Condenser Water System Components

Cooling towers

Cooling towers reject energy (building load and heat of compression) from water-cooled chilled-water systems. To reject heat, water is passed through the cooling tower where a portion of it evaporates. How close the leaving tower water temperature is to the outdoor air wet-bulb temperature is called the approach. The approach changes as outdoor condition, heat rejection, and cooling tower airflow vary. The lower the approach temperature, the colder the water temperature will be leaving the cooling tower. The approach is important because the tower leaving water temperature is the same as the chiller entering condenser water temperature.

During operation, approach changes as follows:¹

• At a decreased heat rejection load, approach decreases.

• At a constant heat rejection load, as wet bulb decreases, approach increases.

• As water flow decreases, approach decreases.*

• As tower fan speed is reduced, approach increases.

Let’s turn our attention to cooling tower performance as fan speed is reduced. It is important to understand how tower fan speed and airflow reduction affect cooling tower fan energy.

At off-design conditions, cooling tower fan speed may be modulated. Figure 1 shows that the tower fan power varies with the cube of the speed – so reducing fan speed by just 30 per cent (i.e., to 70 per cent) results in a power reduction of almost 66 per cent.

Note that even when the tower fan is off, due to convection there is still heat rejection available – often a bit above 15 per cent of the tower capacity. 

Condenser water pumps

Condenser water pumps supply the required condenser water flow rate and overcome the pressure drop through the chiller’s condenser, the condenser water pipes, elbows and valves, and lift the water from the basin to the top of the cooling tower (the static lift). As flow varies, the pressure drop through the system and the condenser vary approximately with the square, but static lift remains constant.

The equation for pump power is shown below. As flow rate and pressure drop decrease, the pump power decreases. The effect of condenser pump power reduction on optimal system operation will be discussed in HPAC’s May/June 2013 issue.

Condenser water pump kW = gpm x ΔP x 0.746/3960 x PE x PME x PDE†

Chillers

A chiller’s compressor must produce the pressure difference (lift) between the evaporator refrigerant pressure and the condenser refrigerant pressure. The evaporator pressure is determined primarily by the chilled water leaving temperature. Often people think of the entering condenser water temperature as setting the pressure the chiller’s compressor must produce. That is incorrect. The condenser refrigerant temperature and pressure are determined primarily by the leaving condenser water temperature—which in turn is determined by the entering condenser water temperature, chiller heat rejection, and flow rate.

As shown in the first two columns of Table 1, for a given cooling tower, as flow rate is reduced, the entering tower water temperature rises. Note, the range difference is 4.7F, but since the tower approach temperature improves, the entering tower water temperature only increases by 2.8F. Entering tower water temperature is the same as the chiller leaving condenser water temperature. So, as flow rate is reduced, the chiller leaving condenser water temperature rises, as does the chiller power. <>

REFERENCE

1. SPX Cooling Technologies. 1986. “Cooling Tower Performance: Basic Theory and Practice.” Marley Cooling Tower Information Index. Available at http://spxcooling.com/pdf/CTII-1.pdf.

In HPAC May/June look for Condenser water system savings: Optimizing flow rates and control.

Sidebar

Cooling tower cells

A cooling tower cell consists of the structure, media, and fan. It should be noted that it is more efficient to operate multiple tower cells at part speed than one tower cell at full speed. For example, one cell operating at full speed (40 hp) and the other off gives about 58 per cent of the tower’s capacity. Two cells with fans each operating at 60 per cent—a total of 20 hp—gives 60 per cent of the tower’s capacity.