Selecting an Ammonia-Water Absorption Heat Pump System

Steam use and cooling load remain high even though one process rejects heat while another needs heat between 140 and 220°F. An ammonia-water absorption system can couple those duties, but selection starts with temperatures, heat rates, and pressure definitions—not with a nominal equipment capacity.

Cycle mechanism and boundary definitions

An absorption cycle replaces the vapor compressor with an absorber, a liquid-solution pump, and a thermally driven generator. Ammonia acts as the refrigerant and water as the absorbent. Refrigerant vapor enters the absorber and dissolves into the solution; absorption releases heat. The pump raises the pressure of the liquid solution. Generator heat then separates ammonia-rich vapor from the solution so the refrigerant can condense, expand, and evaporate again.

The term heat of absorption here means the absorber heat duty resulting from vapor absorption, solution mixing, and sensible enthalpy changes. It is not a single constant applicable across 0–50 psig and 140–220°F. Calculate it from the inlet and outlet states using ammonia-water equilibrium and enthalpy data at the actual pressure, temperature, and concentration.

Check 1: Temperature lift and heat matching

Plot both process streams as temperature versus cumulative heat duty. A heat pump cannot be selected from the endpoint range alone because the controlling condition is the temperature approach throughout each exchanger, including any boiling, condensing, or solution-temperature glide.

1. Read the cooling-source inlet and required outlet temperatures. Expect a defined heat-removal profile rather than only “140–220°F.” If the source temperature falls during cooling, use the cold-end condition for the limiting branch.
2. Read the heating-sink inlet and required outlet temperatures. Expect the refrigerant or solution temperature to remain above the sink temperature by a positive exchanger approach at every point. A temperature crossover means the proposed state points cannot deliver the duty.
3. Read the generator heat-source temperature. Expect it to exceed the required solution-generation temperature after exchanger approach and losses. If it does not, an absorption cycle cannot regenerate the solution at that operating point.
4. Compare simultaneous heat rates. Expect the recoverable cooling duty and usable heating duty to overlap in time. If either process operates alone, define heat rejection, supplemental heating, storage, or turndown for the unmatched period.

If the temperature profiles cross, change the process targets, split the duties, or evaluate another cycle architecture. If they remain feasible, continue with pressure and equilibrium calculations.

Check 2: Pressure basis and equilibrium states

The stated 0–50 psig range is ambiguous until assigned to a fluid and location. Gauge pressure is referenced to local atmospheric pressure, while refrigerant property calculations require absolute pressure. Convert each measured value with P_abs = P_gauge + P_atmospheric, using the atmospheric pressure applicable to the installation.

1. Identify every pressure tap. Expect separate readings for the low-pressure evaporator/absorber side and the high-pressure generator/condenser side. One range applied to the entire cycle is not a usable design basis.
2. Convert gauge pressure to absolute pressure. Expect positive absolute values recorded with units. Using psig directly in an equilibrium model produces incorrect saturation and concentration states.
3. Check ammonia-water equilibrium at each state. Expect a feasible refrigerant-rich solution leaving the absorber and a refrigerant-lean solution leaving the generator at their respective temperatures and pressures.
4. Check vapor composition leaving the generator. Expect sufficient ammonia purity for the condenser and evaporator duty. Water carried with the vapor changes condensation and evaporation behavior, so the separation or rectification requirement belongs in the cycle model.

If equilibrium states cannot be closed at the measured pressures, correct the pressure basis first. If they still do not close, the proposed temperatures and concentrations do not define a viable ammonia-water cycle.

Check 3: Absorber and system heat balances

Use measured or calculated stream enthalpies; a tabulated heat of absorption alone omits sensible heat and concentration effects. Define m_ref as ammonia-rich refrigerant vapor flow, m_lean as refrigerant-lean solution flow, and m_rich as refrigerant-rich solution flow.

For a steady absorber with negligible heat loss, calculate the removed heat as Q_abs = m_ref h_ref,in + m_lean h_lean,in - m_rich h_rich,out, with m_rich = m_ref + m_lean. Keep one enthalpy reference and one concentration convention across all property data. A sign reversal usually indicates a mixed convention or an incorrectly assigned inlet state.

Close the remaining components with the same method. The evaporator duty is Q_evap = m_ref(h_out - h_in). The generator balance must include rich-solution inlet, lean-solution outlet, refrigerant-vapor outlet, and generator heat input. The condenser balance uses the refrigerant inlet and outlet enthalpies. Add solution-pump electrical input and measured heat losses when closing the complete system.

1. Check mass closure. Expect total mass and ammonia mass entering each component to equal the amounts leaving it.
2. Check component energy closure. Expect each residual to fall within the combined accuracy of the flow, temperature, pressure, composition, and property calculations.
3. Check useful-duty accounting. Expect only heat accepted by the process sink to count as delivered heat; rejected heat is not a steam saving.

Check 4: Working-pair and equipment screening

Do not select another refrigerant or absorption pair from the 140–220°F range alone. Screen each candidate against all cycle state points, required pressure, solution stability, material compatibility, toxicity controls, vapor purity, available property data, and service capability.

Resolving procedure and acceptance verification

1. Instrument both process streams for inlet temperature, outlet temperature, and flow; collect coincident operating data across the production range.
2. Label every pressure as process-side or working-fluid pressure and as gauge or absolute. Convert refrigerant calculations to absolute pressure.
3. Calculate source and sink heat rates from measured flow and enthalpy change. Plot both temperature-duty curves.
4. Select trial evaporator, absorber, generator, and condenser state points. Obtain ammonia-water equilibrium and enthalpy values from a validated property source covering those states.
5. Close total mass, ammonia mass, and energy balances. Size the absorber from Q_abs, not from a constant heat-of-absorption value.
6. Calculate heating performance using a declared boundary, such as COP_h = useful process heat delivered / total generator heat supplied. Report pump electricity separately or include it through a clearly stated energy conversion basis.
7. Check exchanger approaches, off-design conditions, startup, shutdown, turndown, unmatched heating or cooling operation, materials, containment, and relief requirements before requesting equipment selection.

Verification check 1: Expect measured process heat removal to match Q_evap within the stated measurement uncertainty.

Verification check 2: Expect useful absorber and condenser heat accepted by the sink to match the process-side heat gain within the stated uncertainty.

Verification check 3: Expect the complete cycle energy balance to close after generator heat, pump input, useful heat, cooling duty, and measured losses are included.

Frequently asked questions