# How much energy does each leather drying method use per kilogram of water, and what does a heat pump change?

Canonical: https://senseible.earth/answers/tannery-drying-heat-pump-mj-per-kg-water

The European tanning reference document lists drying energy in megajoules (MJ) per kilogram of water evaporated: toggling 8.17, vacuum 7.20, chamber 5.83 and through-feed 5.22, against a theoretical minimum of 2.48. With a heat pump, the same table gives 1.62 for chamber and 1.12 for through-feed.

## The table, MJ per kilogram of water evaporated

- Theoretical minimum: 2.48
- Toggling: 8.17. Pasting: 6.37. High-frequency drying: 6.84 (no heat pump figure).
- Chamber drying: 5.83 without a heat pump, 1.62 with one.
- Vacuum drying: 7.20 without, 1.37 with.
- Through-feed drying: 5.22 without, 1.12 with.
- The document takes these from a 1986 German-language paper, so treat them as old.

## Which energy is it?

Without a heat pump the document says the energy is mainly thermal. The exception is high-frequency drying, which uses electricity only. For the heat pump columns it does not say which energy is counted.

My reading: the heat pump figures sit below the 2.48 theoretical minimum, which is possible when they count only what the heat pump draws, because a heat pump delivers more heat than the electricity it uses. If so, a heat pump moves energy from a fuel invoice to the electricity bill. Confirm with the maker before you plan the Scope 1 and Scope 2 change.

## When the document says a heat pump pays

It says generating heat with heat pumps is normally only economical when the price of a gigajoule (GJ) of electricity is less than 2 to 2.5 times the price of a GJ of heat from fuel. Work that ratio out from your own bills before you ask for quotes.

## Cheaper steps the document names first

- Squeezing out more water in samming before drying can save 0.5 to 1 GJ per tonne of raw hide in drying.
- Up to 75% of the waste heat from drying may be recovered with heat exchangers.
- Clean cooling water from a vacuum dryer, fed into the hot water supply, may recover 10 to 20% of the vacuum drying energy.
- Natural drying uses the least energy, but needs time and a favourable climate.

## Example: 1,000 kilograms of water evaporated in a chamber dryer

- Without a heat pump: 5.83 x 1,000 = 5,830 MJ
- With a heat pump: 1.62 x 1,000 = 1,620 MJ, which is 4,210 MJ less, or 72.2%
- Vacuum: (7.20 minus 1.37) / 7.20 = 81.0% less. Through-feed: (5.22 minus 1.12) / 5.22 = 78.5% less
- If the 1,620 MJ were all electricity (an assumption): 1,620 / 3.6 = 450 kilowatt hours (kWh) x 0.675 kilograms of carbon dioxide (CO2) per kWh for financial year (FY) 2025-26 = 303.8 kilograms (kg)

Table values are the document's, from 1986 literature. Your dryer, leather and load will differ; measure your own fuel and kWh per month before and after.

## What Senseible does here

Senseible reads fuel invoices and electricity bills into records by billing period, so a before and after comparison of fuel and kWh is on file. It does not meter a dryer or measure water evaporated.

- [Where a tannery's energy goes](https://senseible.earth/answers/tannery-energy-split-drying-hot-water-heating)
- [Tannery energy per tonne of raw hide](https://senseible.earth/answers/tannery-energy-per-tonne-raw-hide-european-range)
- [Green loan readiness check](https://senseible.earth/green-loan-check)

Sources: [European Commission, Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Tanning of Hides and Skins, February 2003 edition (filed by the publisher as superseded)](https://eippcb.jrc.ec.europa.eu/sites/default/files/2020-03/superseded_tan_bref-0203.pdf), [CEA, CO2 Baseline Database for the Indian Power Sector, version 22.0](https://cea.nic.in/cdm-co2-baseline-view?mappingId=234&lang=en)

Last reviewed 2026-11-06. Figures are from the February 2003 edition of the European tanning reference document, which the publisher files as superseded; its energy and effluent tables rest on European tanneries and on older studies. Check the current edition and your own records before you quote a number to a buyer.