Pulp mill liquors heated with condenser heat
How a 60 tonne-per-day agro-residue pulp mill can preheat weak black liquor, wash water and caustic with a water-source heat pump on evaporator condenser water, and what it saves against a biomass-fired steam boiler.
▶Video: Waste-heat recovery heat pumps
V10 · 2–2.5 minLifting warm water from condensers, effluent and process cooling to useful heat
Video coming soon01Problem statement
- Live steam for preheating. Weak black liquor, washer shower water and caustic make-up are heated with live steam that could otherwise drive the evaporator.
- Condenser heat lost. The evaporator's surface condenser rejects heat at 45–55 °C to a cooling tower.
- Washing needs hot water. Brown stock washing works better and uses less water with hot showers, but heating that water uses a lot of steam.
- Losses between fuel and process. Burning briquettes in a boiler or heater and distributing the heat loses a quarter or more of the fuel's energy before it reaches the process.
- Compliance, safety and handling. Fired equipment brings boiler inspections, fuel storage and unloading, flue emissions and an attendant on every shift.
02Use case
The reference plant pulps wheat straw and bagasse, making about 60 tonnes of pulp a day. Weak black liquor goes to a multiple-effect evaporator and recovery boiler, and the mill runs round the clock.
| Heat user | Process temp. | Basis | Heat, kWh/day |
|---|---|---|---|
| Brown stock washer shower water | 35 → 70 °C | 25,000 L/day to final washer, 35 → 70 °C | 1,016 |
| Weak black liquor preheat | 70 → 85 °C | 60,000 L/day, ρ 1.1, cp 3.6, 70 → 85 °C | 990 |
| Caustic make-up and dilution | 30 → 80 °C | 10,000 L/day, ρ 1.1, cp 3.7, 30 → 80 °C | 565 |
| Storage and pipework standing losses | – | about 5% | 129 |
| Total useful heat | 2,700 | ||
Averaged over 20 running hours, the heat pumps need about 135 kW.
03P&ID
Tags: HP heat pump, TK tank, SG steam generator, P pump, PHE plate heat exchanger, TCV temperature control valve, TIC temperature indicating controller (panel), TT temperature transmitter, MT moisture transmitter, PT pressure transmitter, LT level transmitter, FS flow switch, NC normally closed. Process fluids are heated through their own exchanger, coil or jacket.
04Solution
System description
- 5 × SE-WWUHT-27 (135 kW total) (SE-WWUHT water-source, R245fa) delivering 95 °C, covering the day's 2,700 kWh in about 20.0 running hours.
- A 4,500 L insulated storage tank decouples the heat pumps from the process peaks.
- Heat source: evaporator surface-condenser water at about 50 °C. Cooling the surface-condenser water improves evaporator vacuum and reduces cooling-tower load.
- The biomass-fired steam boiler stays connected behind a normally closed valve as standby.
- Surface-condenser cooling water at about 50 °C passes through the heat pump evaporator before returning to the tower.
- The heat pump supplies 95 °C water to a wide-gap black liquor preheater, the washer shower water exchanger and the caustic tank coil.
- Existing steam heaters stay in place as standby and for trim.
Selected equipment
| Model | SE-WWUHT-27 × 5 |
|---|---|
| Heating capacity | 27 kW each, 135 kW total |
| COP | 2.93 estimated for a 50 °C source (the data sheet is rated on a 60/55 °C source) |
| Rating | water or steam to 120 °C, R245fa |
| Supply temperature | 95 °C |
| Storage | 4,500 L insulated tank |
Capacity is sized from the heat demand above. COP is an estimate until Sunniva engineering confirms the design.
Control philosophy
TT-101 at the tank top starts the heat pump below 90 °C and stops it at 95 °C, if the condenser water is above 42 °C. TIC-201 holds black liquor feed at 85 °C. TIC-202 holds shower water at 70 °C and TIC-203 holds the caustic tank. Steam heaters cut in only if a user stays 3 K below set point.
05Energy saving and ROI
Change any input to match a customer's site. Defaults are typical Indian prices for October 2026. The installed cost is a placeholder; replace it with the actual quotation.
| Existing | Heat pump | |
|---|---|---|
| Daily consumption | – | – |
| Annual energy cost | – | – |
| Cost per tonne of pulp | – | – |
| CO₂ per year | – | – |
06Benefits
More steam for evaporation
Steam no longer used for preheating is available to the evaporator, raising capacity or cutting boiler load.
Better washing
Hot showers improve brown stock washing and reduce soda loss to the paper machine.
Condenser load reduced
Taking heat from the condenser water helps evaporator vacuum and eases the cooling tower.
Lower energy cost
Each kWh of electricity delivers about 2.9 kWh of heat at the required temperature, against well under one kWh of useful heat per kWh of fuel burnt.
No flame on site
No burner, flue or fuel tank for this duty, and no fuel deliveries to schedule.
Built-in redundancy
Several machines share the load. If one is serviced, the others and the storage carry most of the day, with the existing heater in reserve.
07Engineering pointers
Site data to collect
- Pulp production and raw material
- Weak black liquor flow, solids and temperature
- Surface condenser water flow and temperatures
- Washer shower and caustic make-up volumes
- Steam used for preheating
- Present fuel or electricity use and monthly bill
- Sanctioned electrical load and spare capacity
- Space for the heat pumps and storage, with service access
Design notes
- Black liquor fouling. Agro-residue liquors carry silica and fibres. Use wide-gap or spiral exchangers and plan regular caustic boil-outs.
- Materials. Use stainless steel 316 or duplex on liquor and caustic sides, and follow caustic handling safety for the make-up tank.
- Source stream. The source must give up roughly two-thirds of the heat delivered. Confirm its flow and temperature over a full production day, and filter it before the evaporator.
- Water quality. Scale forms quickly above 60 °C. Use softened make-up and keep the heat pump loop closed, with process fluids on the far side of an exchanger or coil.
- Performance figure. The COP on this page is an engineering estimate, not a data sheet value. Sunniva engineering will confirm it for the actual source and supply temperatures.
Service and warranty
Sunniva has supplied 5,000+ heat pumps since 2012, with nine service technicians across India, including remote sites such as Leh. All components carry a one-year warranty.
Talk to us
Pulp mill liquors heated with condenser heat
How a 60 tonne-per-day agro-residue pulp mill can preheat weak black liquor, wash water and caustic with a water-source heat pump on evaporator condenser water, and what it saves against a biomass-fired steam boiler.
▶Video: Waste-heat recovery heat pumps
V10 · 2–2.5 minLifting warm water from condensers, effluent and process cooling to useful heat
Video coming soon01Problem statement
- Live steam for preheating. Weak black liquor, washer shower water and caustic make-up are heated with live steam that could otherwise drive the evaporator.
- Condenser heat lost. The evaporator's surface condenser rejects heat at 45–55 °C to a cooling tower.
- Washing needs hot water. Brown stock washing works better and uses less water with hot showers, but heating that water uses a lot of steam.
- Losses between fuel and process. Burning briquettes in a boiler or heater and distributing the heat loses a quarter or more of the fuel's energy before it reaches the process.
- Compliance, safety and handling. Fired equipment brings boiler inspections, fuel storage and unloading, flue emissions and an attendant on every shift.
02Use case
The reference plant pulps wheat straw and bagasse, making about 60 tonnes of pulp a day. Weak black liquor goes to a multiple-effect evaporator and recovery boiler, and the mill runs round the clock.
| Heat user | Process temp. | Basis | Heat, kWh/day |
|---|---|---|---|
| Brown stock washer shower water | 35 → 70 °C | 25,000 L/day to final washer, 35 → 70 °C | 1,016 |
| Weak black liquor preheat | 70 → 85 °C | 60,000 L/day, ρ 1.1, cp 3.6, 70 → 85 °C | 990 |
| Caustic make-up and dilution | 30 → 80 °C | 10,000 L/day, ρ 1.1, cp 3.7, 30 → 80 °C | 565 |
| Storage and pipework standing losses | – | about 5% | 129 |
| Total useful heat | 2,700 | ||
Averaged over 20 running hours, the heat pumps need about 135 kW.
03P&ID
Tags: HP heat pump, TK tank, SG steam generator, P pump, PHE plate heat exchanger, TCV temperature control valve, TIC temperature indicating controller (panel), TT temperature transmitter, MT moisture transmitter, PT pressure transmitter, LT level transmitter, FS flow switch, NC normally closed. Process fluids are heated through their own exchanger, coil or jacket.
04Solution
System description
- 5 × SE-WWUHT-27 (135 kW total) (SE-WWUHT water-source, R245fa) delivering 95 °C, covering the day's 2,700 kWh in about 20.0 running hours.
- A 4,500 L insulated storage tank decouples the heat pumps from the process peaks.
- Heat source: evaporator surface-condenser water at about 50 °C. Cooling the surface-condenser water improves evaporator vacuum and reduces cooling-tower load.
- The biomass-fired steam boiler stays connected behind a normally closed valve as standby.
- Surface-condenser cooling water at about 50 °C passes through the heat pump evaporator before returning to the tower.
- The heat pump supplies 95 °C water to a wide-gap black liquor preheater, the washer shower water exchanger and the caustic tank coil.
- Existing steam heaters stay in place as standby and for trim.
Selected equipment
| Model | SE-WWUHT-27 × 5 |
|---|---|
| Heating capacity | 27 kW each, 135 kW total |
| COP | 2.93 estimated for a 50 °C source (the data sheet is rated on a 60/55 °C source) |
| Rating | water or steam to 120 °C, R245fa |
| Supply temperature | 95 °C |
| Storage | 4,500 L insulated tank |
Capacity is sized from the heat demand above. COP is an estimate until Sunniva engineering confirms the design.
Control philosophy
TT-101 at the tank top starts the heat pump below 90 °C and stops it at 95 °C, if the condenser water is above 42 °C. TIC-201 holds black liquor feed at 85 °C. TIC-202 holds shower water at 70 °C and TIC-203 holds the caustic tank. Steam heaters cut in only if a user stays 3 K below set point.
05Energy saving and ROI
Change any input to match a customer's site. Defaults are typical Indian prices for October 2026. The installed cost is a placeholder; replace it with the actual quotation.
| Existing | Heat pump | |
|---|---|---|
| Daily consumption | – | – |
| Annual energy cost | – | – |
| Cost per tonne of pulp | – | – |
| CO₂ per year | – | – |
06Benefits
More steam for evaporation
Steam no longer used for preheating is available to the evaporator, raising capacity or cutting boiler load.
Better washing
Hot showers improve brown stock washing and reduce soda loss to the paper machine.
Condenser load reduced
Taking heat from the condenser water helps evaporator vacuum and eases the cooling tower.
Lower energy cost
Each kWh of electricity delivers about 2.9 kWh of heat at the required temperature, against well under one kWh of useful heat per kWh of fuel burnt.
No flame on site
No burner, flue or fuel tank for this duty, and no fuel deliveries to schedule.
Built-in redundancy
Several machines share the load. If one is serviced, the others and the storage carry most of the day, with the existing heater in reserve.
07Engineering pointers
Site data to collect
- Pulp production and raw material
- Weak black liquor flow, solids and temperature
- Surface condenser water flow and temperatures
- Washer shower and caustic make-up volumes
- Steam used for preheating
- Present fuel or electricity use and monthly bill
- Sanctioned electrical load and spare capacity
- Space for the heat pumps and storage, with service access
Design notes
- Black liquor fouling. Agro-residue liquors carry silica and fibres. Use wide-gap or spiral exchangers and plan regular caustic boil-outs.
- Materials. Use stainless steel 316 or duplex on liquor and caustic sides, and follow caustic handling safety for the make-up tank.
- Source stream. The source must give up roughly two-thirds of the heat delivered. Confirm its flow and temperature over a full production day, and filter it before the evaporator.
- Water quality. Scale forms quickly above 60 °C. Use softened make-up and keep the heat pump loop closed, with process fluids on the far side of an exchanger or coil.
- Performance figure. The COP on this page is an engineering estimate, not a data sheet value. Sunniva engineering will confirm it for the actual source and supply temperatures.
Service and warranty
Sunniva has supplied 5,000+ heat pumps since 2012, with nine service technicians across India, including remote sites such as Leh. All components carry a one-year warranty.