Don't Get Frozen Out
Low charge ammonia spray chillers cool fluids without freezeup.
Spray evaporators have been used successfully in the winery and poultry industries for decades. One of the positive attributes of a spray evaporator is its ability to work under limited refrigerant charge. These chillers are used to cool fluids as close to the freeze point as is possible without freezeup.
To avoid freezup, a system must be designed to operate at suction temperatures close to the process-fluid freeze point and still be efficient under close approach temperatures. Obviously, plain surface tubes have a limit on approach temperature. For this reason, enhanced surface tubes that would require low wall superheat often are used.
Some key features of spray evaporators are:
- They have relatively higher heat transfer coefficients, so for the same size and load, the log-mean temperature difference (LMTD) is lower.
- The refrigerant suction temperature can be raised, resulting in augmented compressor capacity, thus maintaining the suction temperature at close to or higher than the freeze point of process fluid.
- The refrigerant charge is orders of magnitude lower than the same capacity flooded evaporator, making it an environmentally attractive option.
- Because of the low charge, which means the shell is devoid of liquid refrigerant, spray evaporators are well guarded against freezeup if process pumps or refrigerant control fail.
- There is no hydrostatic head penalty, so there is no adverse affect on the LMTD.
Spray evaporators work effectively if the feed ratio is higher than 2:1 so that no section of the bundle is starved of liquid refrigerant. An appropriate refrigerant distribution system, as proposed by Zeng, et al.1, is an essential element of an optimized evaporator with full-cone spray nozzles. Consider liquid refrigerant being distributed through an array of full-cone nozzles above a tube bundle (figure 1), wherer is radius of the area that each spray nozzle covers.
Figure 1. Liquid refrigerant is distributed through an array of full-cone nozzles above a tube bundle. The equations help determine the refrigerant charge.
w is the tube bundle width.A1 is the portion of the sprayed liquid that will miss the bundle. The area of A1 should be minimized to reduce the parasitic pumping power.
A2 is the area covered by two neighboring nozzles. This also should also be minimized to avoid thicker film.
Hence, for an optimized design, A1+A2 should be minimized as much as possible. A1 and A2 are expressed as:
A1 = 0.5r( – sin)
A2 = r( – – sin)
d(A1+A2)/d = 0
The resultant equation can be solved for cone angle ().
In order to minimize (A1+A2), a second derivative must be greater than zero:
This results in angle, , equal to a constant 109.5°. Thus, the optimal distance between the two adjacent nozzles is
L = 2r sin(/2) = 1.63r = 1.41w
In this example, the optimal radius of coverage by each nozzle is
r = 0.866w
The corresponding height of the nozzle above the top row of tubes is
d = 0.866w cot (/2)
Because none of the previous falling film correlations were applicable due to the mechanics of the flow and the wide range of saturation pressure, Zeng2 adopted the modified Chun and Seban3 function but substituted a non-dimensional heat flux, represented by , for a dimensional heat flux, represented by q”, as proposed by Parken4. To account for the effect of saturation temperature, a reduced pressure ratio was also added to the correlation as follows:
Nu = 0.0568 Re-0.0058 Pr0.193 pr0.323 1.034
(-10°F < TS < 50°F)
Nu = h/k (2/g)1/3
Re = 2Γf/µ
= q”D/(Tcr – Ts)k
Spray Evaporators in Action
Spray evaporators can be used in many industrial process cooling applications. A few examples help illustrate how.
Water Chiller Used at a Chemical Plant. An ammonia spray evaporator for cooling water was installed at a chemical plant with double-enhanced surface tubes. The top half of the bundle had low fintubes and the lower half carried structured surface tubes. This concept helped in distributing the cascading liquid ammonia in the lower section of the bundle.
For optimum efficiency of the spray evaporator, it is vital to keep the entire tube bundle wet. Water Chiller Used at a Food Plant. A custom-designed ammonia spray evaporator was supplied to a herring processor. The design was based on capacity with ammonia suction at -18°F (-7°C) and sodium chloride brine flow rate of 3,500 gal/min with outlet temperature of -4°F (-20°C). The total ammonia charge in the evaporator is less than 500 lb.
According to the end-user, the chiller has exceeded all expectations. The first year after installation, 750 to 800 tons a day were processed, compared to 450 tons a day the year before, making it the world’s largest herring processor. During the last season, 917 tons per day were logged-in with even a higher suction temperature of -12°F (-24°C).
In view of the current trend toward higher efficiency, sustainability and environmental concerns, the expanded use of ammonia seems a reasonable approach. In order to handle the only drawback regarding ammonia - its toxicity - a case can be made if systems are designed with extremely low charge such as spray evaporators. PC
1. Zeng, X., Chyu, M.-C., and Ayub, Z. H., 1995, Evaporation Heat Transfer Performance of Nozzle-Sprayed Ammonia on a Horizontal Tube, ASHRAE Transactions, Vol. 101, pt. 1: 136-149.
2. Zeng, X., Chyu, M.-C., and Ayub, Z. H., 1995, Nozzle-Sprayed Flow Rate Distribution on Horizontal Tube Bundle, ASHRAE Transactions, Vol. 101, pt. 2: 443-353.
3. Chun, K.R. and Seban, R.A., 1971, Heat Transfer to Evaporating Liquid Films, J. Heat Transfer: 391-396.
4. Parken, W.H., Fletcher, L.S., Sernas, V., and Han, J.C., 1990, Heat Transfer through Falling Film Evaporation and Boiling on Horizontal Tubes, J. Heat Transfer, 112: 744-750.