A pipe freezing apparatus 2 has pipe freezer heads 12a, 12b supplied with refrigerant by a compressor 4. The freezer heads 12a, 12b comprise expansion chambers in which the refrigerant expands, to cool the heads. A valve 10 alternately diverts flow of the refrigerant to the freezer heads 12a, 12b. When a supply to a head 12a, 12b is shut off, the compressor lowers the pressure in the head, resulting in enhanced cooling when supply to the head is restored. The compressor 4 can be run continuously while the cooling at the heads is controlled.
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