REFRIGERATION & AIR-CONDITIONING

Module 6. Vapour absorption refrigeration system

LESSON 24
PROPERTIES OF REFRIGERANT – ABSORBENT COMBINATION

24.1 Introduction

1. Refrigerant should be much more volatile than absorbent.

2. Refrigerant properties must provide moderate +ve pressures same as VC

3. Both should be chemically stable at all operating conditions same as VC

4. They should not form solid phase in the operating conditions same as VC

5. Absorbent should have strong affinity for refrigerant.

6. Both should not cause corrosion in the range of conditions same as VC

7. Should not be toxic and inflammable.

8. Low viscosity to promote heat and mass transfer.

9. Refrigerant should have high latent heat to reduce mass flow same as VC rate.

10. Both must be completely miscible in liquid and vapour phases and no range of concentration values where a heterogeneous mixture would exist.

24.2 Simple Vapour & Improved Vapour

● Lot of heat need to bring strong liquor to Generator temperature.
● Hot weak liquor had lesser capacity to absorb NH3 & cooling water will be needed more.
● Substantial carry – over of water with NH3 from generator to condenser. This water raises the boiling point higher in Evaporator.

Table 24.1

Table 24.2

Table 24.3

Table 24.4

Analysis of Aqua-Ammonia Refrigeration System

The analysis and design of aqua-ammonia refrigeration system, can be done by representing the processes and condition of aqua on X-h (concentration-ethalpy) chart at various components of the system. The input and output of the system being in the form of heat (enthalpy) only, the system performance can be calculated once the enthalpies at different points are known.
Once the temperatures at condenser & evaporator are known and the load to be taken by the system is known, then we can decide the circulation of aqua as well as pure NH₃ to be used in the system. Design of the sizes and arrangements of the different components of the system will follow.

Representation of Absorption system on Concentration-Enthalpy chart of Ammonia - Water

The working components of the system and concentration chart are shown in Fig. (a) and Fig: (b). The X-axis represents concentration (X) and Y-axis represents enthalpies in kJ/kg. The concentration is defined as amount of NH₃ in kg per kg of mixture. The lower part of the diagram is for the mixture of NlH₃ & water (liquid form) whereas the upper part of the diagram corresponds to NH₃ & water (vapour form) mixture. Both the parts of the diagram are for the same pressure lines (full lines). On the lower part of the diagram, constant temperature lines (dotted) are also drawn. In addition to this, auxiliary lines are also drawn as shown in Figure which are only pressure lines. They do not help to get any properties of solution of gaseous mixture but their use is to locate the points in gaseous mixture as per given conditions.

1) The concentration X = 0 means, there is no NH₃ and mixture is pure water.

Fig. 24.1

Fig. 24.2

(2) The concentration X = 1 means, there is no water and mixture is pure NH₃

(3) If the concentration X = 0.4 (say) & pressure is 12 bar and then the temperature of the mixture will be 90 °C, (from chart) if the mixture is saturated as it will have only one & one temperature at saturated condition. But if the temperature of the aqua is less than 90 °C, then it indicates that the aqua NH₃ is at sub-cooled condition.

The basic components of NH₃ - absorption system and the conditions of aqua at different components of the aqua NH₃ system.

Point -1 This represents pure NH₃ - saturated vapour at condenser pressure (P_c) having a concentration X = 1. This point can be marked on X-h chart in vapour region as X & P_c are known.

Point - 2 This represents pure NH₃ - saturated liquid at P_c & X = 1. This point is marked in liquid region.

Point - 3 This represents the condition of pure NH₃ (wet) but at pressure P_c (evaporator pressure) but X = 1 and h remains constant as 2-3 is throtting process. The point - 2 also represents point 3.

Point - 4 This represents the condition of pure NH₃ at pressure P_e but saturated vapour which absorbs heat in the evaporator and converts from wet vapour to saturated vapour. This point is marked in vapour region.

Point - 5 This represents the strong aqua-solution coming out of absorber after absorbing vapour coming in from evaporator. Say X_s (concentration of strong solution = 0.6) and pressure is P_e. This point can be marked as X_s & P_e at point 5 are known. It is always assumed as saturated aqua at point '5' if not mentioned.

Point - 6 This is the condition of the aqua solution whose X_s = 0.6 (say) but the pressure is increased from P_e to P_c as it passes through the pump. when the aqua pressure increases passing through the pump.

Point - 7 As the strong low temperature aqua solution passes through heat exchanger, it gains the heat and its enthalpy increases but its concentration C_s remains same as well as pressure Pc also remains same.

Point - 8 The point '8' represents hot weak liquid (X_w) and this point can be located under the following two conditions.

(1) If degasifying factor is known

The degasifying factor is the amount of NH₃-gas removed from the strong liquid entering in the generator maintaining the pressure constant by supplying the heat Q_g (per kg of pure NH₃) in the generator. This gasification factor lies between 0.05 to 0.1 only even high gasification factor is desirable. This is because, higher gasification factor can also evaporate more water vapour & it creates troubles in evaporator & it is necessary to be removed completely before entering into the condenser.

If say the gasification factor is known and it is 0.1, then the concentration of aqua at point 8 = 0.6 - 0.1 = 0.5 as X_s (concentration at point 7) = 0.6 is known. The point '8' can be marked as X_s = 0.5 and pressure at the point 8 is P_c. The condition at point '8' can be considered as saturated liquid if not mentioned.

(2) If T _g (temperature in the generator) is known

The point 8 can be marked as it is the cross­ section of the pressure line P_c and temperature line T_g.

Point - 9 This shows the condition of weak liquid coming out of heat exchanger after giving heat to the strong solution so, the enthalpy is reduced. Deducting the heat lost by the weak solution in heat exchanger, the point 9 can be marked as concentration does not change.

Point -10 The point 10 represents the same enthalpy as at point 9 but at reduced pressure (p_e) as it passes through the pressure reducing valve.

Now join the points 8 and 7 and extend till it cuts to Y-axis l(b). Then join point-a and 5 and extend till it cuts the vertical line passing through '8' as shown. This also decides the position of the point 9 or 10.

Absorber

In absorber, the pure NH₃ gas enters at condition 4 and weak aqua solution enters at condition 10 and after mixing, strong aqua comes out at condition 5. The mixing occurring inside is undefined but aqua condition coming out is definitely known, Join the points 10 and 4 and extend the vertical line passing through point 7 till it cuts at point 7". Now we can say that mixing is taking place along the line 4-10 and at pressure P_c & resulting aqua is coming out at 5 after losing heat in the absorber. Joining the points 4 & 10 and marking point 7" is not necessary for solving the problems or designing the system components.

Generator

In generator, strong aqua is heated by supplying heat Q_g. The strong aqua enters into the generator at condition 7 and pure NH₃ vapour comes out at condition 1 and weak aqua at condition 8. Now join the points 8 and 1 and extend the vertical line through point 7 to mark the point 7' which cuts the line 1-8. Now, we can say that the separation is taking place along the line 1-8 and at pressure P_c. Joining the points 1 and 8 and marking the point 7' is not necessary for solving the problems or designing the system components.

Determination of Enthalpy change at Different Components of the System

The enthalpy change Q at different components and specific enthalpy ‘q’ are calculated in the following steps.

(1) Heat Removed in Condenser

The amount of heat removed in the condenser is given by

Q_c = (h₂ – h₁) kJ/kg of NH₃.

As NH₃ saturated vapour enters in and NH₃ saturated liquid comes out.

This can be directly red from the X-h chart

(2) Heat Absorbed in the Evaporator

The amount of heat removed in the evaporator is given by.

Q_c = (h₄ - h₃) kJ/kg of NH₃.

Where h₄ is the heat of saturated vapour at P_c and h₃ is the heat of mixture of NH₃ liquid & vapour at P_e or heat of NH₃ liquid at point ‘2’ as 2 - 3 is constant enthalpy throttling process.

(3) Heat Removed from the Absorber

When NH₃ vapour at 4 and aqua at 10 are mixed, the resulting condition of the mixture in the absorber is represented by 7” and after losing the heat in the absorber (as it is cooled), the aqua comes out at condition 5. Therefore, the heat removed in the absorber is given by.

q_a = (h ^“₇ - h₅) kJ/kg of aqua.

By extending the triangle ∆ 10 - 7”- 5 towards right till 10 - 7” cuts at 4 and 10-5 cuts at point - a on y - axis. Therefore heat removed per kg of NH₃ is given by

Q_a = (h₄ – h_a) kJ/kg of NH₃.

(4) Heat Given in the Generator

Q_g is the heat supplied in the generator and Q_d is the beat removed from the water vapour, then the net heat removed per kg of aqua is given by

(q_g - q_d)= (h₇^’ - h₇) kJ/kg of aqua.

as the aqua goes in at 7 and comes out at condition 8 and 1 which can be considered a combined condition at 7'.

By extending the triangle ∆ 8 - 7 - 7' towards right till 8 - 7' cuts at 1 and 8 - 7 cuts at 'a' on Y- axis then, the heat removed per kg of NH₃ is given by

(Q_g - Q_d) = (h₁ - h_a) kJ/kg of NH₃.

Now for finding out Q_d separately, extend the verticle line 7 - 7' till it cuts the auxilary P_c line and mark the point ‘b'. Then draw a horizontal line through 'b' which cuts the P_c line (in vapour region) at point 11. Then join the points ’7 and 11 and extend that line till it cuts Y-axis at 12. Then the Q_d is given by

(Q_d = (h₁₂ - h₁) kJ/kg of Nh₃.

From the above two equations, Q_g can also be calculated.

Now Q_c Q_e Q_a Q_g and Q_d per kg of NH₃ flowing in the system will be known from the X-h chart.