Methodology for selecting KKB for air handling units

Increasingly popular in the implementation of projects for central cooling systems are compressor-condensing units (CCU).

Used for cooling ventilation systems, they have a number of advantages:

• the cost of obtaining 1 kW of cold is less than with the installation of chiller cooling systems;
• convenient regulation with the help of standard modifications of controllers;
• simple installation without additional equipment: air ducts, fans and other equipment.

In the practical activities of various organizations, there are many illustrative examples of ventilation systems air conditioning, in which KKB either do not work or quickly fail. The reason for this is incorrectly performed calculation and selection of system elements.

When using typical methods for selecting units, there are often cases of errors that affect the operation of the system.

Let’s consider an example of the selection of a KKB and an evaporator according to the most common method that gives incorrect and distorted results. The calculation is carried out taking into account conditional parameters that clearly show the selection algorithm:

1. For the calculation, the initial data are taken — the air consumption of the supply unit, we accept 4500 m3 / h.
2. A direct-flow type supply unit is adopted, without recirculation, operating on the use of outside air.
3. The region of construction of the construction site is Moscow, the air temperature is +28 °С, humidity is 45%, at the outlet is +13 °С.
4. Using id diagram, the required amount of cold flow is determined. For this example, it is equal to 33.4 kW.
5. Taking into account the obtained value of the cold consumption (33.4 kW), the KKB is selected, considering the nearest major and minor modification.
6. A brand with a cooling capacity of 35.3 kW (TSA120/380-3) is accepted.

In the process of operation of the supply unit and the KKB model, the selection of which was carried out according to the described method, the following problematic issues arise:

KKB has an overestimated performance

When selecting a ventilation air conditioner, the parameters +28°С and 45% humidity were taken into account. But the operation of the air conditioner will be carried out at other temperatures.

The controller is set to +20°C or lower. KKB gives performance in the amount of 100% or 0%. In the event of a decrease in the parameters of the external air, the KKB does not reduce the performance, but can even increase it, using more subcooling in the condenser.

With a decrease in the inlet temperature, the KKB will also produce a lower outlet temperature of the air mass from the evaporator, which, according to the accepted calculated data at the outlet, is +3°C. This value is not true, given that the boiling point of freon in the evaporator is + 5 ° C.

A decrease in the indicator at the inlet to the evaporator to +22°C and below leads to an overestimation of the performance of the KKB. As a result of the freon not boiling in the evaporator, the refrigerant is returned to the compressor, which can lead to compressor breakdown and failure due to mechanical damage.

Selection of an underestimated evaporator

When selecting an air handling unit, it is required to set a number of parameters for the functioning of the evaporator: the inlet temperature is +28°C, humidity 45%, outlet +13°C, that is, the selection of the evaporator corresponding to these parameters is performed. When the temperature at the inlet to the evaporator changes (not +28°C, but +25°C), the amount of heat will be less proportional to the temperature change. But KKB still continues to give out 100% performance. Freon returns to the compressor, which can lead to compressor failure.

As follows from the calculations, the design temperature of the evaporator is the minimum operating parameter of the KKB.

Selection of KKB with a «margin»

When selecting a KKB, a negative factor is the value of the margin in terms of performance, taking into account the fact that the margin is a liquid freon on the compressor. This can cause the compressor to seize. The following rule should be observed: the evaporator capacity must exceed the compressor capacity.

For the correct selection of KKB for supply systems, it is necessary to take into account the following features:

First of all, it should be understood that the KKB cannot be the only source of cold in the building. The ventilation system conditioning process allows to reduce the maximum load. In any case, the use of local closers (VRF units or fan coil units) is required to maintain a given temperature regime inside the room.

Therefore, the function of the KKB is not to maintain a certain temperature regime in case of ventilation cooling (this cannot be done due to on-off regulation), but to reduce the heat supply to the premises when the specified outdoor temperature is exceeded.

Consider an example of calculating a ventilation system with air conditioning.

We accept the initial parameters for the calculation: the city of Moscow, temperature +28°C and 45% humidity. The flow rate of supply air is 4500 m3/h. Excess heat in the room from the operation of computer equipment and other devices and mechanisms, people in the room, solar UV radiation, etc. are about 50 kW. The level of the calculated temperature index in the premises is +22°С.

The performance of the air conditioning system must be selected in such a way that it is sufficient to create the worst conditions (at maximum temperature conditions). But, besides this, trouble-free operation of ventilation air conditioners is required with various intermediate options for operation. At the same time, ventilation air conditioning systems are operated for the predominant period of time at a load value 60-80%.

The algorithm for performing the calculation consists in carrying out the following operations:

• We accept as initial parameters the calculated temperature of the external air and the calculated temperature in the room. The main fundamentally important task of the KKB is the process of cooling the supply air to the level of the temperature regime in the room. In the event that the outdoor air temperature indicator is less than the set room air temperature indicator, the inclusion of the KKB is not required.

For the Moscow region from +28°С to the set room temperature of +22°С, the temperature difference is 6°С. The difference in temperature values ​​on the evaporator should not exceed more than 10 ° C, because the supply air temperature indicator cannot be less than the boiling point of freon;

• Taking into account the conditions for performing cooling of the supply air, the required productivity of the KKB is determined from the level of +28°С to +22°С. According to the chart id we find the value, which is 13.3 kW of cold;
• Based on the required performance level of 13.3 kW, we select the KKB model. At the same time, the closest positions are considered, represented by modifications of the popular manufacturer LENNOX. We select the closest KKB in size — the TSA036 / 380-3 model, the performance of which is 12.2 kW;
• We carry out the selection of the supply evaporator, taking into account the worst operating parameters for it. This is the equivalent outdoor air temperature corresponding to the room temperature — in this example +22°C. The cold productivity value of the evaporator is equal to the productivity of the KKB, i.e. 12.2 kW. Taking into account the possible contamination of the evaporator, a margin is added within 10-20%;
• We set the supply air temperature taking into account the outside temperature +22°C and get 15°C. It exceeds the boiling point of freon + 5 ° C and above the dew point level + 10 ° C, therefore, it is possible not to insulate the supply air ducts;
• We carry out the determination of the remaining heat surpluses of the premises. We have 50 kW of internal excess heat and a small value of heat from the supply air 13.3-12.2=1.1 kW. A total of 51.1 kW is the value of the calculated performance for local control ventilation systems.

Conclusions:

The principle of the performed calculation, which should be noted, provides for the need to calculate the KKB not for the maximum temperature regime, but for the minimum, considered in the interval of operation of the ventilation air conditioner.