A cooling system comprising: an indirect evaporative heat exchanger including: a heat exchanger assembly including: a plurality of tubes, each tube having a first end, a second end, and an outer surface, each tube being configured to (i) have a process fluid flow therethrough in a first direction from the first end to the second end and (ii) have a scavenger cooling medium flow over the outer surface of each tube in a second direction, the second direction intersecting the first direction; and a plurality of sections aligned in the first direction, each section of the plurality of sections including a portion of each of the plurality of tubes; and a plurality of nozzles located above the plurality of tubes, at least one nozzle of the plurality of nozzles being (i) located in each of the plurality of sections and (ii) configured to selectively discharge coolant onto the portion of the tube in that section of the heat exchanger assembly; characterised by a mechanical cooling system including evaporator coils configured to (i) have the process fluid flow therethrough and (ii) cool the process fluid; and a controller having a water loss mode corresponding to a loss of water supply event, the controller being configured to, in the water loss mode: (i) selectively discharge water from the nozzles located in one section of the indirect evaporative heat exchanger to cool the process fluid; and (ii) operate the mechanical cooling system to cool the process fluid. an indirect evaporative heat exchanger including: a heat exchanger assembly including: a plurality of tubes, each tube having a first end, a second end, and an outer surface, each tube being configured to (i) have a process fluid flow therethrough in a first direction from the first end to the second end and (ii) have a scavenger cooling medium flow over the outer surface of each tube in a second direction, the second direction intersecting the first direction; and a plurality of sections aligned in the first direction, each section of the plurality of sections including a portion of each of the plurality of tubes; and a plurality of nozzles located above the plurality of tubes, at least one nozzle of the plurality of nozzles being (i) located in each of the plurality of sections and (ii) configured to selectively discharge coolant onto the portion of the tube in that section of the heat exchanger assembly; characterised by a heat exchanger assembly including: a plurality of tubes, each tube having a first end, a second end, and an outer surface, each tube being configured to (i) have a process fluid flow therethrough in a first direction from the first end to the second end and (ii) have a scavenger cooling medium flow over the outer surface of each tube in a second direction, the second direction intersecting the first direction; and a plurality of sections aligned in the first direction, each section of the plurality of sections including a portion of each of the plurality of tubes; and a plurality of tubes, each tube having a first end, a second end, and an outer surface, each tube being configured to (i) have a process fluid flow therethrough in a first direction from the first end to the second end and (ii) have a scavenger cooling medium flow over the outer surface of each tube in a second direction, the second direction intersecting the first direction; and a plurality of sections aligned in the first direction, each section of the plurality of sections including a portion of each of the plurality of tubes; and a plurality of nozzles located above the plurality of tubes, at least one nozzle of the plurality of nozzles being (i) located in each of the plurality of sections and (ii) configured to selectively discharge coolant onto the portion of the tube in that section of the heat exchanger assembly; characterised by a mechanical cooling system including evaporator coils configured to (i) have the process fluid flow therethrough and (ii) cool the process fluid; and a controller having a water loss mode corresponding to a loss of water supply event, the controller being configured to, in the water loss mode: (i) selectively discharge water from the nozzles located in one section of the indirect evaporative heat exchanger to cool the process fluid; and (ii) operate the mechanical cooling system to cool the process fluid. (i) selectively discharge water from the nozzles located in one section of the indirect evaporative heat exchanger to cool the process fluid; and (ii) operate the mechanical cooling system to cool the process fluid.