Tumble dryer dehumidifiers

I know that being office based I am a bit out of touch with the technical side but look at this snippet I got from a post on UK-DIY about tumble dryer dehumidifiers

The question is, who on this site could work this one out? (and explain to a customer)

and would it not be easier (and cheaper in the long run) to sell them a condensor dryer..

>>I’ve never used one but:
>>- They seem to come with ice packs that you put in the freezer overnight
>>before use.
>
>
> At best – the amount of heat released by water condensing is about
> twice (per kilo) that of ice melting.
> So, you’r at best going to need double the amount of water you remove
> from the clothes.
> To get a “reasonable” result, say 10C output at 100{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d} RH, you are probably
> looking at an ice pack that will absorb around 4Kw (heat added by dryer
> and heat that would be needed to heat the air from 10C-room temp).
>
> Say the dryer runs for 1 hour, 4Kwh is around 15MJ, so will melt around
> 15Kg of ice.

In fact it’s worse than that – at 273K, latent heat of fusion is
3.3×10^5 J/kg, latent heat of vapourisation 2.5×10^6 J/kg.

Also specific heat capacity of liquid water is 4.2×10^3 J/kg/K, of ice
2.1×10^3 J/kg/K.

In more detail, and looking at it slightly differently to you, each
kilogramme of water to be condensed and cooled (from say 60 degC) to
room temperature (say 20 degC) will release 2.5×10^6 + (4.2×10^3 x 40) =
2668 kJ (assumes condensation happens at 60). If this is removed by
heating ice (starting at -15 degC) to 0 degC and then melting it we will
absorb 361.5 kJ per kilo.

2668/361.5=7.38

In other words we would need ~7kg of ice per kilo (~litre) of water
soaked into the clothes.[quote:2k2r4vlr]

And I thought the VAT return was bad.

John