Scroll compressors COPELAND SCROLL ™ ZF * EVI and ZF Summit can use an economizer with compression of the resulting vapor. The use of an economizer contributes to an increase in refrigeration capacity and COP when compared with a conventional refrigeration cycle.

Technological features of steam injection give tangible results under conditions of low boiling points, where an increase in refrigeration capacity and COP is most needed. This technology allows a smaller compressor to be used for a given heat load. Plus steam injection creates additional cooling conditions, which affects the expansion of the operating range of the compressor, allowing it to operate in low-temperature conditions with high compression ratios.

In the simplest case, the circuit design is shown in Fig. one.

Part of the condensed liquid “i” passes through the expansion valve and enters a counter-flow plate heat exchanger (economizer), where it evaporates, while supercooling the main stream of liquid “m”. Superheated steam from the economizer is injected into the compression chamber of the scroll compressor at an intermediate point in the compression process.

Subcooling the liquid in the economizer increases the cooling capacity of the evaporator and the entire refrigeration system due to a decrease in the temperature of the liquid and subsequently enthalpy. We want to emphasize this process: the refrigeration capacity of the system increases due to an increase in the enthalpy difference between the supercooled liquid refrigerant at the inlet to the evaporator and the superheated vapor at the outlet from it. The additional mass flow “i” also increases heat recovery in the condenser.

The energy efficiency of this system is higher than a standard system of the same capacity. Using an economizer, part of the refrigeration capacity is generated, spending less energy because part of the steam is compressed to the condensing pressure not from low evaporating pressure, but a higher intermediate injection pressure. This effect is achieved by adding steam to the compression process with intermediate pressure and temperature, controlled by the expansion valve of the economizer.

The design features of these compressors, which make it possible to implement a cycle with an economizer.

Savings in numbers and graphs

The increase in the refrigerating capacity of models ZF * EVI and ZF Summit compressors (R404A, t 0 / t к = –35 / + 40 о C) with an economizer reaches an average of 46% (Fig. 2).

Example. (the prices shown are indicated at the time of this writing and may differ in any direction at the time of reading)

Required refrigerating capacity – 6 kW at conditions: R404A, t 0 = –35 о C; t to = +40 about C; Sc = 2 K, Sh = 15 K.

Using the ZF scroll compressors, the required cooling capacity can be obtained in two ways:

•   compressor ZF34K5E-TFD without steam injection with a refrigerating capacity of 6.05 kW (price 3505 euros);
•   compressor ZF18KVE-TFD EVI with steam injection, cooling capacity of 5.93 kW (price 2,743 euros).

A compressor with steam injection, with the same refrigerating capacity, costs 762 euros, or 28%, less than a compressor without steam injection.

The increase in the COP of ZF * EVI and ZF Summit Scroll compressors with economizer is illustrated in fig. 3.

The increase in COP under the conditions described above is 26%. Naturally, the cost of electricity significantly depends on the region and the consumer, yet it is advisable to demonstrate a reduction in operating costs using a specific example. For the compressors in the above example, and under the same operating conditions, we get:

•   compressor ZF34K5E-TFD without steam injection: energy consumption – 27015 kWh per year, energy cost – 1350 euros
•   compressor ZF18KVE-TFD EVI with steam injection: energy consumption – 27015 kWh per year, energy cost – 1093 euros

In the calculations, the time utilization factor was 0.6; the conditional cost of electricity was 0.05 euros per 1 kWh. For some, electricity is more expensive, for others, it is cheaper than this value, but it is important to note that the savings in electricity per compressor amounted to 23%, or 257 euros, per year.

Let us compare the COP of the ZF * EVI and ZF Summit compressors with the popular competitors in the market, as shown in fig. four.

Compressors in the ZF * EVI and ZF Summit series with economizer have a tangible energy efficiency advantage over competitors.

Compressor stations: It is clear that using several ZF * EVI and ZF Summit compressors as part of a compressor station, energy savings are multiplied, and investment costs are significantly reduced. Several circuit solutions are possible, each of which has its advantages and disadvantages. One solution is using an individual (one heat exchanger for each compressor) and a common economizer. Let’s consider a scheme with a common economizer (Fig. 5).

It is recommended to use an electronic expansion valve (EEV) as an expansion device for a common economizer (see Fig. 5). The steam injection line for each compressor must be equipped with an individual solenoid valve (EK) to be able to shut off the steam injection when the compressor is off. This is necessary to avoid getting a large amount of liquid into it while parked. The use of an economizer allows in this case a reduction in the number of necessary compressors and therefore lowers the initial investment in the refrigeration system.

As noted in the article, COPELAND SCROLL ™ ZF compressors with an economizer have significant advantages over their counterparts and competitors.

The main advantages in their use in a low-temperature compressor-condensing unit (CCA) or compressor station (CC) are as follows:

•   a compressor of lower refrigerating capacity is required (for KKA)
•   fewer compressors are needed (for compressor stations)
•   the capacitor can be smaller
•   the dimensions and weight of the compressor station and the spacecraft are reduced
•   the noise level is reduced
•   energy efficiency (COP) is significantly improved

Thus, the use of an economizer with ZF * EVI and ZF Summit compressors in low-temperature refrigeration systems results in a significant reduction in both initial costs and operating costs.