled below 8°C (minimum possible temperature of chilled water. 5°C + minimum driving force required for heat transfer in shell and tube heat exchanger, 3°C), brines are used as cooling medium. Brine may be
1. An aqueous solutions of inorganic salts, e.g., sodium chloride solution, calcium chloride solution, etc.
2. Aqueous solutions of organic compounds such as alcohols or glycols,
e.g. ethanol, methanol, ethylene glycol, propylene glycol, etc.
3. Chlorinated or fluorinated hydrocarbons, e.g. methylene chloride, trichloroethylene, trichlorofluoromethane, halogenated hydrocarbons as refrigerants, etc.
STEP 3 Energy Balance and Heat Transfer Calculations
Establish energy balance or heat duty balance and based on that find the mass flow rate of heating medium or cooling medium.
For example in case of condenser
where,
m = Mass flow rate of vapour condensed, kg/s
𝜆 = Latent heat of condensation kJ/kg
mw = Mass flow rate of cooling medium required kg/s
CLw = Specific heat of cooling medium kJ/(kg . °C)
t'2, t'1 = Outlet/inlet temperatures of cooling medium, °C
STEP 4 Mean Temperature Difference
Mean temperature difference (MTD) is calculated from equation
Where,
𝚫Tm = Logarithmic mean temperature difference, °C
Where, 𝚫T2 and 𝚫T1 are terminal temperature differences.
For 1-1 heat exchanger (one shell side pass and one tube side pass), Ft= 1 When shell side passes and/or tube side passes are more than one. Ft must be determined.
STEP 5 Estimation of Overall Heat Transfer Coefficient
Assume the value of U. overall heat transfer coefficient.
For this, use the standard tables, in which the range of the values of U for the different cases are given.
Refer to Table for the recommended values.
Typical Overall Coefficients for Shell and Tube Heat Exchangers.
Find out anticipated heat transfer area based on this selected or assumed value of U.
For the first tria cinnamon, as A is the heat transfer area provided or actual transfer area of available or selected heat exchanger.
Where,
do = Outside diameter of tube, m
L = Tube length, m
Nt = Total number of tubes
Values of do and L are decided by designer and from Eq, value of Nt is determined.
Then for the first trial, number of tube side passes and shell side passes are decided.
Tube arrangement and type of heat exchanger are now selected.
STEP 6 Finding Shell Diameter
Based on all these informations find the inside diameter of shell. Select the type of baffle. First choice is 25% cut segmental baffle.
Fix the value of baffle spacing.
If there is cooling or heating on shell side then for the first trial baffle spacing Bs could be 0.3 to 0.5 times shell ID and if there is condensation or boiling on shell side, Bs could be equal to shell ID.