Wednesday, December 21, 2011

Condensing heat pump dryers

In my last blog post, I discussed why the Barton Drying Engine has a remarkable Coefficient of Performance as a heat pump.  Moreover the BDE is a drying device as well – it heats and dries the air at the same time.  That leads to a question – how good are existing condensing heat pump dryers?  Perhaps the answer to that question will encourage me to pursue putative financiers with greater gusto.

The answer is now provided …

The figure below is a schematic of a conventional condensing heat pump dryer, such as is now widely available.  In this design, moist air from the tumble dryer is cooled in the evaporator for the refrigerant circuit.  This causes condensation of some water vapour to water, with water removed from the circuit.  The dry cold air is then warmed in the condenser for the refrigerant circuit, before being returned to the tumble dryer where it again picks up moisture.  Two necessary components not depicted in the figure are an inline filter and a blower to keep the air circulating. 
Schematic of a heat pump dryer.  Here black lines indicate airflow, blue lines indicate flow of refrigerant, and the purple squares are heat exchangers (evaporator and condenser for refrigerant).  The X on the refrigerant circuit is a throttling valve that lets condensed high-pressure refrigerant escape to the evaporator.

Palandre & Clodic [1] have experimentally studied the performance of condensing heat pump dryers.  Their paper also includes dryers based on mechanical steam compression, but I won’t discuss that aspect here.

For the conventional dryer studied by P&C, temperatures at the various parts of the circuit are:

drying temperature, Td                   40°C
evaporator temperature, Te            15°C
condenser temperature, Tc             60°C
blowing temperature, Tb                slightly less than Tc

Introduce the notation:

Qe                                heat transfer in evaporator [J_th/sec]
Qc                               heat transfer in condenser [J_th/sec]
W                                work done by compressor in refrigerant circuit [J_e/sec]
W′                               work done by blower and tumble motor [J_e/sec]
COPheating                           Qc / W

global COPheating          Qc / (W+W′)

P&C give the results

COPheating = 3.35 ([1], Table 3),
global COPheating = 2.2 ([1], Figure 13).

From these it follows that

(W+W′) / W = 3.35 / 2.2   which gives   W′/W = 0.52.

P&C Figure 14 gives the total energy consumption of the heat pump dryer as 1,730 Wh = 1730 × 3600 J in 105 × 60 seconds.  It follows that

W+W′ = 1730 × 3600 / (105 × 60) = 989 J /sec.

Then since W′/W = 0.52, the various quantities can be calculated explicitly:
W = 650 J_e/sec,
W′ = 338 J_e/sec,
Qc = 2,179 J_th/sec.

That tells us the nameplate performance of this particular condensing heat pump dryer.  Let’s now interpret the results …


Neglecting heat exchanger losses, heat transfer Qc raises the temperature of the air from Te to Tc, an increase of 45°C.  But note that the overall effect is to raise the air temperature from Td to Tc, an increase of only 20°C from inlet to outlet.  Thus to compare P&C results with my drying engine (BDE), COP_heating  as calculated by P&C needs to be multiplied by 20/45:

(1)        COPheating, comparison = 3.35 × 20/45 ≈ 1.49.

Note also that the comparative global COP would be even lower, namely

(2)        global COPheating, comparison = 2.2 × 20/45 ≈ 0.98.

Heat exchanger inefficiencies are not expected to dramatically alter the above conclusions.  These would mean that the temperature of the air circuit would not reach Te after the evaporator; rather it would be Te + δe.  Nor would the temperature reach Tc after the condenser; rather it would be Tcδc.  Thus the temperature increase over the condenser would be Tcδc – (Te + δe) = TcTe – (δc+ δe).  Equation (1) then would become
(3)        COPheating, comparison = 3.35 × (20 – δc) / (45 – {δc + δe}).

Provided the condenser and evaporator are both reasonably efficient, then estimates (1) and (3) should not differ greatly.

To sum up …

The COP quoted by P&C is as conventionally defined in heat pumps.  However it overstates the actual performance of condensing heat pump drying by a factor of 45/20 = 2.25.  Equation (1) is a better way to assess the heat pump effect for drying.  Once other power requirements are taken into account, equation (2) gives the actual performance that a user would see.

I have compared P&C’s results with my BDE condensation heat pump (in continuous-flow form).  At the present time, the results are a commercial secret.

Acknowledgement

Thanks to Anthony Kitchener for comments on this work and for providing the reference by Palandre & Clodic.

Reference

[1]  L Palandre & D Clodic, “Comparison of heat pump dryer and mechanical steam compression dryer”, International Congress of Refrigeration, Washington, D.C. (2003).

7 comments:

  1. Very insightful calculations! I spent a while mulling it over and I realize it actually doesn't make sense to think of heating efficiency necessarily. It makes the most sense to look at the amount of energy consumed in order to condense one gram of water as compared to the heat of vaporization. It is indeed true that the heat produced by the heat pump is the actual energy source used to evaporate the water and that it is expected for that to be very similar to the hvap used to condense... But that is still missing the point. Because the heat pump here is a closed system it is actually impossible to get a cop over one net!!! There are no fans or vents.

    Instead the beauty of this system is that it is mostly adiabatic. Regular condenser dryers lose a majority of the energy to the heat sink while this recycles it.

    That said... I would hope for a cop much larger than this in a dehumidifier. It is working with rather than against the temperature gradient after all.

    Thanks!

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