As the name suggests, heat pumps are equipment which pump heat from a lower temperature heat source to a comparatively higher temperature heat sink. As the direction of heat flow is against the natural course, external work has to be supplied to achieve the required pumping.
The vapour absorption based heat pump essentially operates with three utilities
- Low temperature heat source – from which heat is absorbed
- Driving heat source – which supplies work for the pumping
- Product hot water stream – to which heat is added in the heat pump
Depending on the relative temperatures of the low temperature heat source, driving heat source and desired hot water temperatures, the heat pump is operated using different cycles to cater to the altered requirement.
Accordingly, Heat pumps can be classified into:
- Heat Pump Type I
- Heat Pump Type II (Heat Transformer)
In Type I Heat Pump, a low-grade heat source (40°C – 35°C), typically cooling water is circulated in the evaporator. A high-grade driving heat source (180°C – 160°C) is utilized in the generator by means of which heat rejection is done to a medium grade heat medium (60°C – 90°C).
That is, the heat absorbed in the evaporator from the low-grade heat source & the heat absorbed in the generator from a high-grade driving heat source is rejected to medium grade heat medium circulated in the absorber and condenser.
Heat Balance equation Co-efficient of performance
Q1 = Q2 + Q3 COP = Q2/Q1
= 0.45
Heat Pump Type II
Heat pump Type II, better known as Heat Transformer, transforms a waste heat source to a more useful heat source by raising its temperature. A medium grade heat source (110°C – 100°C) is circulated in evaporator as well as generator, where heat is absorbed into the heat pump.
Part of this absorbed heat is rejected to a higher-grade heat medium (150°C – 160°C) in the absorber which is the actual output of the heat pump, and the remaining heat is rejected to cooling water (30°C) circulated in the condenser.
That is, medium grade waste heat is added to high-grade heat utilizing medium grade waste heat source.
That is, 45% of waste heat that was initially rejected to atmosphere can be pumped to a useful heat source (product hot water/steam), thus enhancing overall process efficiency greatly. The heat pump output hot water can be flashed in a flash tank to provide steam as per the desired pressure.
Heat Pump Type II Cycle Description
In the evaporator, refrigerant evaporates by absorbing heat from the waste heat source circulated through the evaporator tubes.
The vapour thus generated is absorbed by the strong LiBr solution being sprayed in the absorber. This absorption of water vapour by LiBr solution is an exothermic process; that is, heat is released during dilution of the LiBr solution. The hot water which is circulated through the absorber tubes absorbs the heat released during dilution of LiBr solution. As such, the hot water temperature increases.
Once the LiBr solution has absorbed water vapour in the absorber section, its concentration reduces; the dilute LiBr solution is then pumped to the generator where the absorbed water vapour is boiled off and the concentration of LiBr solution is increased; the heat for boiling is provided by the waste heat source circulated through the generator tubes; the strong LiBr solution is then brought back to the absorber for spraying.
The water vapour boiled off in the generator is condensed in the condenser by rejecting heat to the cooling water circulated through the condenser tubes. The condensed water vapour, refrigerant, is brought back to the evaporator for cycle continuation.
As cooling water is acting as a heat sink for condensation, the condensing temperature is lower and as such generator pressure (generator and condenser are in the same shell, and hence at equal pressure) is also low.
As boiling point is a function of pressure, for the lower generator pressures, LiBr solution boils at lower temperatures and as such the lower temperature waste heat source can provide the required heat.
Advantages of Heat Pump Type II:
- 45% of waste energy is converted to useful energy.
- That is, energy efficiency of the process or system shall increase due to internal heat recovery.
- Correspondingly, the heat rejection to atmosphere of the process/system shall also reduce, which means fin fanned coolers/ dry coolers/ Cooling tower heat duties shall reduce.
- Further, benefit of reduction in CO2 emissions (carbon credits) can be attained due to higher energy efficiency and lower energy requirements.
- Additionally, vapour absorption machines attract higher rate of depreciation as an energy conservation device and hence the feasibility can be further improved due to tax savings.
Applications
Poly film manufacturing process for photovoltaic cells
Thermax has successfully commissioned a heat transformer in Poly film manufacturing process for photovoltaic cells for a customer based in China.
The manufacturing process requires 100oC water. During the process, this water temperature increases to 110oC. For reutilization, the 110oC water leaving the process was cooled to 100oC in a dry cooler where the heat was simply rejected to the atmosphere.
Using Heat Pump Type II, the available heat was utilized for generation of 4 barg steam, which finds use within the manufacturing process.
Operating Parameters
Heat source
Medium (waste) heat source inlet temp = 108°C
Medium (waste) heat source outlet temp = 100°C
Medium (waste) heat source flow rate = 1000 m3/h
Product
Hot water inlet temp = 152°C
Hot water outlet temp = 157°C
Hot water flow rate = 765 m3/h
Flash steam generation = 6.5 TPH @ 4 bar(g) (from hot water)
Food industries
Pasta Manufacturing Process
In this application, Gas engine’s jacket water is cooled in the Type II heat pump. Using the 93oC jacket water as heat source, hot water of 125oC is heated to 135oC.This hot water of 135oC is then used in a pasta manufacturing process.
Pre-cooked Meal Manufacturing Process
In this application also, Gas engine’s jacket water is cooled in the Type II heat pump. Using the 90°C jacket water as heat source, hot water of 95°C is heated to 105°C. This hot water of 105°C is then used in Pre-cooked Meal manufacturing process.
Similarly, in food & beverage industries, many product streams are at elevated temperature after processing and have to be cooled before packing & storage. Also, the heat sources for the same processing would normally include hot water or steam up to 2-4 bar g. In such cases, Heat Pump Type II can be used for heat recovery from the product streams and enhance the overall efficiency of the process.
Refineries & Petrochemicals
The product streams of many processes in refineries are at elevated temperatures, these have to be cooled before taking to filling & storing on account of high volatilities to avoid evaporation losses. Also, high temperature heat sources i.e. steam @ 4-8 bar are required in substantial quantities for innumerable processes. Here, Heat pump Type II can be used to pump heat from the medium temperature streams to higher temperature, to provide steam @ desired pressure.
This will not only reduce steam costs but also reduce the costs incurred for product cooling which may include use of dry coolers or any other such indirect heat exchangers where effectively the heat is rejected to the atmosphere.
Engine jacket water cooling
Normally, the jacket water @ 93°C-95 °C has to be cooled up to 78°C-80°C. This heat can be used to generate steam 1 bar(g) – 1.5 bar(g) steam depending on the available cooling water temperatures.
For GE 1.063 kW el. Engine
Epilogue
Heat pumps help increase heat recovery within the processes, ultimately, enhancing the energy efficiency of the system. Obviously, heat rejections to the atmosphere reduce, making the systems more environment-friendly. Also, if heat pump integration is considered meticulously during the plant process design stage itself, design loads for hot water generating systems, boilers, fin-fanned coolers, condensers or cooling towers can be optimized. Accordingly, the capital investment for these equipment shall reduce along with the ensured operational savings. Again, heat pumps are compact like chillers, they are easy to erect, commission and operate; supplied as single unit, erection simply encompasses connecting the utility circuits to the Heat pump; The Evolved PLC controlled operation allows single command start-stop and fully automated operation. As such, Heat pump is a simple solution to increase the energy efficiency of the process.
AUTHORS CREDIT & PHOTOGRAPH
P Babu
Head – Technology & Innovation, Thermax
