Energy spending by a heat pump is mainly used for compressing the refrigerant vapor,
or mechanical energy (compression heat pump) or thermal energy (absorption heat pump).
In the system described, this is the vaporization (by spraying and atomization) of the refrigerant in a high pressure carrier gas which provides the high-pressure refrigerant : the energy necessary for spraying (atomization) being less than the energy required for compression (by comparing with a compression system), the coefficient of performance will be higher.
The NH3 / H2O couple was chosen as an example but other couples may agree.
Similarly, the carrier gas for the vaporization can be a gas other than air.
Four main elements constitute the system (Figure 1):
- In the low pressure zone:
1) the desorber which receives heat from the environment and releases NH 3 gas from the solution,
2) the condenser wherein the NH3 gas is condensed by cooling in the vaporizer, a heat exchanger connecting the two elements,
- In the high pressure zone:
3) the spray in which the liquid refrigerant (liquid NH3) is vaporized by spraying in the carrier gas (air) with decrease in temperature and supply of heat from the condenser,
4) the absorber wherein the NH3 gas contained in the air is absorbed by the solution with supply of heat to the hot source.
It must be added:
- a heat exchanger solution absorber / desorber solution,
- a heat exchanger air / air + NH3,
- pumps and regulation.
2. SPRAY - CONDENSEUR.
See Figure 2.
Main condition: the temperature in the vaporizer must be low enough to allow the NH3 condensation in the condenser.
t3: NH3 gas temperature entering the condenser
tc: NH3 condensation temperature in the condenser (tc <t3)
t1: temperature of the air entering the vaporizer
tmin: minimum temperature in the vaporizer
t2: temperature of the saturated air by NH3 at the outlet of the vaporizer.
a) t3 = t2 + pinch
t1 = t2 + pinch
-> t1 = t3.
b) Calculation tmin.
1) Content x NH3 in the air (kg / kg).
If we accept the approximation of ideal gases, we have:
for a kg of dry air: P (air) * V = R (air) * T
for x kg NH3 gas: P (NH3) * V = R * x (NH3) * T
R (air) = 29.24 m /) K
R (NH3) = 49.87 m / ° K
P (total) = P (air) + P (NH3) (Dalton's Law)
-> x = 0.586 * P (NH3) / (P (total) -P (NH3)) (1)
See Figure 2.
For 1 kg of dry air: i = 0.24 * t1 (Kcal / kg).
For 1 kg of air saturated with x NH3 kg : i = 0.24 * t min + x * (i (vap sat) - i (liq sat)) = 0.24 * t min + x * Q (vap)
(X and Q (vap) considered at the tmin temperature).
The transformation is isenthalpically.
-> tmin = t1 - (x * Q (vap)) / 0.24 (2)
The system should consist of two stages A and B (see Figure 3) to obtain a sufficient temperature above.
Can validly estimate the difference of temperature between tc (NH3 condensation temperature in the condenser) and tmin (minimum temperature in the
spray) at least 20 ° C to ensure condensation of NH3.
t1 = t3 = -25 ° C
tc = -42 ° C
lower pressure: 0.7 kg / cm²
upper pressure: 5 kg / cm²
By successive approximations, we obtain : t min = -42 -23 = -65 ° C
Data of the table of constants of the saturated vapor NH3 at -65 ° C:
p '= 0.223 kg / cm²
Q (vap) = 343.9 kcal / kg
(1) x = 0.586 * 0.223 / (5 - 0.223) = 0.0274 kg / kg
(2) tmin = -25 - (0.0274 * 343.9) / 0.24 = -25 - 39.26 = -64.26 ° C
t1 = t3 = 10 ° C
tc = -10 ° C
lower pressure: 3 kg / cm²
pressure above 20 kg / cm²
By successive approximations, we obtain = t min = -10 -23 = -33 ° C
Data of the table of constants of the saturated vapor NH3 at -33 ° C:
p '= 1.0576 kg / cm²
Q (vap) = 326.54 kcal / kg
(1) x = 0.586 * 1.0576 / (20-1.0576) = 0.0327 kg / kg
(2) tmin = 10 - (0.0327 * 326.54) / 0.24 = 10 - 44.49 = -34.49 ° C
So we get well, for each stage A and B, a difference of temperature a little above 20 ° C, between the condensation temperature of NH3
and the minimum temperature in the vaporizer.
Note: in practice, the element "condenser" can be simply constituted by a spiral tube (or other form of heat exchanger) within the element "spray".