How can Polypropylene transfer heat as good as metals?

Many people have the opinion that heat exchangers of plastics should have a very poor performance because of the poor thermal conductivity of plastics. This is not the case! 

 

Figure 1. Heat transfer through a single wall.

 

The heat transfer through a single wall is dependent on three factors; the overall heat transfer coefficient, U, the heat transferring area, A, and the temperature difference between the heat emitting and the heat absorbing media.

 

In a heat exchanger the overall heat transfer coefficient, U, is dependent on; the convective heat transfer coefficient for both the hot and the cold fluid and the thermal conductivity of the wall material.

 

It can be shown that the overall heat transfer coefficient, U, is governed by the individual convection heat transfer coefficient, h, on the gas (air) side of the walls in the heat exchanger.

 

This means that the heat transfer in this case is relatively insensitive to the type of material used in the walls. If the heat exchanger was made of aluminum or steel instead of polypropylene, the overall heat transfer would not change significantly. 

 

The surface characteristics of the heat exchanger is also important, affecting pressure drop and fouling tendencies. Organic and mineral deposits are less adherent to the plastic surface, compared to metals, which gives less fouling and that the exchangers are easier to clean. Fouling adds an extra thermal resistance that lowers the overall heat transfer.

 

Definition of the overall heat transfer coefficient, U

The overall heat transfer coefficient for a plate heat exchanger is calculated by;

where

U [W/m2K] is the overall heat  transfer coefficient 

A [m2] is the contact area for each fluid side

k [W/mK] is the thermal conductivity of the material

h [W/m2K] is the individual convection heat transfer coefficient for each fluid

Δxw [m] is the wall thickness

 

 

Thermal resistance

The overall heat transfer coefficient can also be calculated by the view of thermal resistances. This means that the wall is split in areas of thermal resistance, i.e. the heat transfer between the fluid and the wall is one resistance, the wall it self is also one and lastly the transfer between the wall and the second fluid is the last thermal resistance. 

Surface coatings, like fouling and epoxy that is commonly used with aluminium in heat exchangers, adds extra thermal resistances decreasing the overall heat transfer.

 

 

Thermal conductivity, k

The thermal conductivity, k, for some typical materials used in plate heat exchangers is shown below.    

 

Polypropylene: 0.12 W/mK

Stainless steel:   21 W/mK

Aluminium:       221 W/mK

 

 

Convection heat transfer coefficient, h

The convection heat transfer coefficient, h, is dependent on the type of media, gas or liquid, the flow properties such as velocity and other flow and temperature dependent properties.

 

Air:         10- 100   W/m2K

Water:   500-10 000 W/m2K

 

 

Simple examples

Consider a single wall with media 1 on the left side that transfers heat to media 2 on the right side of the wall. The wall thickness is assumed to be 0.1mm and the material is PP, aluminium or stainless steel.

 

The overall heat transfer coefficient, U, for a single wall (with equal areas) is;

 

Example 1

Assume that media 1 and 2 are air with the convection heat transfer coefficient hAir=50 W/m2K

 

The overall heat transfer coefficient becomes:

PP:      U=24.5 W/m2K

STEEL: U=25.0 W/m2K

ALU:    U=25.0 W/m2K

 

If we ignore the influence of the wall U becomes:

Thus with air-to-air the wall material is irrelevant, since U is governed by hAir.

 

Example 2

Assume that media 2 is a liquid (water) with the convection heat transfer coefficient h2=1000 W/m2K

The overall heat transfer coefficient becomes:

PP:      U=45.8 W/m2K

STEEL: U=47.6 W/m2K

ALU:    U=47.6 W/m2K

 

Once again we see that U is governed by hAir, and the variation between different materials is small.

 

Example 3

Assume that media 1 and 2 are water with the convection heat transfer coefficient hWater=1000 W/m2K

The overall heat transfer coefficient now becomes:

PP:      U=353 W/m2K

STEEL: U=499 W/m2K

ALU:    U=500 W/m2K

 

If the thermal conductivity is very large or the thickness of the wall is very thin, then the theoretical max. heat transfer coefficient becomes: