BIZOMAT MAKEOS · Hydraulics
Heat Exchanger Sizing (Duty, Outlet Temperature and Area)
How much heat does this exchanger move, what will the cold stream leave at, and is the installed area enough?
Sizing check of a liquid-to-liquid heat exchanger: duty from the hot stream’s flow, specific heat and temperature drop, the cold stream outlet temperature from the energy balance, the log-mean temperature difference for counterflow or parallel flow (as the heat exchanger LMTD tool), the area needed with your overall coefficient, and checks of the installed area and the closest approach temperature.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Hot stream | ||
| Hot flow | L/min | Volume flow of the hot stream. |
| Hot density | kg/m³ | Density of the hot fluid. |
| Hot specific heat | kJ/kg·K | Specific heat of the hot fluid (about 1.9–2.1 for mineral oil). |
| Hot inlet | °C | Hot stream inlet temperature. |
| Hot outlet | °C | Hot stream outlet temperature wanted. |
| Cold stream | ||
| Cold flow | L/min | Volume flow of the cooling stream. |
| Cold density | kg/m³ | Density of the cold fluid. |
| Cold specific heat | kJ/kg·K | Specific heat of the cold fluid. |
| Cold inlet | °C | Cold stream inlet temperature. |
| Exchanger | ||
| Flow arrangement | choice | Whether the streams run in opposite directions (counterflow) or the same direction. |
| Overall heat transfer coefficient | W/m²·K | Overall heat transfer coefficient for the exchanger type and fluids, including fouling (from the maker or your data). |
| Installed area | m² | Heat transfer area of the exchanger fitted or offered. |
| Closest approach temperature allowed | K | Smallest temperature difference you accept between the streams at either end. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
Hot stream
What this means: Hot flow
- What it is
- The volume of hot fluid (the stream being cooled) passing through the exchanger each minute, from the pump or circuit flow.
- Units and format
- Enter a number in L/min: litres per minute. The form accepts 0.000001 L/min to 1000000 L/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Duty rises in proportion and the cold outlet gets warmer, so the area needed rises.
What this means: Hot density
- What it is
- The mass per unit volume of the hot fluid at its working temperature, used to turn its volume flow into mass flow.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 3000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Hot specific heat
- What it is
- The heat one kilogram of the hot fluid gives up per degree it cools, from the fluid’s data sheet.
- Units and format
- Enter a number in kJ/kg·K: kilojoules per kilogram per kelvin. The form accepts 0.000001 kJ/kg·K to 10 kJ/kg·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4.18 kJ/kg·K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Duty rises in proportion.
- Get it right
- kJ/kg·K, not J/kg·K.
What this means: Hot inlet
- What it is
- The temperature of the hot fluid as it enters the exchanger.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 400 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 70 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Hot outlet
- What it is
- The temperature you want the hot fluid to leave at. It must be below the hot inlet.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 400 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower outlet raises the duty and the area needed.
Cold stream
What this means: Cold flow
- What it is
- The volume of cooling fluid passing through the exchanger each minute, from the cooling water supply or pump.
- Units and format
- Enter a number in L/min: litres per minute. The form accepts 0.000001 L/min to 1000000 L/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More cooling flow lowers the cold outlet temperature, which raises the LMTD and lowers the area needed.
What this means: Cold density
- What it is
- The mass per unit volume of the cooling fluid at its working temperature, used to turn its volume flow into mass flow.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 3000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Cold specific heat
- What it is
- The heat one kilogram of the cooling fluid absorbs per degree it warms, from the fluid’s data sheet.
- Units and format
- Enter a number in kJ/kg·K: kilojoules per kilogram per kelvin. The form accepts 0.000001 kJ/kg·K to 10 kJ/kg·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4.18 kJ/kg·K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- kJ/kg·K, not J/kg·K.
What this means: Cold inlet
- What it is
- The temperature of the cooling fluid as it enters the exchanger, for example the cooling water supply temperature on the hottest day.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 400 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the warmest supply temperature you expect, not an average.
Exchanger
What this means: Flow arrangement
- What it is
- Whether the two streams travel through the exchanger in opposite directions (counterflow) or the same direction (parallel flow). It sets the end temperature differences.
- Units and format
- Choose one of 2 options from the list: Counterflow; Parallel flow.
- Example
- The form opens with “Counterflow” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Counterflow gives a larger LMTD for the same temperatures, so less area.
What this means: Overall heat transfer coefficient
- What it is
- The overall rate of heat transfer per square metre per degree of temperature difference, through both fluid films, the wall and any fouling, from the maker or your data.
- Units and format
- Enter a number in W/m²·K: watts per square metre per kelvin. The form accepts 0.000001 W/m²·K to 10000 W/m²·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 400 W/m²·K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The area needed is inversely proportional to U.
- Get it right
- Use a value that includes fouling; a clean value makes the exchanger look larger than it will be in service.
What this means: Installed area
- What it is
- The heat transfer surface of the exchanger fitted or offered, from its data sheet. It is compared with the area needed.
- Units and format
- Enter a number in m²: square metres. The form accepts 0.000001 m² to 100000 m². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 m² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Closest approach temperature allowed
- What it is
- The smallest temperature difference you accept between the hot and cold streams at either end of the exchanger.
- Units and format
- Enter a number in K: kelvin (a difference of 1 K equals 1 °C). The form accepts 0 K to 100 K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- This is a difference in kelvin, not a temperature.
Method and formulas
- Duty = ṁ_h c_ph (T_hi − T_ho), ṁ = flow × density.
- Cold outlet T_co = T_ci + duty ÷ (ṁ_c c_pc).
- LMTD = (ΔT₁ − ΔT₂) ÷ ln(ΔT₁ ÷ ΔT₂) with the end differences for the flow arrangement (as the LMTD tool).
- Area needed = duty ÷ (U × LMTD).
- Example: cooling 30 L/min from 70 to 50 °C moves 41.8 kW; the water leaves at 35 °C and 3.5161 m² is needed.
Limitations
- Pure counterflow or parallel flow; shell-and-tube with several passes needs an LMTD correction factor.
- Constant specific heats and no phase change.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size an oil or coolant cooler for a machine
- Find the water outlet temperature of a cooler
- Check whether an existing exchanger has enough area
Who it is for
- Plant engineers
- Hydraulic engineers
- Maintenance engineers
What you get
- Duty and cold outlet temperature
- LMTD and area needed
- Area and approach checks
Interpreting the result
- Counterflow needs less area for the same duty and lets the cold outlet exceed the hot outlet.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 371–378. Topic reference; no source tables or recommended settings are reproduced.
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