use of refridgeration of warm air to dry crops -

Hullo all

I am building an experimentle small scale dryer to dry grass and similar crops, with the intention of upscaling to a full scale dryer depending on the results, I am now researching if it will be worthwhile using heat recovery and or drying the incoming air. It would appear that heat pumps can work on a 3 to 1 ratio, say 15kw recovered heat would cost about 5kw electric input. Is this, although obviously a huge over simplification is it basically achievable? I estimate after the air is exhausted it will be 80 to 100% humidity, with a temp difference over ambient of only 5-10 deg C. some expected parameters ( if useful for background info ):

incoming air ambient say 18-28 deg C. humidity 60-70%
drying air requires 60+ deg C.
air flow probably somewhere around 40 m3 / minute

Any thoughts / advice will be gratefully received.


Thanks for all the advice and suggestions. I always intended to use electric heat elements as the main source, particularly on this prototype, it would appear to be sensible to use heat only at the first stage, as a proof of concept, financial considerations dictate the cheapest options to begin with ( am talking financial outlay to begin with with running costs a secondary consideration at this point) when/if I think this 1st stage looks promising then heat recovery etc. will follow. When I said that RH will increase I was referring to the the warm air passing through the wet crop, drawing moisture out of the crop and dropping in temperature.
Some interesting lines to follow up re Perrys and woodchip drying though, thanks again

Maybe this company could assist.
Heat pumps are very efficient as you say, although this company do not offer it, but sounds reasonable idea to investigate.

https://www.perryofoakley.co.uk/prod...SAAEgJrgfD_BwE

I doubt that you will achieve air off temperatures in the region of +60C from a heat pump alone.

Having said that, the use of a fridge system would be cheaper to run as opposed to oil, gas or electric means, due to the greater COP.

I would suggest that a fridge system acting as a dehumidifier, with re-heat would be a better solution. I.E., externally mounted AHU with input and extract ductwork, filtration, heat pump and re-heat (possibly electric or gas boiler and water coil with 3 port valve arrangement)

To ensure that you ended up with a workable installation, I suggest that you engage a Building Services Consultant to fully design the system, as it will involve selecting the correctly sized components and control strategy to suit your needs.

Perhaps have a look at the use of heat pump timber kilns as an example.

https://www.eberl-trocknungsanlagen.de/en/products/air-classic-drying-container-kiln-with-heat-pump.html

Heat pump will not be able to increase air temperature to 60C. You need an electrical heater. When you heat air, relative humidity will decrease. You mentioned that it will change from 60-70% to 80-100%. This is not correct.

Think there is a tumble dryer that uses a refrigeration system to dry the air and hence the clothes.

We do work at a herb growers, they use a Ravnsborg dry air generator to pre dry the herbs though they finish them with a conveyor with gas burners. Just some ideas.

It depends if you are planning to force heated air or dry air through the product
The problem with heated air on grass etc is fire risk at 60 dc once humidity has disappeared your left with a very dry product and 60 dc on it would be interested to sea how long before it catches into a ball of flames
It also depends what time of the year you are doing this as this reflects what air you are bringing in
I would perhaps suggest a internal blower unit like a grain dryer that recycles the air in the room with some form of refrigeration dehumidification this will recycle the air and dry the air without fire risk if by any chance you have a milk tank on site this could also be used to supply a drying effect via condensors

This is NOT my area of expertise, However I did work on a project that used Munters Dehumidifiers Could this be applied to your situation?
https://www.munters.com/en/solutions/dehumidification/