Frequently Asked Questions:

What is dew point?
How is heat transfer duty calculated?
How is heat exchanger performance calculated?
How is mean temperature difference calculated?
How is a U value calculated?
 

Q. What is Dew Point?
The dew point is the temperature to which a given parcel of air must be cooled, at constant pressure, for water vapor to start to condense into liquid.  The condensed water is called dew. The dew point is a saturation point.  The dew point is associated with relative humidity; high relative humidity indicates that the dew point is close to the temperature.  Relative humidity of 100% indicates the dew point is equal to the temperature and the water vapor (humidity) is saturated.  When the dew point remains constant and temperature increases, relative humidity will decrease.  At a given pressure, independent of temperature, the dew point indicates the mole fraction of water vapor in the air, and therefore determines the specific humidity of the air.

Find data for your area on Climate Charts world-wide map.  Click on a nearby station to view a chart of daily high and low temperatures.  The overnight low is often a few degrees below the daytime dew point.

The highest ambient dew point ever recorded was 95°F, in Dhahran, Saudi Arabia, on the Persian Gulf on July 8, 2003.  In the USA, the highest dew points (above 80°F) occur near the Gulf of Mexico and in parts of the upper Mississippi Valley.  Dew points higher than 80°F are rare, even in the tropics.  The most humid USA design condition published by ASHRAE (American Society of Heating, Refrigeration & Air Conditioning Engineers) is Galveston, Tx: a 75.2 °F dew point, which is exceeded only 1% of the average summer, or about 30 hours.

The heat index below gives some perspective on dew points.  Beware of a specification of "100 F, 100% RH" ...as you can see it's off the chart; it simply does not happen.

NOAA's National Weather Service
Heat Index

Humidity specified in terms of dew point is much simpler and less error-prone than the often misused term relative humidity.  Relative humidity is often specified without an accurate reference temperature (to which the humidity is relative).  For instance, it's common to see something like: "Average temperature 55 to 94°F and relative humidity of 35% to 100%" for a location ...but no mention of the fact that the 94°F occurs at 3 PM and the 100% relative humidity occurs at 3 AM when the air temperature is 65°F.  Given the exponential shape of the dew point curve, it would grossly overstate the amount of moisture in the air to take data like this and presume the average high temperature/humidity is 94°F and 100% relative humidity.

The capital and operating costs (e.g. refrigeration) of heat exchangers that cool below dew point are exponentially related to the specified dew point.  Notice how the curve above is trending towards a vertical line as temperature increases.  Overly conservative humidity specifications make projects financially unjustifiable and have kept a lot of beneficial systems on the drawing board.  Please take care to specify a realistic dew point, or ask us and we'll look up the ASHRAE design climate data for the installation site.

 

Q. How is heat transfer duty calculated?
Heat transfer can be classified in two ways:

Sensible heat transfer, also known as temperature change.   For example, it takes one BTU to raise one pound of water one °F.  The key relationship here is:   

Q = C_p • M • TD

Where:
  Q is BTU/hr
  C_p is heat capacity in BTU/lb-F (the C_p of water is 1)
  M is mass flow in lb/hr
  TD is Temperature difference in °F

Latent heat transfer, also known as phase change.  For example, it takes 1000 BTUs to boil one pound of liquid water into steam - at the same temperature.  The key relationship here is:

Q = H_fg • M

Where:
  Q is BTU/hr
  H_fg is the latent heat of vaporization in BTU/lb (the H_fg of water is 1000)
  M is mass flow in lb/hr

 

Q. How is heat exchanger performance calculated?
Heat exchanger design starts looking just as simple, but it gets much more interesting!  Heat exchangers are machines that get fluids to transfer their heat.  Most heat exchangers work with two fluids flowing separate passages, for example cold water flowing inside a tube and warm air flowing outside the tube.  When this happens, the cold fluid warms up and the hot fluid cools off.  The key relationship here is:

Where:
  A is the required amount of surface area in ft²
  Q is duty in BTU/hr
  U is the performance of the heat exchanger in BTU/hr-ft²-°F 
  LMTD is the mean temperature difference throughout the heat exchanger in °F

 

Q. How is mean temperature difference calculated?
The LMTD is a computation that takes the inlet and outlet temperatures of both fluids and reduces them to one number, which is the average temperature difference:



     T₁ = Hot Stream Inlet Temp.
     T₂ = Hot Stream Outlet Temp.
       t₁ = Cold Stream Inlet Temp.
       t₂ = Cold Stream Outlet Temp.

 

Q. How is a U value calculated?
The U value is the heat exchanger's performance coefficient, it's based on the unit's design, materials and the fluids that flow through.  A heat exchanger made from aluminum, will have a higher U value than one made of plastic, because aluminum is a better conductor of heat.  A heat exchanger using water as coolant will have a higher U value than it would using air as coolant, because water is a better coolant.  The key relationship here is:

 
Where:
  U is the performance coefficient for the heat exchanger in BTU/hr-ft²-°F
  h₁, h₂, h₃, etc. for a fin tube, air-to-water heat exchanger are heat transfer coefficients typically:
air, fin, tube, water.
 R₁, R₂, R₃, etc. is the thermal resistance of the various aspects of the heat exchanger in hr-ft²-°F/BTU, typically:
Internal fouling factor, metal contact resistance, external fouling factor

The number and type of heat transfer coefficients and thermal resistances will vary from one heat exchanger design to the next.  In a simple unit where two fluids flow on opposite sides of a metal plate, there might be an h_cold-fluid, R_metal-plate and h_hot-fluid.  The metal resistance is trivial because the thermal conductivity of metals is well known.  The fluid heat transfer coefficients depend not only on the fluid, but on it's level of turbulence as it flows through the heat exchanger.  Determining fluid heat transfer coefficients is often the crux of designing heat exchangers.